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The temperature outside is below freezing, the winds are howling, and you can barely see the tree in your front yard. A large amount of snow has been falling for more than three hours-a blizzard is under way.

According to the National Weather Service, blizzards are snowstorms with winds higher than 35 miles an hour and visibility (meaning, how far you can see) of less than a quarter mile. Winds from a blizzard can travel over 120 miles an hour-that's faster than a cheetah, the world's fastest land animal.

These conditions can be dangerous-low visibility can cause drivers to crash on roads, and high winds can topple power lines, which means some homes will be without heat. Large amounts of snowfall can trap people inside their homes. And those stuck outside in freezing temperatures can face frostbite or hypothermia, a condition in which your body temperature becomes dangerously low.

Learn more about these dangerous winter storms-and what to do if one comes your way.

How blizzards develop

Before you can have a blizzard, you first need snow. And for snow, you need clouds.

Clouds form when part of the air becomes cold enough that water vapor, which is the gas form of water, condenses and becomes tiny water droplets that form around things like dust and sea salt. When those droplets collect more water, they fall as rain. At temperatures between about minus 40°F and 32°F, the water droplets crystallize. When enough crystals stick together, they form snowflakes and fall to the ground.

What makes a snowstorm a blizzard is strong wind. First, warm air from the Equator mixes with cold air from the Arctic. Wind forms when warm air rises over the cold air; the faster the warm air rises over the cold air, the stronger the wind becomes.

Another type of winter storm is called a ground blizzard. This is when gusty winds-often 50 to 60 miles an hour-lift up snow that's already on the ground. Both types of blizzards can cause whiteouts, a condition in which so much snow is blowing so fast that it's hard to see anything. Sometimes you can't even tell the difference between the sky and the ground!

Blizzards in the future

Blizzards occur all over the world. But because they're closer to the Arctic, places like Russia, central and northeastern Asia, northern Europe, Canada, and the northern United States experience more blizzards than other parts of the globe.

And though most blizzards occur during winter, they can happen during other seasons, too. One of the most severe blizzards in the United States-known as the Great Blizzard of '88-happened in March 1888, killing more than 400 people.

Scientists believe that as global temperatures continue to rise because of climate change, blizzards could become more extreme. A warmer atmosphere means more moisture in the air that will eventually fall as snow (or rain); many also believe that warmer temperatures will create higher winds.

How to survive a blizzard

The best way to survive a blizzard is to prepare for extreme weather. Meteorologists use certain words and phrases that will help you know when a blizzard is coming. For instance:

Wind chill factor: This is what the temperature feels like outside, even if the real temperature is higher. Blizzards usually have wind chill factors of 10°F or lower.
Winter storm outlook or watch: This means that conditions are right for a snowstorm or blizzard to form.
Winter weather advisory: This means that dangerous winter weather is happening, which could be a blizzard.

Learn more about ways you can protect your family from dangerous winter weather.
Before a blizzard:
* Ask your parents to "winterize" your house: Check that heating sources are clean and working, and make sure windows are insulated so cold air can't come in. (If they're not, apply plastic sheets on the inside.)
* If you have a fireplace, stock up on firewood or coal.
* Make sure your family has a home emergency kit: water (at least a gallon for each person per day for several days), food (non-perishable or canned items for several days), flashlight, extra batteries, first-aid kit, manual can opener, local maps, battery-powered radio, and cell phone with a backup battery.
* Charge devices like cell phones.
During a blizzard:
* Listen to local radio or news for updated weather information.
* Layer up if you lose power. Try a t-shirt under a long-sleeved shirt under a sweater under a coat. That way you can take off items if you start to sweat. (Being cold and wet will just make you colder!) Other must-haves: two pairs of socks, mittens instead of gloves, a thick scarf, and a hat.
* Drink lots of water-or hot cocoa, or tea, or whatever-to stay hydrated. You might not think you need as much liquid in the winter as you do in summer, but staying hydrated helps your body generate enough heat to maintain a healthy body temperature.
* Evacuate only if you lose power and your home becomes too cold to stay in. Try to find a shelter-like a nearby school or library-that has heat and electricity.
If you must leave your house:
* Try to travel during the day when visibility will be better.
* Load up on blankets, canned fruits, bottles of water, and warm fluids in a thermos; bring an external battery in case you need to recharge your cell phone.
* Don't forget the layers!
* If you get stuck in your car, huddle with your companions for extra warmth.
* If you're caught outside, huddle together against a tree or a rock ledge to block the wind, and do not sit down on snow. (Your non-frozen butt will thank you.) And don't eat the snow-that could dangerously lower your body temperature.
* If you or your companions show signs of hypothermia or frostbite (shivering, feeling very tired, confusion, drowsiness, memory loss, slurred speech), seek help immediately.

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Learn how these winter snowstorms form-and how you can stay safe if one blows your way.

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Tornado research at the National Severe Storms Laboratory, with information about current research into tornadoes and its role in improving tornado forecasts and warnings to help save lives.

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Although they can occur at any time of the day or night, most tornadoes form in the late afternoon. By this time the sun has heated the ground and the atmosphere enough to produce thunderstorms.

Tornadoes form when warm, humid air collides with cold, dry air.

The denser cold air is pushed over the warm air, usually producing thunderstorms. The warm air rises through the colder air, causing an updraft. The updraft will begin to rotate if winds vary sharply in speed or direction.

As the rotating updraft, called a mesocycle, draws in more warm air from the moving thunderstorm, its rotation speed increases. Cool air fed by the jet stream, a strong band of wind in the atmosphere, provides even more energy.

Water droplets from the mesocyclone's moist air form a funnel cloud. The funnel continues to grow and eventually it descends from the cloud. When it touches the ground, it becomes a tornado.

Ron Johnson, Peoria Journal Star/AP

Characteristics of tornadoes

Twisters are usually accompanied or preceded by severe thunderstorms and high wlnds. Hail is also common.

Once a tornado hits the ground, it may live for as little as a few seconds or as long as three hours.

The average twister is about 660 feet wide and moves about 30 miles an hour. Most don't travel more than six miles before dying out.

Massive tornadoes, however-the ones capable of widespread destruction and many deaths-can roar along as fast as 300 miles an hour.

These measurements are scientists' best estimations. Anemometers, which measure wind speed, cannot withstand the enormous force of tornadoes to record them.

Using units F0 to F5, the Fujita scale measures a tornado's intensity by analyzing the damage the twister has done and then matching that to the wind speeds estimated to produce comparable damage. The United States now uses the EF (Enhanced Fujita) scale, which takes more variables into account when assigning wind speeds to a tornado.

Tornado destruction

Every year in the United States, tornadoes do about 400 million dollars in damage and kill about 70 people on average.

Extremely high winds tear homes and businesses apart. Winds can also destroy bridges, flip trains, send cars and trucks flying, tear the bark off trees, and suck all the water from a riverbed.

High winds sometimes kill or injure people by rolling them along the ground or dropping them from dangerous heights. But most tornado victims are struck by flying debris-roofing shingles, broken glass, doors, metal rods.

The number of average deaths per year in the United States used to be higher before improved forecasting and warning systems were put into place.

How tornadoes are forecasted

Meteorologists at the U.S. National Weather Service use Doppler radar, satellites, weather balloons, and computer modeling to watch the skies for severe storms and tornadic activity.

Doppler radars record wind speeds and identify areas of rotation within thunderstorms. Since Doppler radar has been in use, the warning time for tornadoes has grown from fewer than five minutes in the 1980s to an average of 13 minutes by the late 2000s.

When weather conditions are conducive for tornado formation, the National Weather Service issues a tornado watch. When a tornado has been sighted or indicated on radar, a tornado warning is issued.

Some scientists, meteorology buffs, and adrenaline junkies hit the road during tornado season to chase storms. Researchers race to place sensors in tornadoes' paths. The sensors measure data such as wind speed, barometric pressure, humidity, and temperature.

The challenge for researchers is being in the right place at the right time. Every morning they study weather conditions and head for the area that seems most likely to spawn a twister. They drive through severe storms, dodge lightning, face flash floods, and get pounded by hail-sometimes for years-before ever spotting a tornado.

All at considerable risk. In 2013, National Geographic Explorer Tim Samaras and his team were killed while trying to study a tornado in Oklahoma. ( See photos of Samaras's work.)

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Learn how these deadly storms form and wreak havoc, and how you can reduce your risk.

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In the Atlantic and Northeast Pacific, we use the term "hurricane" to describe severe storms with high-velocity winds that rotate around a central, low-pressure core. The same type of disturbance in the Northwest Pacific is called a "typhoon" and "cyclones" occur in the South Pacific and Indian Ocean.

In order for a hurricane to form, two things must be present: a weather disturbance, such as a thunderstorm, that pulls in warm surface air from all directions and water at the ocean's surface that is at least 80° Fahrenheit (27° Celsius). Because it is the interaction of warm air and warm seawater that spawns these storms, they form over tropical oceans between about 5 and 20 degrees of latitude. At these latitudes, seawater is hot enough to give the storms strength and the rotation of the Earth makes them spin.

Hurricanes start simply with the evaporation of warm seawater, which pumps water into the lower atmosphere. This humid air is then dragged aloft when converging winds collide and turn upwards. At higher altitudes, water vapor starts to condense into clouds and rain, releasing heat that warms the surrounding air, causing it to rise as well. As the air far above the sea rushes upward, even more warm moist air spirals in from along the surface to replace it.

As long as the base of this weather system remains over warm water and its top is not sheared apart by high-altitude winds, it will strengthen and grow. More and more heat and water will be pumped into the air. The pressure at its core will drop further and further, sucking in wind at ever increasing speeds. Over several hours to days, the storm will intensify, finally reaching hurricane status when the winds that swirl around it reach sustained speeds of 74 miles per hour or more.

Eventually, hurricanes turn away from the tropics and into mid-latitudes. Once they move over cold water or over land and lose touch with the hot water that powers them, these storms weaken and break apart.

Recent studies have shown a link between ocean surface temperatures and tropical storm intensity - warmer waters fuel more energetic storms.

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For More Information

Hurricanes - Multimedia Discovery Missions
National Hurricane Center

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In the Atlantic and Northeast Pacific, we use the term "hurricane" to describe severe storms with high-velocity winds that rotate around a central, low-pressure core. The same type of disturbance in the Northwest Pacific is called a "typhoon" and "cyclones" occur in the South Pacific and Indian Ocean.

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El Niño & La Niña (El Niño-Southern Oscillation)

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El Niño-the warm phase of the El Niño-Southern Oscillation ("ENSO") climate pattern-is currently chugging along, and forecasters expect it to continue for the next several months, with a 62% chance of lasting through April­-June 2024. It's looking like it will be a strong event.

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Welcome to Britannica Library , it provides your patrons with access to three sites in one, with age appropriate material from the child-friendly collection through to the adult general reference collection.

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This article was most recently revised and updated by

blizzard, severe weather condition that is distinguished by low temperatures, strong winds, and large quantities of either falling or blowing snow. The National Weather Service of the United States defines a blizzard as a storm with winds of more than 56 km (35 miles) per hour for at least three hours and enough snow to limit visibility to 0.4 km (0.25 mile) or less. A severe blizzard has winds of over 72 km (45 miles) per hour, visibility near zero, and temperatures of −12 °C (10 °F) or lower. A ground blizzard occurs when there is no falling snow, but snow is drifting and blowing near the ground.

The name originated in the central United States, where blizzards are brought by northwesterly winds following winter depressions, or low-pressure systems. In the United States and in England, the term is often used for any strong, heavy snowstorm with wind. In Antarctica, blizzards are associated with winds spilling over the edge of the ice plateau at an average velocity of 160 km (about 100 miles) per hour.

John P. Rafferty.

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Blizzard | Snowstorms, Winter Storms, Arctic Winds

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Blizzard, severe weather condition that is distinguished by low temperatures, strong winds, and large quantities of either falling or blowing snow. The National Weather Service of the United States defines a blizzard as a storm with winds of more than 56 km (35 miles) per hour for at least three

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thunderstorm, a violent short-lived weather disturbance that is almost always associated with lightning, thunder, dense clouds, heavy rain or hail, and strong gusty winds. Thunderstorms arise when layers of warm, moist air rise in a large, swift updraft to cooler regions of the atmosphere. There the moisture contained in the updraft condenses to form towering cumulonimbus clouds and, eventually, precipitation. Columns of cooled air then sink earthward, striking the ground with strong downdrafts and horizontal winds. At the same time, electrical charges accumulate on cloud particles (water droplets and ice). Lightning discharges occur when the accumulated electric charge becomes sufficiently large. Lightning heats the air it passes through so intensely and quickly that shock waves are produced; these shock waves are heard as claps and rolls of thunder. On occasion, severe thunderstorms are accompanied by swirling vortices of air that become concentrated and powerful enough to form tornadoes.

Thunderstorms are known to occur in almost every region of the world, though they are rare in polar regions and infrequent at latitudes higher than 50° N and 50° S. The temperate and tropical regions of the world, therefore, are the most prone to thunderstorms. In the United States the areas of maximum thunderstorm activity are the Florida peninsula (more than 80 thunderstorm days per year, with some areas over 100), the Gulf Coast (60-90 days per year), and the mountains of New Mexico (50-80 days per year). Central Europe and Asia average 20 to 60 thunderstorm days per year. It has been estimated that at any one moment there are approximately 1,800 thunderstorms in progress throughout the world.

This article covers two major aspects of thunderstorms: their meteorology (i.e., their formation, structure, and distribution) and their electrification (i.e., the generation of lightning and thunder). For separate coverage of related phenomena not covered in this article, see tornado, ball lightning, bead lightning, and red sprites and blue jets.

Thunderstorm formation and structure

Vertical atmospheric motion

Most brief but violent disturbances in Earth's wind systems involve large areas of ascending and descending air. Thunderstorms are no exception to this pattern. In technical terms, a thunderstorm is said to develop when the atmosphere becomes "unstable to vertical motion." Such an instability can arise whenever relatively warm, light air is overlain by cooler, heavier air. Under such conditions the cooler air tends to sink, displacing the warmer air upward. If a sufficiently large volume of air rises, an updraft (a strong current of rising air) will be produced. If the updraft is moist, the water will condense and form clouds; condensation in turn will release latent heat energy, further fueling upward air motion and increasing the instability.

Once upward air motions are initiated in an unstable atmosphere, rising parcels of warm air accelerate as they rise through their cooler surroundings because they have a lower density and are more buoyant. This motion can set up a pattern of convection wherein heat and moisture are transported upward and cooler and drier air is transported downward. Areas of the atmosphere where vertical motion is relatively strong are called cells, and when they carry air to the upper troposphere (the lowest layer of the atmosphere), they are called deep cells. Thunderstorms develop when deep cells of moist convection become organized and merge, and then produce precipitation and ultimately lightning and thunder.

Upward motions can be initiated in a variety of ways in the atmosphere. A common mechanism is by the heating of a land surface and the adjacent layers of air by sunlight. If surface heating is sufficient, the temperatures of the lowest layers of air will rise faster than those of layers aloft, and the air will become unstable. The ability of the ground to heat up quickly is why most thunderstorms form over land rather than oceans . Instability can also occur when layers of cool air are warmed from below after they move over a warm ocean surface or over layers of warm air. Mountains, too, can trigger upward atmospheric motion by acting as topographic barriers that force winds to rise. Mountains also act as high-level sources of heat and instability when their surfaces are heated by the Sun.

The huge clouds associated with thunderstorms typically start as isolated cumulus clouds (clouds formed by convection, as described above) that develop vertically into domes and towers. If there is enough instability and moisture and the background winds are favourable, the heat released by condensation will further enhance the buoyancy of the rising air mass. The cumulus clouds will grow and merge with other cells to form a cumulus congestus cloud extending even higher into the atmosphere (6,000 metres [20,000 feet] or more above the surface). Ultimately, a cumulonimbus cloud will form, with its characteristic anvil-shaped top, billowing sides, and dark base. Cumulonimbus clouds typically produce large amounts of precipitation.

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thunderstorm | Definition, Types, Structure, & Facts

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thunderstorm, a violent short-lived weather disturbance that is almost always associated with lightning, thunder, dense clouds, heavy rain or hail, and strong gusty winds. Thunderstorms arise when layers of warm, moist air rise in a large, swift updraft to cooler regions of the atmosphere.

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NOAA's Atlantic Oceanographic and Meteorological Laboratory

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Radar

Each WP-3D aircraft has three radars: nose, lower fuselage, and tail. The nose radar (a solid-state C-band radar with a 5° circular beam) is used strictly for flight safety and is not recorded for research purposes. The lower fuselage and tail radars are used for operational and research purposes. The G-IV aircraft has a nose and a tail radar too.

Expendables
Dropwindsondes are deployed from the aircraft and drift down on a parachute measuring vertical profiles of pressure, temperature, humidity and wind as they fall. They are released from the both the WP-3D and G-IV aircraft over data-sparse oceanic regions.
Airborne eXpendable BathyThermographs (AXBT)
Airborne eXpendable Current Profilers (AXCP)
Airborne eXpendable Conductivity Temperature and Depth probes (AXCTD)
Drifting buoys

Oceanographic instruments may be deployed from the WP-3D aircraft either from external chutes using explosive cads or from an internal drop chute. They activate upon hitting the ocean surface and radio sea temperature, salinity, and current information back to computers aboard the aircraft.

Visit Expendables Page

Remote Sensing

Among the suite of airborne remote sensing instruments available on the WP-3D aircraft for the purpose of measuring surface winds in and around tropical cyclones are the Stepped Frequency Microwave Radiometer and the C-band scatterometer (C-SCAT). The C-SCAT conically scans the ocean surfaceobtaining backscatter measurements from 20° to 50° off nadir

C-Band Scatterometer (C-SCAT) The C-SCAT antenna is a microstrip phased array whose main lobe can be pointed at 20°, 30°, 40°, and 50° off nadir. The antenna is rotated in azimuth at 30 rpm. Thus, conical scans of the ocean surface are repeated every 2 s (0.25 km at 125 m/s ground speed).

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AOML's hurricane research division has been in operation longer than AOML itself, and is developing next generation hurricane modelling.

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Extreme events are occurrences of unusually severe weather or climate conditions that can cause devastating impacts on communities and agricultural and natural ecosystems. Weather-related extreme events are often short-lived and include heat waves, freezes, heavy downpours, tornadoes, tropical cyclones and floods. Climate-related extreme events either persist longer than weather events or emerge from the accumulation of weather or climate events that persist over a longer period of time. Examples include drought resulting from long periods of below-normal precipitation or wildfire outbreaks when a prolonged dry, warm period follows an abnormally wet and productive growing season.

Scientists usually define an extreme event using either of two approaches. The first approach examines the probability or chance of an event of a given magnitude occurring within a certain reference period (e.g., 1961 - 1990); here an extreme event has a low probability of occurring at a given location (< 10%) and is typically of a high intensity. This type of probabilistic approach is applied in extreme event attribution to determine whether global warming is driving changes in the frequency and intensity of extreme events.

The second approach is more widely applied by the climate adaptation community and uses impact-related thresholds to determine if an event is extreme, and to inform adaptive solutions. A common example here is the number of consecutive days over 100˚F, which can be used to quantify heat waves. Of course, impacts associated with a 100˚F threshold will vary according to location - what might be an extreme event in one place (e.g., Burlington, VT) may be within the normal range somewhere else (e.g., Phoenix, AZ). Thus, thresholds are often location-specific.

This portion of the Climate Hubs website expands on the types of extreme events that impact the different climate hub regions. For example, strong tropical cyclones (hurricanes or typhoons) pound Atlantic and Pacific coastal regions and islands with torrential rain, powerful winds and flooding while drought and wildfire regularly impact Western U.S. States.

The resources listed below also provide more details on the definition and monitoring of extreme climate and weather events, extreme event attribution, and extreme weather events in the context of climate change.

Herring, S.C., Christidis, N., Hoell. A., Kossin, J.P., Schreck C.J., Stott, P.A. (Eds.), 2018. Explaining Extreme Events of 2016 from a Climate Perspective. Special Supplement to the Bulletin of the American Meteorological Society, Vol. 99, No. 1.

IPCC, 2012: Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation. A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change [C.B. Field, V. Barros, T.F. Stocker, D. Qin, D.J. Dokken, K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen, M. Tignor, and P.M. Midgley (eds.)], Cambridge University Press, Cambridge, UK, and New York, NY, USA, 582 pp.

Lindsey, R., 2016. Extreme event attribution: the climate versus weather blame game, https://www.climate.gov/news-features/understanding-climate/extreme-event-attribution-climate-versus-weather-blame-game, ClimateWatch Magazine, NOAA.

National Academies of Sciences, Engineering, and Medicine, 2016. Attribution of Extreme Weather Events in the Context of Climate Change, Washington, DC: The National Academies Press. doi: 10.17226/21852.

Walsh, J., et al., 2014: Ch. 2: Our Changing Climate. Climate Change Impacts in the United States: The Third National Climate Assessment, [J. M. Melillo, T.C. Richmond, and G. W. Yohe (Eds.)], U.S. Global Change Research Program, 19-67. doi:10.7930/J0KW5CXT.

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Extreme events are occurrences of unusually severe weather or climate conditions that can cause devastating impacts on communities and agricultural and natural ecosystems. Weather-related extreme events are often short-lived and include heat waves, freezes, heavy downpours, tornadoes, tropical cyclo…

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El Niño is a climate pattern that describes the unusual warming of surface waters in the eastern tropical Pacific Ocean. El Niño is the "warm phase" of a larger phenomenon called the El Niño-Southern Oscillation (ENSO). La Niña, the "cool phase" of ENSO, is a pattern that describes the unusual cooling of the region's surface waters. El Niño and La Niña are considered the ocean part of ENSO, while the Southern Oscillation is its atmospheric changes.

El Niño has an impact on ocean temperatures, the speed and strength of ocean currents, the health of coastal fisheries, and local weather from Australia to South America and beyond. El Niño events occur irregularly at two- to seven-year intervals. However, El Niño is not a regular cycle, or predictable in the sense that ocean tides are.

El Niño was recognized by fishers off the coast of Peru as the appearance of unusually warm water. We have no real record of what indigenous Peruvians called the phenomenon, but Spanish immigrants called it El Niño, meaning "the little boy" in Spanish. When capitalized, El Niño means the Christ Child, and was used because the phenomenon often arrived around Christmas. El Niño soon came to describe irregular and intense climate changes rather than just the warming of coastal surface waters.

Led by the work of Sir Gilbert Walker in the 1930s, climatologists determined that El Niño occurs simultaneously with the Southern Oscillation. The Southern Oscillation is a change in air pressure over the tropical Pacific Ocean. When coastal waters become warmer in the eastern tropical Pacific (El Niño), the atmospheric pressure above the ocean decreases. Climatologists define these linked phenomena as El Niño-Southern Oscillation (ENSO). Today, most scientists use the terms El Niño and ENSO interchangeably.

Scientists use the Oceanic Nino Index (ONI) to measure deviations from normal sea-surface temperatures. El Niño events are indicated by sea surface temperature increases of more than 0.9° Fahrenheit for at least five successive three-month seasons. The intensity of El Niño events varies from weak temperature increases (about 4-5° F) with only moderate local effects on weather and climate to very strong increases (14-18° F) associated with worldwide climatic changes.

In order to understand the development of El Niño, it's important to be familiar with non-El Niño conditions in the Pacific Ocean. Normally, strong trade winds blow westward across the tropical Pacific, the region of the Pacific Ocean located between the Tropic of Cancer and the Tropic of Capricorn. These winds push warm surface water toward the western Pacific, where it borders Asia and Australia.

Due to the warm trade winds, the sea surface is normally about 0.5 meter (1.5 feet) higher and 7.2° C (12.96° F) warmer in Indonesia than Ecuador. The westward movement of warmer waters causes cooler waters to rise up toward the surface on the coasts of Ecuador, Peru, and Chile. This process is known as upwelling.

Upwelling elevates cold, nutrient-rich water to the euphotic zone, the upper layer of the ocean. Nutrients in the cold water include nitrates and phosphates. Tiny organisms called phytoplankton use them for photosynthesis, the process that creates chemical energy from sunlight. Other organisms, such as clams, eat the plankton, while predators like fish or marine mammals prey on clams.

Upwelling provides food for a wide variety of marine life, including most major fisheries. Fishing is one of the primary industries of Peru, Ecuador, and Chile. Some of the fisheries include anchovy, sardine, mackerel, shrimp, tuna, and hake.

The upwelling process also influences global climate. The warm ocean temperature in the western Pacific contributes to increased rainfall around the islands of Indonesia and New Guinea. The air influenced by the cool eastern Pacific, along the coast of South America, remains relatively dry.

El Niño Events

El Niño events are defined by their wide-ranging teleconnections. Teleconnections are large-scale, long-lasting climate anomalies or patterns that are related to each other and can affect much of the globe.

During an El Niño event, westward-blowing trade winds weaken along the Equator. These changes in air pressure and wind speed cause warm surface water to move eastward along the Equator, from the western Pacific to the coast of northern South America.

These warm surface waters deepen the thermocline, the level of ocean depth that separates warm surface water from the colder water below. During an El Niño event, the thermocline can dip as far as 152 meters (500 feet).

This thick layer of warm water does not allow normal upwelling to occur. Without an upwelling of nutrient-rich cold water, the euphotic zone of the eastern Pacific can no longer support its normally productive coastal ecosystem. Fish populations die or migrate. El Niño has a devastating impact on Ecuadorian and Peruvian economies.

El Niño also produces widespread and sometimes severe changes in the climate. Convection above warmer surface waters bring increased precipitation. Rainfall increases drastically in Ecuador and northern Peru, contributing to coastal flooding and erosion. Rains and floods may destroy homes, schools, hospitals, and businesses. They also limit transportation and destroy crops.

As El Niño brings rain to South America, it brings droughts to Indonesia and Australia. These droughts threaten the region's water supplies, as reservoirs dry and rivers carry less water. Agriculture, which depends on water for irrigation, is threatened.

Stronger El Niño events also disrupt global atmospheric circulation. Global atmospheric circulation is the large-scale movement of air that helps distribute thermal energy (heat) across the surface of Earth. The eastward movement of oceanic and atmospheric heat sources cause unusually severe winter weather at the higher latitudes of North and South America. Regions as far north as the U.S. states of California and Washington may experience longer, colder winters because of El Niño.

El Niño events of 1982-83 and 1997-98 were the most intense of the 20th century. During the 1982-83 event, sea-surface temperatures in the eastern tropical Pacific were 7.8-12.8° C (9-18° F) above normal. These strong temperature increases caused severe climatic changes: Australia experienced harsh drought conditions; typhoons occurred in Tahiti; and record rainfall and flooding hit central Chile. The west coast of North America was unusually stormy during the winter season, and fish catches were dramatically reduced from Chile to the U.S. state of Alaska.

The El Niño event of 1997-98 was the first El Niño event to be scientifically monitored from beginning to end. The 1997-98 event produced drought conditions in Indonesia, Malaysia, and the Philippines. Peru experienced very heavy rains and severe flooding. In the United States, increased winter rainfall hit California, while the Midwest experienced record-breaking warm temperatures during a period known as "the year without a winter."

El Niño-related disruption of global atmospheric circulation extends beyond Pacific Rim nations. Strong El Niño events contribute to weaker monsoons in India and Southeast Asia. ENSO has even contributed to increased rainfall during the rainy season in sub-Saharan Africa.

Diseases thrive in communities devastated by natural hazards such as flood or drought. El Niño-related flooding is associated with increases in cholera, dengue, and malaria in some parts of the world, while drought can lead to wildfires that produce respiratory problems.

'Flavors' of El Niño

Variations of El Niño are referred to as "flavors." The transition period of an El Niño event, for instance, is called a "Trans Niño." Trans Niño events occur at the onset and closing of an El Niño event. Trans Niño events often include increased tornado activity in the American Midwest.

Another "flavor" of El Niño is the El Niño Modoki, or Modoki Niño. Modoki is a Japanese word meaning "similar, but different." Modoki Niño, also called the Central Pacific Niño, is characterized by changes in sea-surface temperatures in the central, not eastern, Pacific. Some Modoki Niño events are distinct from traditional El Niño events, such as increased hurricane activity in the Atlantic and Gulf of Mexico. Many meteorologists are critical of the Modoki Niño, calling for more climate models to study the proposed phenomenon.
Monitoring El Niño
Scientists, governments, and nongovernmental organizations (NGOs) collect data about El Niño using a number of technologies. The National Oceanic and Atmospheric Administration (NOAA), for instance, operates a network of scientific buoys. These buoys measure ocean and air temperatures, currents, winds, and humidity. The buoys are located at about 70 locations in the southern Pacific Ocean, from the Galapagos Islands to Australia.

These buoys transmit data daily to researchers and forecasters around the world. Using data from the buoys, along with visual imagery they receive from satellite imagery, scientists are able to more accurately predict El Niño and visualize its development and impact around the globe.

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El Niño is a climate pattern that describes the unusual warming of surface waters in the eastern Pacific Ocean.

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The National Environmental Education Foundation (NEEF)

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While you have probably heard the terms El Niño and La Niña used to describe these complex weather patterns, it can be difficult to know the difference between the two. These periodic weather patterns occur as a result of fluctuating ocean temperatures in one part of the world, namely the east-central equatorial Pacific Ocean. This can lead to extreme weather (think intense rains and severe droughts) in other parts of the world, such as the United States.

How Do El Niño and La Niña Affect the Weather?

The surface of the ocean warms and cools intermittently as it interacts with the strength of the trade winds, which blow from east to west in normal conditions. As the ocean surface temperature changes in response to atmospheric conditions, it modifies rainfall patterns. It's a bit of a scientific dance between ocean and atmosphere, with the opposite ends of the spectrum known as El Niño (the warm phase) and La Niña (the cold phase). Together, these extreme phases are called the El Niño-Southern Oscillation (ENSO) cycle.

What Happens During an El Niño?

When warm water builds up along the central and eastern tropical Pacific Ocean, an El Niño occurs. This increases moisture rising in the air, resulting in more rainstorms. In the US, winter temperatures tend to be warmer than normal in the Northwest with decreased rainfall, while the Southeast experiences wetter-than-average conditions.

For example, in 1997, the unusually warm water was up to 6 °C warmer than average, resulting in above-average rainfall in certain parts of the world. You may recall that parts of the Southwest, including California, experienced historic flooding and landslides during 1997. Similarly, in 2015, Southern California's sea levels were more than six inches above normal, leading to higher-than-normal tides and increased coastal flooding.

What Happens During a La Niña?

Conversely, when cool water builds up along the same region, a La Niña occurs with the opposite impact. In the US, winter temperatures tend to be cooler than average in the Northwest and warmer than average in the Southeast.

In 1988, the unusually cool water caused the atmosphere to cool in response. As less water evaporated, the air became dryer, cooler, and denser. Because this dense air didn't rise or develop into storms, less rain fell in certain parts of the world, such as the eastern Pacific, Ecuador, Peru, and the southeastern United States.

Why Are El Niño and La Niña Important?

Together, El Niño and La Niña are part of a natural cycle that can significantly impact not only global weather, climate, and ocean conditions but also food production, human health, and water supply. These systems typically last about one to two years, with the cycle alternating every three to seven years.

As we near the tail-end of multiple El Niño years, what can we expect for the upcoming La Niña winter? According to NOAA's winter forecast, the US will experience warmer, drier conditions across the Southeast, and cooler, wetter conditions in the North. Also, NOAA's Climate Prediction Center is closely monitoring persistent drought, since nearly half of the continental US is experiencing drier-than-normal conditions.

Learn More About Seasonal Weather Patterns

Get a closer look at how these weather patterns work by watching a few helpful videos from the National Oceanic and Atmospheric Administration (NOAA). In particular, check out El Niño and La Niña Explained and Observing El Niño.

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El Niño and La Niña: What's the Difference?

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You have probably heard the terms El Niño and La Niña before but it can be difficult to know the difference between the two. Get a deeper understanding of how these periodic weather patterns have an effect on the US.

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[MUSIC PLAYING] MATT: Hi, John. Thanks for joining us again.

JOHN RAFFERTY: Hey, Matt. How are you?

MATT: Good. Good. So a few weeks ago, we discussed climate change, specifically how the pandemic was affecting climate change. So I thought I would do a little bit of a follow-up and ask you some other weather-related questions. As we get into the summer months, I think people are often used to, OK, well, the heat is going up, the temperature is going up. But there are also some other weather systems that happen or they start happening in the summer months, such as thunderstorms, tornadoes. And it's the start of hurricane season. How come this happens now?

JOHN RAFFERTY: How come this happens now? Well, why don't we start with thunderstorms?

MATT: Sure.

JOHN RAFFERTY: Just because they're kind-of part and parcel of some of these other systems you talk about. It has a lot to do with how much liquid water is available and how much energy there is available. And specifically, the contrast between the energy available at the surface and the cooler air conditions aloft above the surface.

So let's just talk about liquid water for a second. In the winter months in a lot of parts of the Northern Hemisphere, the Southern Hemisphere-- a lot of that water is locked up as ice. So it's not available to evaporate quite as easily. But in the summertime, because temperatures are such that things are warmer, you do have greater liquid water availability.

So what happens is it allows the water to evaporate. And when water evaporates and it's warm, warm water vapor rises to a point where at the upper levels of the atmosphere, it cools and condenses into clouds. And when that happens, if the condensation is great enough, it forms the water droplets. And if those water droplets are heavy enough, they fall out as rain.

MATT: Thunderstorms are usually shorter in length, but stronger in force. Why is that?

JOHN RAFFERTY: Well, it has to do with how rapidly that convection, condensation process takes place. OK, so in the summer months around here in North America, we get our prevailing winds from the Southwest. And that's where the warmer temperatures are. So that's the energy source.

Well, in addition, you've got all these growing plants. You've got open water on lakes and rivers. So you've got a lot of moisture that's available. Over the course of the day, that water evaporates, temperatures heat up. And so there is that perfect storm of those two things together that causes this convection happen.

The strong thunderstorms result in this heated air that really wants to rise. And it's also taking the water vapor with it. As this warm parcel is warming the cooler air above it, the cooler air is also interacting with it as well. So it's cooling that parcel. The more energy is available, the higher it goes. So the higher it goes, the clouds happen. You get more intense situations like a thunderstorm that would arise.

As quickly as it comes up in the late afternoon, it dissipates as it moves along. Because we're running out of sunlight at that time. That energy source is starting to go down in the early evening. So that's why the greatest chance of thunderstorms is right at that peak sunlight hour, peak sun time in the late afternoon. That's where the heat is the greatest.

MATT: Right. Usually, when we experience intense thunderstorms, it is that late afternoon, early evening. And then oftentimes, now we get all these alerts on our phone. You get a severe thunderstorm warning. Sometimes you may even get a tornado watch or tornado warning, depending on where you live. Do thunderstorms and tornadoes go hand in hand? Or in other words, can a tornado form without a thunderstorm? Or does it need to-- does a thunderstorm have to happen first, before you get to a tornado?

JOHN RAFFERTY: Well, it actually comes down a lot to the definition. OK, a thunderstorm is only called a thunderstorm if there's lightning that produces thunder. So you could still have a cloud that does everything a thunderstorm does, except for thunder and lightning. And it wouldn't technically be called a thunderstorm. But those kinds of things can produce tornadoes as well. By and large, though, in practice, tornadoes and thunderstorms typically go hand in hand.

MATT: Got it. And as I was doing some research before talking to you, I did learn that the US has the most tornadoes out of anywhere in the world. We actually even nicknamed an area in the Central United States called Tornado Alley. Why do more tornadoes form in this area?

JOHN RAFFERTY: It has to do with just the recipe of conditions. And it's not just the source of water and the energy, but it's also the topography-- the lay of the land, the landscape. Tornado Alley occurs in the Great Plains. It's an area that is pretty flat. Some hills here and there, not a whole lot of forests.

And you also have the Gulf of Mexico that's nearby. And that's the moisture source. And with those southwesterly winds, southernly winds, you get the moisture from the Gulf of Mexico coming over this frying pan. And you get that convective storm action that we discussed earlier.

MATT: So tornadoes form over land. If we move on to hurricanes, they actually form over oceans.

JOHN RAFFERTY: Yes.

MATT: Why do they need the oceans in order to form the tropical system?

JOHN RAFFERTY: Well, it's interesting. Because if you think about it, if you compare tornadoes and hurricanes, the recipes for them are fairly similar. You're going to need a heat source. You're going to need a moisture source. However, with hurricanes in the tropical oceans, the tropical oceans provide the heat source and an endless supply of moisture. That water vapor is going to rise and form thunderstorms and ultimately, hurricanes based on those same kinds of things that are going on over land with tornadoes.

MATT: We live in the United States, so we're used to the systems that we see here. We call hurricanes hurricanes, but they have different names, depending on where they developed across the globe, correct?

JOHN RAFFERTY: Yes.

MATT: So what's the difference between a hurricane, a cyclone, and a typhoon?

JOHN RAFFERTY: Nothing. Nothing. Actually, the technical term for all of these things, all of these phenomenon that everybody has different names for is tropical cyclone. So if you're talking to a nerdy scientist like myself, we might say tropical cyclone. But in different regions, they call them different things.

So here in North America in the North Atlantic, we talk more about hurricanes than we do about typhoons, because that's just what we call them. Around China, Taiwan, Japan-- that's the typhoon zone, but it's basically the same thing. The storms form the same way. It's just what they call them. And then typically, everywhere else it's a matter of a mix between typhoon and cyclone. Pretty much more cyclone than anything else, because that's just what they call them.

MATT: Got it. So they all start the same way. They're just a tropical depression. And it really depends on if it intensifies enough and becomes a storm and then if it strengthens all the way to what we refer to it as a Category 1 hurricane. The name can just change, depending on where this formation happens around the globe.

JOHN RAFFERTY: Yes. Everybody has got different names for things. In the common parlance between these different regions, they call them cyclones.

MATT: So, John, Hollywood loves a good disaster movie. Oftentimes, these disaster movies are rooted in some type of weather event. And in the movie, they depict the meteorologist or the storm chasers as action heroes. They are exaggerated a lot of the time for entertainment value.

But there is some truth to what these experts have to do in order to get the data to best predict these systems and hopefully protect us as best they can. Do you think that we'll ever get to a point with that technology, will be so advanced that we don't have to send people into the eye of a storm?

JOHN RAFFERTY: What we need to do is we need to take measurements in order to get a sense of the storm. And some of those measurements-- you can get from space using satellites, get a sense of the size of the storm, the heat level of the storm, infrared that's coming off of the storm, those kinds of things. But you're still going to need something to go through the cross section to see how high the winds are, to gather pressure readings, et cetera. So that's why we use those hurricane hunter aircraft to go through.

Now, are we going to ever get to a point where the satellites will be so sophisticated that they can just take a snapshot of a developing storm and go, OK, this is how strong the hurricane is going to be in a week and this is where it's going to go? Maybe. But it's pretty speculative right now. It's a combination of remote sensing from satellites, but also ground truth or close to ground truth measurements through hurricane hunter aircraft, and the measurements that those take.

MATT: John, thanks again for joining us. I'm sure we'll talk again-- next couple of months. It's been a very eventful 2020, but hopefully, the hurricane season is not too intense this year.

JOHN RAFFERTY: Matt, thanks for having me on. I share your sympathies. Let's hope we catch a break this year.

MATT: All right. Thanks, John.

JOHN RAFFERTY: You're welcome.

[MUSIC PLAYING]

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Learn about why thunderstorms, tornadoes, hurricanes, and other weather phenomena typically form during the summer.

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This is very helpful

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Welcome to Britannica Library, it provides your patrons with access to three sites in one, with age appropriate material from the child-friendly collection through to the adult general reference collection.

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Thunderstorms typically develop in the warmer months of spring, summer, and fall, but they can occur at any time of the year over most of the United States. There are three basic ingredients needed for thunderstorm development: moisture, an unstable atmosphere, and some way to start the atmosphere moving.

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This article was most recently revised and updated by

blizzard, severe weather condition that is distinguished by low temperatures, strong winds, and large quantities of either falling or blowing snow. The National Weather Service of the United States defines a blizzard as a storm with winds of more than 56 km (35 miles) per hour for at least three hours and enough snow to limit visibility to 0.4 km (0.25 mile) or less. A severe blizzard has winds of over 72 km (45 miles) per hour, visibility near zero, and temperatures of −12 °C (10 °F) or lower. A ground blizzard occurs when there is no falling snow, but snow is drifting and blowing near the ground.

The name originated in the central United States, where blizzards are brought by northwesterly winds following winter depressions, or low-pressure systems. In the United States and in England, the term is often used for any strong, heavy snowstorm with wind. In Antarctica, blizzards are associated with winds spilling over the edge of the ice plateau at an average velocity of 160 km (about 100 miles) per hour.

John P. Rafferty.

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Blizzard | Snowstorms, Winter Storms, Arctic Winds

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Blizzard, severe weather condition that is distinguished by low temperatures, strong winds, and large quantities of either falling or blowing snow. The National Weather Service of the United States defines a blizzard as a storm with winds of more than 56 km (35 miles) per hour for at least three

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Extreme Weather | National Wildlife Federation

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Learn how extreme weather events—including hurricanes, wildfires, floods, and droughts—are threatening wildlife in the United States.

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Hurricanes are the most awesome, violent storms on Earth. People call these storms by other names, such as typhoons or cyclones, depending on where they occur. Whatever they are called, tropical cyclones all form the same way.

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How do Hurricanes Form? | Precipitation Education

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Hurricanes are the most awesome, violent storms on Earth. People call these storms by other names, such as typhoons or cyclones, depending on where they occur.

# data[].attributes.attachment_link.authors[].name
US Department of Commerce, National Oceanic and Atmospheric Administration

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El El Nino and La Nina are complex weather patterns resulting from variations in ocean temperatures in the Equatorial Pacific.

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What is "El Niño" and what are its effects?

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The term El Niño (Spanish for 'the Christ Child') refers to a warming of the ocean surface, or above-average sea surface temperatures, in the central and eastern tropical Pacific Ocean. The low-level surface winds, which normally blow from east to west along the equator ("easterly winds"), instead weaken or, in some cases, start blowing the other direction (from west to east or "westerly winds").

# data[].attributes.attachment_link.authors[].name
US Department of Commerce, National Oceanic and Atmospheric Administration

# data[].attributes.attachment_link.description
El El Nino and La Nina are complex weather patterns resulting from variations in ocean temperatures in the Equatorial Pacific.

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This article was most recently revised and updated by

La Niña, cyclic counterpart to El Niño, consisting of a cooling of surface waters of the Pacific Ocean along the western coast of South America. While its local effects on weather and climate are generally the opposite of those associated with El Niño, its global effects can be more complex. La Niña events often follow El Niño events, which occur at irregular intervals of about two to seven years.

The Oceanic Niño Index (ONI), a measure of the departure from normal sea surface temperature in the east-central Pacific Ocean, is the standard means by which each La Niña episode is determined, gauged, and forecast. La Niña episodes are indicated by sea surface temperature decreases of more than 0.5 °C (0.9 °F) for at least five successive overlapping three-month seasons.

John P. Rafferty.

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La Niña | El Niño, Pacific Ocean, Climate Change

# data[].attributes.attachment_link.description
La Niña, cyclic counterpart to El Niño, consisting of a cooling of surface waters of the Pacific Ocean along the western coast of South America. While its local effects on weather and climate are generally the opposite of those associated with El Niño, its global effects can be more complex.

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Snow whirling around in wind blowing faster than highway speed limits makes the whole world look white. Imagine snow so deep that second graders can't see over the top of it. The wind and the snow may cause a power outage and collapse roofs. That's what you can expect from a blizzard.

What Is a Blizzard?

The National Weather Service defines a blizzard as a storm with large amounts of snow or blowing snow, winds greater than 35 mph (56 kph), and visibility of less than ¼ mile (0.4 km) for at least three hours. Some blizzards, called ground blizzards, have no falling snow. Instead, snow that had fallen before the blizzard is blown around or drifts in a way to create these conditions.

Blizzard conditions usually build up on the northwest side of a powerful storm system. The storm produces ample snow while strong winds develop because of a difference in pressure between the low pressure of the storm and the high pressure beyond the storm.

Conditions during a blizzard can be severe. Travel becomes dangerous when the blowing snow causes whiteout conditions and sky and ground look white. Roads can be partially or fully blocked by snowdrifts - piles of snow formed by the wind. Many times cold temperatures that can cause frostbite or hypothermia are part of a blizzard and can last for days after the storm has ended.

Where Do Blizzards Occur?

In the United States, blizzards are common in the upper Midwest and the Great Plains but occur in most areas of the country except for the Gulf Coast and the California coast. Blizzards can occur all over the world, even in the tropics where it is cold on high altitude mountaintops.

What a Blizzard Needs

Three things are needed to make a large snowstorm or blizzard:

1. Cold air (below freezing) is needed to make snow.
For snow to fall to the ground, the temperature must be cold both up in the clouds where snowflakes form and down at ground level. If the air near ground level is too warm, the snow will melt on its way down, changing to rain or freezing rain.

2. Moisture is needed to form clouds and precipitation.
Moisture in the air is called water vapor. Air blowing across a body of water, such as a large lake or the ocean, is an excellent source of water vapor. As wind moves air over the water, some water evaporates from the surface, putting vapor into the air. This is how lake effect snowstorms and nor'easters pick up so much moisture. However, cold air is not able to hold much water vapor. In fact, very cold air does not make very much snow.

3. Moist air needs to rise over very cold air, making clouds and snow.
There are two ways that moist air might rise over cold air. Winds pull cold air toward the equator from the poles and bring warmer, moist air toward the poles from the equator. When warm air and cold air are brought together, a front is formed and snow can form. Warm air can also rise to form clouds and blizzard snows as it flows up a mountainside.

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High winds can send snow flying through the air, reducing visibility and causing power outages. That's just some of what you can expect from a blizzard.

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Snow whirling around in wind blowing faster than highway speed limits makes the whole world look white. Imagine snow so deep that second graders can't see over the top of it. The wind and the snow may cause a power outage and collapse roofs. That's what you can expect from a blizzard.

What is a Blizzard?

The National Weather Service defines a blizzard as a storm with large amounts of snow or blowing snow, winds greater than 35 mph (56 kph), and visibility of less than ¼ mile (0.4 km) for at least three hours. Some blizzards, called ground blizzards, have no falling snow. Instead, snow that had fallen before the blizzard is blown around or drifts in a way to create these conditions.

Blizzard conditions usually build up on the northwest side of a powerful storm system. The storm produces ample snow while strong winds develop because of a difference in pressure between the low pressure of the storm and the high pressure beyond the storm.

Conditions during a blizzard can be severe. Travel becomes dangerous when the blowing snow causes whiteout conditions and sky and ground look white. Roads can be partially or fully blocked by snowdrifts - piles of snow formed by the wind. Many times cold temperatures that can cause frostbite or hypothermia are part of a blizzard and can last for days after the storm has ended.

Where do Blizzards Occur?

In the United States, blizzards are common in the upper Midwest and the Great Plains but occur in most areas of the country except for the Gulf Coast and the California coast. Blizzards can occur all over the world, even in the tropics where it is cold on high altitude mountaintops.

What a Blizzard Needs

Three things are needed to make a large snowstorm or blizzard:

1. Cold air (below freezing) is needed to make snow.
For snow to fall to the ground, the temperature must be cold both up in the clouds where snowflakes form, and down at ground level. If the air near ground level is too warm, the snow will melt on its way down, changing to rain or freezing rain.

2. Moisture is needed to form clouds and precipitation.
Moisture in the air is called water vapor. Air blowing across a body of water, such as a large lake or the ocean, is an excellent source of water vapor. As wind moves air over the water, some water evaporates from the surface, putting vapor into the air. This is how "lake effect snowstorms" and "Nor'easters" pick up so much moisture. However, cold air is not able to hold much water vapor. In fact, very cold air does not make very much snow.

3. Moist air needs to rise, over very cold air, making clouds and snow.
There are two ways that moist air might rise over cold air. Winds pull cold air toward the equator from the poles and bring warmer, moist air toward the poles from the equator. When warm air and cold air are brought together, a front is formed and snow can form. Warm air can also rise to form clouds and blizzard snows as it flows up a mountainside.

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Snow whirling around in wind blowing faster than highway speed limits makes the whole world look white. Imagine snow so deep that second graders can't see over the top of it. The wind and the snow may cause a power outage and collapse roofs. That's what you can expect from a blizzard.

# data[].attributes.attachment_link.readable_text
A blizzard is a dangerous weather event, bringing with it frigid temperatures, howling winds, and decreased visibility. Blizzards can be deadly, which is why it is important that meteorologists accurately measure atmospheric conditions and provide the public with timely warnings.

Subjects

Earth Science, Meteorology, Geography

Image

Car Stuck in Blizzard

Blizzards are not just bad snowstorms; they are specific types of snowstorms that consist of large amounts of snow lasting more than three hours, winds of at least 56 kilometers (35 miles) per hour, and visibility of less than 0.4 kilometers (0.25 miles)

Photograph by Rob Carr/Getty Images

The United States National Weather Service's winter weather advisory, watch, or warning system helps meteorologists determine whether atmospheric conditions should be classified as typical winter weather, a snowstorm, or a severe blizzard.

In order for meteorologists to classify a winter storm as a snowstorm, the air temperature high in the atmosphere and near the ground must be below 0°C (32°F). There also needs to be enough water vapor in the air to form snowflake crystals. While snowstorms do not typically last very long (less than a few hours), they can bring high snow accumulations, which can be hazardous.

For a snowstorm to be considered a blizzard, it must also meet specific, though more severe, conditions. To be categorized as a blizzard, the storm must last for at least three hours and produce a large amount of falling snow. Blizzards also have winds measuring over 56 kilometers (35 miles) per hour. These winds cause a large volume of snow to blow around in the air and near the ground, decreasing visibility. Meteorologists will declare blizzard conditions if the snow limits visibility to the point where it is difficult to see an object more than 0.4 kilometers (0.25 miles) away.

The most famous recorded blizzard is the Great Blizzard of 1888, which occurred between March 11 and 14. This blizzard extended all the way from the American states of Maine to Maryland. About 139 centimeters (55 inches) of snow brought newly growing cities in New England and New York City, New York, to a screeching halt. During this time, about one in four people living in the United States lived in one of the New England states, which made the blizzard all the more devastating. About 400 people died during this storm.

A more recent example of catastrophic blizzards would be when two 2010 blizzards formed back-to-back in the Washington, D.C., area on February 4-7 and February 9-11. The first storm brought snow to Pennsylvania and extended through North Carolina. The total amount of snow from this first storm varied from 25 to 76 centimeters (10 to 30 inches). The storm impacted over 45 million people in the northeastern part of the United States. Then, before the states could completely dig out from the storm, a second blizzard hit. The Washington, D.C., area reported snowfall totals of 15-38 centimeters (6-15 inches) from the new storm, and nearby Maryland reported over 60 centimeters (24 inches) on top of the snow that was already on the ground.

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Director

Tyson Brown, National Geographic Society

Author

National Geographic Society

Production Managers

Gina Borgia, National Geographic Society

Jeanna Sullivan, National Geographic Society

Program Specialists

Sarah Appleton, National Geographic Society, National Geographic Society

Margot Willis, National Geographic Society

other

Last Updated

October 19, 2023

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A blizzard is a dangerous weather event, bringing with it frigid temperatures, howling winds, and decreased visibility. Blizzards can be deadly, which is why it is important that meteorologists accurately measure atmospheric conditions and provide the public with timely warnings.

# data[].attributes.attachment_link.readable_text
Picture a thunderstorm: heavy raindrops beat the roof, lightning flashes through the windows, thunder booms, the dog whines from his hiding spot under your bed. Now picture two thousand thunderstorms!

Right now, at this very moment, there are about two thousand thunderstorms going on around the world. Even though thunderstorms are common, they are still dramatic events with intense rain, hail, wind, lightning, thunder, and even tornadoes.

Thunderstorms form when warm, moist air rises into cold air. The warm air becomes cooler, which causes moisture, called water vapor, to form small water droplets - a process called condensation. The cooled air drops lower in the atmosphere, warms, and rises again. This circuit of rising and falling air is called a convection cell. If this happens a small amount, a cloud will form. If this happens with large amounts of air and moisture, a thunderstorm can form.

Thunderstorm Anatomy

Thunderstorms can consist of just one convection cell, multiple convection cells, or even one extremely large and powerful convection cell. Below is a description of three types of thunderstorms, classified by their structure: single-cell, multi-cell, and supercell.

Single-cell thunderstorms: Thunderstorms created by just one convection cell in the atmosphere are called single-cell storms. Most of these are small, lasting only about an hour, and are also called ordinary thunderstorms. These storms often form during summer and include towering cumulonimbus clouds that can grow 12 kilometers (7.5 miles) high in the atmosphere. Rain and lightning are common. Sometimes hail falls.

Multi-cell thunderstorms: Some thunderstorms are made from many convection cells moving as a single unit. These are called multi-cell thunderstorms. Often the convection cells are arranged as a cluster, with each cell at a different stage of the thunderstorm cycle. Multi-cell storms along a cold or warm front, where warm air is pushed high into the atmosphere above cold air, often form a line, called a squall line. The squall line can be up to 1000 km (600 miles) long. Strong wind gusts often blow just ahead of the storm.

Supercell thunderstorms: Thunderstorms with deep, rotating updraft winds, called supercells, are very large and last for hours releasing huge amounts of rain and sometimes even baseball-sized hail. They include fast moving convection - air zooming upward at as much as 280 kilometers (175 miles) per hour. Rotation in supercells sometimes forms violent tornadoes, the largest and most damaging type, because the storms are so long-lived. Several tornadoes can be produced from one supercell thunderstorm. Supercell clouds grow up to 18 km (11 miles) high in the atmosphere, right up to the bottom of the stratosphere. Supercells are the least common type of thunderstorm, but they are the most destructive.

Where Air Rises to Form a Thunderstorm

All thunderstorms begin with air rising into the atmosphere to form a convection cell, but the air can be lifted in different ways. Another way to classify thunderstorms is by the location where they form and the reason the air rises. The pictures below describe three different ways that the lifting of air can begin: due to a mountain or hillside, within an air mass, or at a storm front.

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Right now there are about two thousand thunderstorms going on around the world. While common, they are dramatic with intense rain, hail, wind, lightning, thunder, and even tornadoes.

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Monsoon, a major wind system that seasonally reverses its direction-such as one that blows for approximately six months from the northeast and six months from the southwest. The most prominent monsoons occur in South Asia, Africa, Australia, and the Pacific coast of Central America. Monsoonal tendencies also are apparent along the Gulf Coast of the United States and in central Europe; however, true monsoons do not occur in those regions.

For further information on prominent monsoon regions, see Indian monsoon, West African monsoon, Malaysian-Australian monsoon, and North American monsoon.

The primary cause of monsoons lies in different warming trends over land and sea, though other factors may be involved. Seasonal changes in temperature are large over land but small over ocean waters, and monsoons blow from atmospheric heat sinks (that is, cold regions with high atmospheric pressure) toward heat sources (warm regions characterized by low atmospheric pressure). Consequently, monsoon winds typically travel from sea to land in summer and from land to sea in winter. For example, the heat source involved in the Indian summer monsoon resides over the Plateau of Tibet and the eastern foothills of the Himalayas, while the heat sink occurs over the southern Indian Ocean and Madagascar, a region where relatively cloud-free air cools by emitting infrared, or "long-wave," radiation into space. Likewise, the heat source for the Australian summer monsoon resides over the area in which many meteorologists call the " Maritime Continent," a region made up of parts of Southeast Asia and the islands of Indonesia and the Philippines, while the heat sink resides over Siberia.

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Most summer monsoons have a dominant westerly component and a strong tendency to ascend and produce copious rainfall, which occurs as a result of the condensation of water vapour in the rapidly rising air. The intensity and duration of these rains, however, are not uniform from year to year. Conversely, the winds of winter monsoons have a prevailing easterly component and a strong tendency to diverge, subside, and cause drought.

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The poleward limits of monsoon systems are often sites of sharp changes in wind direction. In India, for example, the monsoon blows from the southwest in July and August, and north of India the winds are from the east. In northern Australia the monsoon arrives from the northwest during January-February. At the southern limit of the Australian monsoon, the winds turn easterly.

Climatic patterns reminiscent of monsoons also occur in areas outside of the prominent monsoon regions. In central Europe, where the average wind direction in summer differs some 30° to 40° from that of the Atlantic, there are monsoonal tendencies that occur not as a continuous flow but rather intermittently within frontal depressions, bringing cool, cloudy weather, rain, and thunderstorms. Some see in this climatic pattern a true monsoon, but it is obvious that it is only an "embryo monsoon" that results in weather singularities. The latitude is too high for a true monsoon to arise. In addition, the Gulf Coast of the United States is prone to climatic patterns with monsoonal tendencies; however, the seasonally consistent winds characteristic of true monsoons do not emerge.

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Monsoon, a major wind system that seasonally reverses its direction-such as one that blows for approximately six months from the northeast and six months from the southwest. The most prominent monsoons occur in South Asia, Africa, Australia, and the Pacific coast of Central America. Monsoonal

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Tropical cyclones such as hurricanes and tropical storms pose the greatest weather threat to the Florida Keys and its coastal waters. Due to the small land areas and only 1 major highway in and out of the Keys, the Keys are extremely vulnerable to tropical systems. It is estimated that it takes approximately 2 days to fully evacuate the Keys. This is why evacuation orders from local emergency officials begin well in advance before a hurricane is expected to affect the Keys. The NWS recommends that you heed any evacuation orders associated with hurricanes or tropical storms that are issued by local emergency officials.

TROPICAL CYCLONE CLASSIFICATIONS

Tropical cyclones are classified into 3 categories, tropical depressions, tropical storms and hurricanes. A tropical depression is an organized system of showers and thunderstorms that has a circulation center with maximum sustained winds of 38 mph (33 knots) or less. A tropical storm is a better organized system of showers and thunderstorms with a well defined circulation center with maximum sustained winds of 39 to 73 mph (34-63 knots). A hurricane is a well defined system of showers and thunderstorms with a well defined circulation center with maximum winds of 74 mph (64 knots) or greater.

Hurricanes are categorized by the Saffir-Simpson Scale. The categories range from 1 to 5 with 1 being the weakest and 5 being the strongest. The scale takes certain measured parameters of a hurricane and relates those to how much damage a storm can cause. The Saffir-Simpson scale is listed below.

Category Central pressure Winds Storm surge Damage

Millibars Inches Mph Feet

1 >979 > 28.91 74-95 4-5 Minimal

2 965-979 28.50-28.91 96-110 6-8 Moderate

3 945-964 27.91-28.47 111-130 9-12 Extensive

4 920-944 27.17-27.88 131-155 13-18 Extreme

5 <920 <27.17 > 155 >18 Catastrophic

Hurricane Georges struck the Florida Keys in Sept. 1998. The lowest pressure and highest sustained winds measured made Georges a category 1 storm. Damage in the lower Keys from Key West to mile marker 15, as indicated above along houseboat row in Key West, reflected that of a category 1 storm. Damage from Mile Marker 15 to Bahia Honda Key was more indicative of a category 2 storm, so, wind speeds were likely highest in that area.

Damage from a hurricane begins to increase dramatically when it reaches the category 3 stage.

The wind damage caused by a 115 mph storm is much greater than from a 105 mph storm whereas the difference in wind damage between a 95 mph and 105 mph storm is not as great.

There have been only 2 category fives to hit the U.S. One was Hurricane Camille which hit the MS/LA coast in August 1969 and the Labor Day Storm which made landfall near upper Matecumbe Key in the Upper Florida Keys in September 1935.

Near the center of each of these cyclones is generally the strongest winds and heaviest rainfall and the eastern halves of tropical cyclones are usually stronger than the western halves. However, severe and dangerous weather such as tornadoes and coastal flooding can occur well ahead of the center of tropical cyclones and on the western half as well. The entire storm should be treated as dangerous.

FORMATION AND FREQUENCY

The Atlantic Hurricane Season (including the Caribbean Sea) runs from June 1 to November 30 each year. This does not mean that tropical cyclones can not affect the U.S. in months outside of hurricane season, but it is unlikely. Based on the last 100 years or so, tropical cyclones affect the Florida Keys with greater frequency than any other location in the U.S.

Tropical cyclones typically form in the tropics with the tropics being defined from 22.5 degrees S latitude (southern hemisphere) to 22.5 N latitude (northern hemisphere). The U.S. is only affected by cyclones that form in the northern hemisphere. Tropical cyclones do not develop near the equator, 0 degrees, and most originate north of 10 N latitude.

Tropical cyclones in the North Atlantic develop from tropical waves which most often originate over the tropical regions of continental Africa. These waves travel from east to west in the tropical easterlies and if conditions are favorable, they may develop into a tropical cyclone. It is estimated that there are over 100 tropical waves each year that move through the North Atlantic Ocean.

The necessary conditions for tropical cyclone development from an existing tropical wave are: 1) water temperatures must be 80 F or greater, the warmer the water the better potential for development and 2) winds at the mid and upper levels (10 to 50 thousand feet AGL) need to be weak, 20 knots or less, and rotating clockwise (anticyclonic). The direction of the upper level winds is important also. Upper level winds from the east favor tropical cyclone development while west winds do not.

Tropical cyclones weaken and/or dissipate when they encounter water cooler than 80 F or strong mid and upper level westerly winds or when they move over or come close to land.

The Keys and the surrounding coastal waters are most often affected by tropical cyclones during September and October with June having a secondary peak in frequency. The least likely time for tropical cyclones to affect the Keys is during July and early August. Most of the tropical cyclones that affect the Keys form in the Caribbean Sea and move north. Some develop in the south central Atlantic and only a few of the "Cape Verde" type cyclones (those that develop in the far eastern Atlantic near the coast of Africa) have impacted the Keys.

DANGERS FROM TROPICAL CYCLONES

Hurricanes and Tropical Storms develop in many shapes and sizes and pose many different threats.

STORM SURGE and STORM TIDE are the greatest threats from a tropical cyclone along immediate coastal areas to life and property. Storm surge is the abnormal rise of water along a coastal area due to strong winds of a tropical cyclone. Storm tide is the combination of the storm surge and the normal high tide.

Over 6,000 people were killed in the Galveston, TX hurricane in 1900, most by storm tide. Hurricane Camille created a 25 foot storm tide in Mississippi in 1969. Hurricane Hugo generated a 20 foot storm tide in September 1989 along the South Carolina coast. By comparison, Hurricane Georges only produced a maximum storm tide of 6 feet in the lower keys

Hurricane Irene hit the Florida Keys in October 1999 as a weak category 1 storm. Little in the way of damage from winds was observed but heavy flooding rains were a problem.

Between 1970-99, many more people have lost their lives in freshwater flooding associated with tropical cyclones than from storm surges (See graph below).

The slower a storm is moving, the greater the potential for extremely heavy rains and flooding Mountainous terrain also enhences the rainfall potential of a tropical cyclone. It is important to remember that tropical cyclones produce tremendous amounts of rain over inland areas and people need to prepare for this danger.

Outside of strong damaging winds and heavy rain, tropical cyclones can produce other types of severe weather. Most landfalling tropical cyclones produce tornadoes. Tropical Storm Mitch produced 3 tornadoes on the upper Keys in November 1998 causing significant damage and injuring 20 people.

NHC FORECASTS

The National Hurricane Center, NHC, in Miami, Florida, issues all watches/warnings and advisories for landfalling tropical cyclones in the U.S., and for of the countries in the Caribbean and Atlantic Ocean including Cuba and the virgin islands. For more information on the NHC go to http://www.nhc.noaa.gov

A WATCH means that hurricane or tropical storm conditions are possible in the specified area within 36 hours. A WARNING means that hurricane or tropical storm conditions are expectedin the specified area within 24 hours. The NWS in Key West will issue a hurricane/tropical local statement to give greater detail on how the storm will affect the Keys and coastal waters. The NWS in Key West will also issue the short-term forecast to provide updates on the weather conditions associated with the storm in between the local statements as well as any severe weather warnings necessary during a storm's landfall.

NWR AND PREPAREDNESS

The best way to receive these and any NWS products is through NOAA Weather Radio (NWR) broadcasts. The broadcasts originate from the NWS office in Key West, FL. Other media sources such as TV, radio and newspapers are good ways to keep updated on tropical cyclone information as well.

It is important to develop a disaster plan for you and your family in case of a hurricane or tropical storm affecting the Keys. The NWS will always recommend safety precautions when issuing products during tropical cyclone events, but you have to decide what is best for you. See the FEMA's hurricane preparedness link under the tropical cyclone section for more detailed information on hurricane preparedness. If you would like more information, contact the NWS in Key West, Monroe County emergency management office or the local chapter of the Red Cross.

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Tropical cyclones such as hurricanes and tropical storms pose the greatest weather threat to the Florida Keys and its coastal waters. Due to the small land areas and only 1 major highway in and out of the Keys, the Keys are extremely vulnerable to tropical systems. It is estimated that it takes approximately 2 days to fully evacuate the Keys.

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Hurricanes are powerhouse weather events that suck heat from tropical waters to fuel their fury. These violent storms form over the ocean, often beginning as a tropical wave-a low pressure area that moves through the moisture-rich tropics, possibly enhancing shower and thunderstorm activity.

As this weather system moves westward across the tropics, warm ocean air rises into the storm, forming an area of low pressure underneath. This causes more air to rush in. The air then rises and cools, forming clouds and thunderstorms. Up in the clouds, water condenses and forms droplets, releasing even more heat to power the storm.

When wind speeds within such a storm reach 74 mph, it's classified as a hurricane. The terms "hurricane" and "tropical cyclone" refer to the same kind of storm: a rotating, organized system of clouds and thunderstorms that originates over tropical or subtropical waters and has closed, low-level circulation.

During just one hurricane, raging winds can churn out about half as much energy as the electrical generating capacity of the entire world, while cloud and rain formation from the same storm might release a staggering 400 times that amount.

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national oceanic and atmospheric administration

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Warm ocean waters and thunderstorms fuel power-hungry hurricanes.

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Joseph A. Zehnder Professor of Atmospheric Sciences, Creighton University.

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Tropical cyclone | Definition, Causes, Formation, and Effects

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Tropical cyclone, an intense circular storm that originates over warm tropical oceans and is characterized by low atmospheric pressure, high winds, and heavy rain. Also called typhoons and hurricanes, cyclones strike regions as far apart as the Gulf Coast of North America, northwestern Australia, and eastern India.

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Severe Weather 101

Tornado Basics

What is a tornado?

A tornado is a narrow, violently rotating column of air that extends from a thunderstorm to the ground. Because wind is invisible, it is hard to see a tornado unless it forms a condensation funnel made up of water droplets, dust and debris. Tornadoes can be among the most violent phenomena of all atmospheric storms we experience.

Read more about NSSL's tornado research here.

Where do tornadoes occur?

Tornadoes occur in many parts of the world, including Australia, Europe, Africa, Asia, and South America. Even New Zealand reports about 20 tornadoes each year. Two of the highest concentrations of tornadoes outside the U.S. are Argentina and Bangladesh.

How many tornadoes occur in the U.S. each year?

About 1,200 tornadoes hit the U.S. yearly. Since official tornado records only date back to 1950, we do not know the actual average number of tornadoes that occur each year. Plus, tornado spotting and reporting methods have changed a lot over the last several decades, which means that we are observing more tornadoes that actually happen.

Where is Tornado Alley?

Tornado Alley is a nickname invented by the media to refer to a broad area of relatively high tornado occurrence in the central United States. Various "Tornado Alley" maps look different because tornado occurrence can be measured many ways: by all tornadoes, tornado county-segments, strong and violent tornadoes only, and databases with different time periods.

However, the idea of a "tornado alley" can be misleading. The U.S. tornado threat shifts from the Southeast in the cooler months of the year, toward the southern and central Plains in May and June, and the northern Plains and Midwest during early summer. Tornadoes can occur and have been reported in all fifty states!

Please remember, violent tornadoes do happen outside "Tornado Alley" every year.

When are tornadoes most likely?

Tornado season usually refers to the time of year the U.S. sees the most tornadoes. The peak "tornado season" for the southern Plains (e.g., Texas, Oklahoma, and Kansas) is from May into early June. On the Gulf coast, it is earlier in the spring. In the northern Plains and upper Midwest (North and South Dakota, Nebraska, Iowa, Minnesota), tornado season is in June or July. But, remember, tornadoes can happen at any time of year. Tornadoes can also happen at any time of day or night, but most tornadoes occur between 4-9 p.m.

What is the difference between a Tornado WATCH and a Tornado WARNING?

A Tornado WATCH is issued by the NOAA Storm Prediction Center meteorologists who watch the weather 24/7 across the entire U.S. for weather conditions that are favorable for tornadoes and severe weather. A watch can cover parts of a state or several states. Watch and prepare for severe weather and stay tuned to NOAA Weather Radio to know when warnings are issued.

A Tornado WARNING is issued by your localNOAA National Weather Service Forecast Office meteorologists who watch the weather 24/7 over a designated area. This means a tornado has been reported by spotters or indicated by radar and there is a serious threat to life and property to those in the path of the tornado. A tornado warning indicates that you should ACT NOW to find safe shelter! A warning can cover parts of counties or several counties in the path of danger.

Watchthis Youtube video for a great explanation!

How is tornado strength rated?

To determine the strength of a tornado, experts examine the damage it caused. From this information, we can estimate the wind speeds. An "Enhanced Fujita Scale" was implemented by the National Weather Service in 2007 to rate tornadoes in a more consistent and accurate manner. The EF-Scale takes into account more variables than the original Fujita Scale (F-Scale) when assigning a wind speed rating to a tornado, incorporating 28 damage indicators such as building type, structures and trees. For each damage indicator, there are 8 degrees of damage ranging from the beginning of visible damage to complete destruction of the damage indicator. The original F-scale did not take these details into account. The original F-Scale historical data base will not change. An F5 tornado rated years ago is still an F5, but the wind speed associated with the tornado may have been somewhat less than previously estimated. A correlation between the original F-Scale and the EF-Scale has been developed. This makes it possible to express ratings in terms of one scale to the other, preserving the historical database.

How do tornadoes form?

The truth is that we don't fully understand. The most destructive tornadoes occur from supercells, which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone. (Supercells can also produce damaging hail, severe non-tornadic winds, frequent lightning, and flash floods.) Tornado formation is believed to be dictated mainly by things which happen on the storm scale, in and around the mesocyclone. Recent theories and results from the VORTEX2 program suggest that once a mesocyclone is underway, tornado development is related to the temperature differences across the edge of downdraft air wrapping around the mesocyclone. Mathematical modeling studies of tornado formation also indicate that it can happen without such temperature patterns; and in fact, very little temperature variation was observed near some of the most destructive tornadoes in history on 3 May 1999. We still have lots of work to do.

What do storm spotters look for when trying to identify a tornado or a dangerous storm?

Inflow bands are ragged bands of low cumulus clouds extending from the main storm tower usually to the southeast or south. The presence of inflow bands suggests that the storm is gathering low-level air from several miles away. If the inflow bands have a spiraling nature to them, it suggests the presence of rotation.

Thebeaver's tail is a smooth, flat cloud band extending from the eastern edge of the rain-free base to the east or northeast. It usually skirts around the southern edge of the precipitation area. It also suggests the presence of rotation.

Awall cloud is an isolated cloud lowering attached to the rain-free base of the thunderstorm. The wall cloud is usually to the rear of the visible precipitation area.

A wall cloud that may produce a tornado can exist for 10-20 minutes before a tornado appears, but not always. A wall cloud may also persistently rotate (often visibly), have strong surface winds flowing into it, and may have rapid vertical motion indicated by small cloud elements quickly rising into the rain-free base.

As the storm intensifies, the updraft draws in low-level air from several miles around. Some low-level air is pulled into the updraft from the rain area. This rain-cooled air is very humid; the moisture in the rain-cooled air quickly condenses below therain-free base to form the wall cloud.

Therear flank downdraft (RFD) is a downward rush of air on the back side of the storm that descends along with the tornado. The RFD looks like a "clear slot" or "bright slot" just to the rear (southwest) of the wall cloud. It can also look like curtains of rain wrapping around the cloud base circulation. The RFD causes gusty surface winds that occasionally have embedded downbursts. The rear flank downdraft is the motion in the storm that causes the hook echo feature on radar.

Acondensation funnel is made up of water droplets and extends downward from the base of the thunderstorm. If it is in contact with the ground it is a tornado; otherwise it is a funnel cloud. Dust and debris beneath the condensation funnel confirm a tornado's presence.

Be careful! Tornadoes can form and be in contact with the ground without a fully condensed funnel!

An excellent comprehensive list of questions and answers about tornadoes can be found here:http://www.spc.noaa.gov/faq/tornado/

NSSL is working to simulate storms that produce tornadoes in computer models to better understand how they form and behave.

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Basic information about tornadoes, from the NOAA National Severe Storms Laboratory.

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Although they can occur at any time of the day or night, most tornadoes form in the late afternoon. By this time the sun has heated the ground and the atmosphere enough to produce thunderstorms.

Tornadoes form when warm, humid air collides with cold, dry air.

The denser cold air is pushed over the warm air, usually producing thunderstorms. The warm air rises through the colder air, causing an updraft. The updraft will begin to rotate if winds vary sharply in speed or direction.

As the rotating updraft, called a mesocycle, draws in more warm air from the moving thunderstorm, its rotation speed increases. Cool air fed by the jet stream, a strong band of wind in the atmosphere, provides even more energy.

Water droplets from the mesocyclone's moist air form a funnel cloud. The funnel continues to grow and eventually it descends from the cloud. When it touches the ground, it becomes a tornado.

Ron Johnson, Peoria Journal Star/AP

Characteristics of tornadoes

Twisters are usually accompanied or preceded by severe thunderstorms and high wlnds. Hail is also common.

Once a tornado hits the ground, it may live for as little as a few seconds or as long as three hours.

The average twister is about 660 feet wide and moves about 30 miles an hour. Most don't travel more than six miles before dying out.

Massive tornadoes, however-the ones capable of widespread destruction and many deaths-can roar along as fast as 300 miles an hour.

These measurements are scientists' best estimations. Anemometers, which measure wind speed, cannot withstand the enormous force of tornadoes to record them.

Using units F0 to F5, the Fujita scale measures a tornado's intensity by analyzing the damage the twister has done and then matching that to the wind speeds estimated to produce comparable damage. The United States now uses the EF (Enhanced Fujita) scale, which takes more variables into account when assigning wind speeds to a tornado.

Tornado destruction

Every year in the United States, tornadoes do about 400 million dollars in damage and kill about 70 people on average.

Extremely high winds tear homes and businesses apart. Winds can also destroy bridges, flip trains, send cars and trucks flying, tear the bark off trees, and suck all the water from a riverbed.

High winds sometimes kill or injure people by rolling them along the ground or dropping them from dangerous heights. But most tornado victims are struck by flying debris-roofing shingles, broken glass, doors, metal rods.

The number of average deaths per year in the United States used to be higher before improved forecasting and warning systems were put into place.

How tornadoes are forecasted

Meteorologists at the U.S. National Weather Service use Doppler radar, satellites, weather balloons, and computer modeling to watch the skies for severe storms and tornadic activity.

Doppler radars record wind speeds and identify areas of rotation within thunderstorms. Since Doppler radar has been in use, the warning time for tornadoes has grown from fewer than five minutes in the 1980s to an average of 13 minutes by the late 2000s.

When weather conditions are conducive for tornado formation, the National Weather Service issues a tornado watch. When a tornado has been sighted or indicated on radar, a tornado warning is issued.

Some scientists, meteorology buffs, and adrenaline junkies hit the road during tornado season to chase storms. Researchers race to place sensors in tornadoes' paths. The sensors measure data such as wind speed, barometric pressure, humidity, and temperature.

The challenge for researchers is being in the right place at the right time. Every morning they study weather conditions and head for the area that seems most likely to spawn a twister. They drive through severe storms, dodge lightning, face flash floods, and get pounded by hail-sometimes for years-before ever spotting a tornado.

All at considerable risk. In 2013, National Geographic Explorer Tim Samaras and his team were killed while trying to study a tornado in Oklahoma. ( See photos of Samaras's work.)

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Learn how these deadly storms form and wreak havoc, and how you can reduce your risk.

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Tornado, a small-diameter column of violently rotating air developed within a convective cloud and in contact with the ground. Tornadoes occur most often in association with thunderstorms during the spring and summer in the mid-latitudes of both the Northern and Southern Hemispheres. These whirling atmospheric vortices can generate the strongest winds known on Earth: wind speeds in the range of 500 km (300 miles) per hour have been measured in extreme events. When winds of this magnitude strike a populated area, they can cause fantastic destruction and great loss of life, mainly through injuries from flying debris and collapsing structures. Most tornadoes, however, are comparatively weak events that occur in sparsely populated areas and cause minor damage.

This article describes tornado occurrence and formation as products of instability within the Earth's air masses and wind systems. Wind speeds and destructiveness are discussed with special reference to the Enhanced Fujita Scale of tornado intensity. For short, descriptive entries on closely related phenomena not covered in this article, see waterspout, whirlwind, and fire storm.

wind speed range**

EF number

metres per second

kilometres per hour

feet per second

miles per hour

*This scale was implemented as the standard scale of tornado intensity for the United States on February 1, 2007.

**Like the Fujita Scale, the Enhanced Fujita Scale is a set of wind estimates (not measurements of wind at the surface). Each level in the Enhanced Fujita Scale is derived from three-second wind gusts estimated at the point of damage to 28 indicators (such as trees, buildings, and various types of infrastructure) and the degree of damage to each indicator. Wind estimates vary with height and exposure. Each value is converted from miles per hour and rounded to the nearest whole number.

Source: Modified from the Enhanced F Scale for Tornado Damage web page (http://www.spc.noaa.gov/efscale/ef-scale.html), produced by the National Oceanic and Atmospheric Association (NOAA).

Tornado occurrence and distribution

Global occurrence

Tornadoes have been reported on all continents except Antarctica. They are most common on continents in the mid-latitudes (between 20° and 60° N and S), where they are frequently associated with thunderstorms that develop in regions where cold polar air meets warm tropical air.

Calculating which country has the most tornadoes per year depends on how this measurement is defined. The United Kingdom has the most tornadoes per land size, most of them weak. On average, about 33 tornadoes are reported annually there. In absolute numbers, the United States has the most tornadoes by far (more than 1,000 per year have been reported every year since 1990). It also has the most violent tornadoes (about 10 to 20 per year). Tornadoes of this intensity are very infrequent outside of the United States. Canada reports the second largest number of tornadoes (about 80 to 100 annually). Russia may have many tornadoes, but reports are not available to quantify their occurrence. About 20 tornadoes are reported in Australia each year, though the actual number is likely much higher. Many storms occur in uninhabited areas, and so any tornadoes that they produce are undocumented.

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Records of tornado occurrences are fragmentary for many areas, making estimates of global tornado frequency difficult. Insurance records show that tornadoes have caused significant losses in Europe, India, Japan, South Africa, and Australia. Rare but deadly tornadoes have occurred in many other countries, including Bangladesh, China, and Argentina. There are few tornado reports from either the Arctic or the equatorial tropics.

In the United Kingdom almost all reported tornadoes are associated with vigorous convection occurring in advance of and along a cold frontal boundary. Large temperature differences are associated with early winter cold fronts that move rapidly across the country from the north and west, at times spawning widespread outbreaks of small tornadoes. For example, the passage of a very strong frontal boundary across the United Kingdom on November 23, 1981, produced 105 documented tornadoes. Similar phenomena occur in other European countries such as France and Belgium.

Most Southern Hemisphere tornadoes occur in Australia. Many reports come from New South Wales, where there were 173 reported tornadoes from 1901 to 1966. In addition, South Africa and Argentina both reported 191 tornadoes from 1930 to 1979. Because tornado formation is closely tied to the speed and directional shear of the wind with height, tornadoes in the Southern Hemisphere almost exclusively rotate clockwise, opposite to the rotation of their Northern Hemisphere counterparts.

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Tornado, a small-diameter column of violently rotating air developed within a convective cloud and in contact with the ground. Tornadoes occur most often in association with thunderstorms during the spring and summer in the mid-latitudes of both the Northern and Southern Hemispheres. These whirling

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VIDEO: What is a hurricane? Here's what you need to know in less than a minute. Transcript

A tropical cyclone is a rotating low-pressure weather system that has organized thunderstorms but no fronts (a boundary separating two air masses of different densities). Tropical cyclones with maximum sustained surface winds of less than 39 miles per hour (mph) are called tropical depressions. Those with maximum sustained winds of 39 mph or higher are called tropical storms.

When a storm's maximum sustained winds reach 74 mph, it is called a hurricane. The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 rating, or category, based on a hurricane's maximum sustained winds. The higher the category, the greater the hurricane's potential for property damage.

Hurricanes originate in the Atlantic basin, which includes the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico, the eastern North Pacific Ocean, and, less frequently, the central North Pacific Ocean. A six-year rotating list of names, updated and maintained by the World Meteorological Organization, is used to identify these storms.

"Hurricane Season" begins on June 1 and ends on November 30, although hurricanes can, and have, occurred outside of this time frame. NOAA's National Hurricane Center predicts and tracks these massive storm systems, which occur, on average, 12 times a year in the Atlantic basin.

As a world leader in hurricane research, NOAA strives to understand the mechanics of these complex storms in order to protect people, property, commerce, and natural resources.

Video Transcript

When the maximum sustained winds of a tropical storm reach 74 miles per hour, it's called a hurricane. Hurricane Season begins on June 1 and ends on November 30, but these powerful storms can occur before and after the official season. A hurricane can be an awesome and destructive force of nature. Be prepared. Visit Ready.gov

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A tropical cyclone is a rotating low-pressure weather system that has organized thunderstorms but no fronts (a boundary separating two air masses of different densities). Tropical cyclones with maximum sustained surface winds of less than 39 miles per hour (mph) are called tropical depressions.

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What Causes extreme Weather?

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Wondering what's causing all the extreme weather we've seen lately? The short answer, scientists say, is rotten luck and a warmer planet.
It's not easy to shatter a record that has lasted for more than 75 years. But that's what happened last month, when a stubborn heat wave pushed July temperatures in the United States into uncharted territory.

Not since 1895, when national record keeping began, has the thermometer stayed so high. The average temperature in July was 3.3°F (1.8°C) above the 20th-century average, according to the U.S. National Oceanic and Atmospheric Administration (NOAA), breaking a record set in 1936. (Related: " July Hottest Month on Record in the U.S.")

Besides making life miserable for people in places like Kansas City, Missouri, where the temperature hit 107°F (42°C) on July 25, the withering heat is also taking a toll on wildlife and crops.

Streams in the Midwest got so hot in recent weeks, and water levels dropped so low, that the extreme conditions killed tens of thousands of sturgeon and other fishes. Meanwhile, crops such as corn and soybeans, despite welcome rains across the Midwest this week, have been devastated by the recent drought and are now expected to produce the smallest yields in years, according to the U.S. Department of Agriculture.
Warming Supercharging Extreme Weather
The main forces behind these disasters have been natural weather cycles, such as back-to-back La Niñas in 2010 and 2011 that routed storms away from drought-prone parts of North America, and a massive high pressure system, known as a heat ridge, that parked itself over the U.S. this summer and refused to budge.

But there's more to the story, said John Nielsen-Gammon, state climatologist in Texas, which just went through the worst one-year drought in its history.

It wasn't just bad luck that caused these disasters, he explained. Human-induced global warming was also to blame. (Test your global warming knowledge.)

Since 1970, the average global temperature has risen by 0.9°F (0.5°C), primarily as a result of greenhouse gases in the atmosphere, according to most scientists. This additional heat has increased the chances for severe heat waves, drought, and other forms of extreme weather.

"Given how little rain we had, we probably would have had record warmth in Texas in 2011 even without climate change," Nielson-Gammon said. "But climate change added an additional degree or so of heat to it."
Greater Rainfall Expected
And that's not all. As Earth's oceans grow warmer, they evaporate more moisture into the atmosphere-about 4 percent more since 1970, according to recent data. This additional moisture in the air makes it more likely that storms will bring downpours like the ones that hit Beijing in late July.

Authorities there described those rains as the heaviest in six decades. "Everybody knows that if you turn up the fire on your stove, you evaporate the water in a pot more rapidly," said Jay Gulledge, senior scientist at the Center for Climate and Energy Solutions (C2ES), a think tank in Arlington, Virginia. The more water vapor, the greater the potential for intense rainfalls, he said.

Luck will always play a role in determining whether it's sunny or rainy on any particular day. But these two factors, rising temperatures and increased moisture in the atmosphere, make the chances for extreme weather that much greater from here on. (See a video of natural disasters.)

For that, we can thank the rise in greenhouse gases, said Gerald Meehl, a senior scientist at the National Center for Atmospheric Research in Boulder, Colorado.

"By adding just a little bit more carbon dioxide to the climate, it makes things a little bit warmer and shifts the odds toward these more extreme events," he said. "What was once a rare event will become less rare."

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Wondering what's causing all the extreme weather we've seen lately? The short answer, scientists say, is rotten luck and a warmer planet. It's not easy to shatter a record that has lasted for more than 75 years. But that's what happened last month, when a stubborn heat wave pushed July temperatures in the United States into uncharted territory.

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National Climate Assessment

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The National Climate Assessment summarizes the impacts of climate change on the United States, now and in the future.