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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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Tornadoes

What is a tornado?
A tornado is a violent rotating column of air extending from a thunderstorm to the ground. The most violent tornadoes are capable of tremendous destruction with wind speeds of up to 300 mph. They can destroy large buildings, uproot trees and hurl vehicles hundreds of yards. They can also drive straw into trees. Damage paths can be in excess of one mile wide to 50 miles long. In an average year, 1000 tornadoes are reported nationwide.

How do tornadoes form?

Most tornadoes form from thunderstorms. You need warm, moist air from the Gulf of Mexico and cool, dry air from Canada. When these two air masses meet, they create instability in the atmosphere. A change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere. Rising air within the updraft tilts the rotating air from horizontal to vertical. An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.

Click Here to learn more about tornadoes from NOAA.

What are some other factors for tornadoes to form?
Several conditions are required for the development of tornadoes and the thunderstorm clouds with which most tornadoes are associated. Abundant low level moisture is necessary to contribute to the development of a thunderstorm, and a "trigger" (perhaps a cold front or other low level zone of converging winds) is needed to lift the moist air aloft. Once the air begins to rise and becomes saturated, it will continue rising to great heights to produce a thunderstorm cloud, if the atmosphere is unstable. An unstable atmosphere is one where the temperature decreases rapidly with height. Atmospheric instability can also occur when dry air overlays moist air near the earth's surface. Finally, tornadoes usually form in areas where winds at all levels of the atmosphere are not only strong, but also turn with height in a clockwise or veering direction.

What do tornadoes look like?
Tornadoes can appear as a traditional funnel shape, or in a slender rope-like form. Some have a churning, smoky look to them, and other contain "multiple vortices", which are small, individual tornadoes rotating around a common center. Even others may be nearly invisible, with only swirling dust or debris at ground levels as the only indication of the tornado's presence.

What is a funnel cloud? A funnel cloud is a rotating cone-shaped column of air extending downward from the base of a thunderstorm, but not touching the ground. When it reaches the ground it is called a tornado.

It is not fully understood about how exactly tornadoes form, grow and die. Tornado researchers are still trying to solve the tornado puzzle, but for every piece that seems to fit they often uncover new pieces that need to be studied. How do tornadoes stop?

A supercell thunderstorm is a long-lived thunderstorm whose updrafts and downdrafts are in near balance. These storms have the greatest tendency to produce tornadoes that stay on the ground for long periods of time. Supercell thunderstorms can produce violent tornadoes with winds exceeding 200 mph.
What is a supercell thunderstorm?

A mesocyclone is a rotating vortex of air within a supercell thunderstorm. Mesocyclones do not always produce tornadoes.
What is a mesocyclone?

A wall cloud is an abrupt lowering of a rain-free cumulonimbus base into a low-hanging accessory cloud. A wall cloud is usually situated in the southwest portion of the storm. A rotating wall cloud usually develops before tornadoes or funnel clouds. What is a microburst?
A microburst is a downdraft (sinking air) in a thunderstorm that is less than 2.5 miles in scale. Although microbursts are not as widely recognized as tornadoes, they can cause comparable, and in some cases, worse damage than some tornadoes produce. In fact, wind speeds as high as 150 mph are possible in extreme microburst cases.
What is a wall cloud?

A waterspout is just a weak tornado that forms over water. They are most common along the Gulf Coast. Waterspouts can sometimes move inland, becoming tornadoes causing damage and injuries.

What is a waterspout?

What is hail?
Hail is created when small water droplets are caught in the updraft of a thunderstorm. These water droplets are lifted higher and higher into the sky until they freeze into ice. Once they become heavy, they will start to fall. If the smaller hailstones get caught in the updraft again, they will get more water on them and get lifted higher in the sky and get bigger. Once they get lifted again, they freeze and fall. This happens over and over again until the hailstone is too heavy and then falls to the ground.

What is the largest hailstone recorded in the United States?
According to the National Weather Service, the largest hailstone is 8 inches in diameter and weights approximately 2 pounds. It fell in Vivian, South Dakota on July 23, 2010.
A gustnado is a short-lived, relatively weak whirlwind that forms along a gust front. A gust front is the surge of very gusty winds at the leading edge of a thunderstorm's outflow of air. Gustnadoes are not tornadoes. They do not connect with any cloud-base rotation. But because gustnadoes often have a spinning dust cloud at ground level, they are sometimes wrongly reported as tornadoes. Gustnadoes can do minor damage.

What is a gustnado?
A landspout is a very weak tornado that is not associated with a wall cloud or a mesocyclone. It is the land equivalent of a waterspout.

What is a landspout?
A dust devil generally forms in the hot sun during the late morning or early afternoon hours. These are mostly harmless whirlwinds and are triggered by light desert breezes that create a swirling plume of dust with speeds rarely over 70 mph. These differ from tornadoes in that they are not associated with a thunderstorm (or any cloud) and are usually very weak.

What is a dust devil? Tornadoes can happen at any time of the year and at any time of the day. In the southern states, peak tornado season is from March through May. Peak times for tornadoes in the northern states are during the summer. A few southern states have a second peak time for tornado outbreaks in the fall. Tornadoes are most likely to occur between 3 p.m. and 9 p.m.

The geography of the central part of the United States, known as the Great Plains, is suited to bring all of the ingredients together to forms tornadoes. More than 500 tornadoes typically occur in this area every year and is why it is commonly known as "Tornado Alley". Texas, Oklahoma, Kansas, Nebraska, South Dakota, North Dakota, Iowa, Missouri, Arkansas and Louisiana all make up Tornado Alley. When are tornadoes most likely to occur?

Where are tornadoes most likely to occur?

Know the Lingo

TORNADO WATCH - Tornadoes are possible in your area. Stay tuned to the radio or television news.
TORNADO WARNING - A tornado is either on the ground or has been detected by Doppler radar. Seek shelter immediately!

Click Here to see if there are any active warnings in your area.

Know the Facts
Tornadoes can occur at any time of the year. nd winds less than 110 mph.

No terrain is safe from tornadoes.

Never open windows in severe weather situations. This allows damaging wind and debris to enter the structure.

69% of all tornadoes are labeled "weak tornadoes" meaning they have a lifetime of 1-10+ minutes a

SCALE

WIND SPEED

POSSIBLE DAMAGE

Enhanced,
Operational
Fujita Scale

29% of all tornadoes are labeled "strong tornadoes" meaning they last 20 minutes or longer and winds reach 110-205 mph.
2% of all tornadoes are labeled "violent tornadoes" and can last over an hour.

Fujita Scale of Tornado Intensity

Tornado Safety Tips
BEFORE A TORNADO: Have a disaster plan. Make sure everyone knows where to go in case a tornado threatens. Make sure you know which county or parish you live in. Prepare a kit with emergency food for your home. Have enough food and water for at least 3 days.

AFTER A TORNADO: Stay indoors until it is safe to come out. Check for injured or trapped people, without putting yourself in danger. Watch out for downed power lines. Use a flashlight to inspect your home. Tornado Activities

Lesson Plan: Here are some great lesson plans on learning about tornado safety. Note: This is a PDF file, so you need to have Adobe Acrobat Reader. DURING A TORNADO: Go to a basement. If you do not have a basement, go to an interior room without windows on the lowest floor such as a bathroom or closet. If you can, get under a sturdy piece of furniture, like a table. If you live in a mobile home get out. They offer little protection against tornadoes. Get out of automobiles. Do not try to outrun a tornado in your car, leave it immediately. If you're outside, go to a ditch or low lying area and lie flat in it. Stay away from fallen power lines and stay out of damaged areas.
Tornado Experiment: Here is a great experiment that allows the kids to make a tornado in a bottle.
IF YOU'RE AT SCHOOL DURING A TORNADO: Every school should have a disaster plan and have frequent drills. Basements offer the best protection. Schools without basements should use interior rooms and hallways on the lowest floor away from windows. Crouch down on your knees and protect your head with your arms.
Tornado Experiment: Here is a great experiment that allows the kids to make a tornado in a jar.
Pressure Experiment: Here is an experiment that shows how pressure is created in our atmosphere by sucking an egg in a bottle. This is a very cool experiment!
Science Fair Project Ideas: Here is a complete list of science fair project ideas. Discover the science behind the weather that impacts us every day.

Make A Barometer Experiment: Here is an experiment that allows the kids to make a barometer.

Evaporation Experiment: Here is an experiment that shows kids how evaporation takes place.

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Weather Wiz Kids weather information for kids

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Weather Wiz Kids is a fun and safe website for kids about all the weather info they need to know. It contains tools for weather education, including weather games, activities, experiments, photos, a glossary and educational teaching materials for the classroom.

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US Department of Commerce, NOAA, National Weather Service

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Find out what causes these deadly twisters—and how to stay safe.

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Hurricanes only form over really warm ocean water of 80°F or warmer. The atmosphere (the air) must cool off very quickly the higher you go. Also, the wind must be blowing in the same direction and at the same speed to force air upward from the ocean surface. Winds flow outward above the storm allowing the air below to rise. Hurricanes typically form between 5 to 15 degrees latitude north and south of the equator. The To learn more about hurricanes, Prepare a disaster supplies kit for your home and car. Include a first aid kit, canned food and a can opener, bottled water, battery-operated radio, flashlight, protective clothing and written instructions on how to turn off electricity, gas, and water. Have a NOAA weather radio handy with plenty of batteries, so you can listen to storm advisories. Have some cash handy as well, because following a hurricane, banks and ATMs may be temporarily closed. Make sure your car is filled with gasoline. Hurricanes

What is a hurricane?
A hurricane is a huge storm! It can be up to 600 miles across and have strong winds spiraling inward and upward at speeds of 75 to 200 mph. Each hurricane usually lasts for over a week, moving 10-20 miles per hour over the open ocean. Hurricanes gather heat and energy through contact with warm ocean waters. Evaporation from the seawater increases their power. Hurricanes rotate in a counter-clockwise direction around an "eye" in the Northern Hemisphere and clockwise direction in the Southern Hemisphere. The center of the storm or "eye" is the calmest part. It has only light winds and fair weather. When they come onto land, the heavy rain, strong winds and large waves can damage buildings, trees and cars.

>Click Here to get your very own Hurricane Tracking Chart.

How do hurricanes form?
Coriolis Force is needed to create the spin in the hurricane and it becomes too weak near the equator, so hurricanes can never form there.

What is storm surge?
Storm surges are frequently the most devastating element of a hurricane. As a hurricane's winds spiral around and around the storm, they push water into a mound at the storm's center. This mound of water becomes dangerous when the storm reaches land because it causes flooding along the coast. The water piles up, unable to escape anywhere but on land as the storm carries it landward. A hurricane will cause more storm surge in areas where the ocean floor slopes gradually. This causes major flooding.
As you watch the storm-surge animations, notice the effect that the physical geography of each coastline has on storm surge. Also, note the waves on top of the ocean's surface. Wind, waves, and sea-level rise all contribute to storm-surge damage.

Shallow-Water Coastline

Deep-Water Coastline

With technology the way it is, there are computer models that allow forecasters to predict the amount of storm surge that will affect a coastal area. These are called Slosh Models and take into account a storm's strength, its path, how the ocean shallows, and the shape of the land. Then it calculates how much storm surge a hurricane will probably cause.

The Atlantic hurricane season is from June 1 to November 30, but most hurricanes occur during the fall months. The Eastern Pacific hurricane season is from May 15 to November 30. (Below is a graphic that shows you when hurricanes are most active across parts of the world.)
When does hurricane season start?

Who names hurricanes?
From 1950 to 1952, tropical cyclones of the North Atlantic Ocean were identified by the phonetic alphabet (Able-Baker-Charlie-etc.), but in 1953 the US Weather Bureau switched to women's names. The rest of the world eventually caught on, and naming rights now go by the World Meteorological Organization, which uses different sets of names depending on the part of the world the storm is in. Around the U.S., only women's names were used until 1979, when it was decided that they should alternate a list that included men's names too. There's 6 different name lists that alternate each year. If a hurricane does significant damage, its name is retired and replaced with another.
Atlantic Hurricane Names
Eastern Pacific Hurricane Names
Nothing except geography. Tropical storms occur in several of the world's oceans, and except for their names, they are essentially the same type of storm. In the Atlantic Ocean, Gulf of Mexico, and the Eastern Pacific Ocean, they are called hurricanes. In the Western Pacific Ocean, they are called typhoons. In the Indian Ocean, the Bay of Bengal, and Australia, these types of storms are called cyclones.
What is the difference between a hurricane and a typhoon?
(This is a satellite animation of Hurricane Georges, which struck the Mississippi Gulf coast in 1998.)

The brave "hurricane hunters" work for the National Oceanic and Atmospheric Administration (NOAA). Each mission lasts about ten hours, with the crews passing four to six times through the storm. The planes carry radar, sophisticated computers, and weather instruments that determine characteristics such as temperature, air pressure, wind speed, and wind direction inside the hurricane. The crews also release instruments that measure temperature, air pressure, and wind at different levels as the devices drop through the hurricane toward the ocean. By mission's end, NOAA can warn everyone in the hurricane's path. (Below is a satellite image of Hurricane Mitch back in October 1998. The Hurricane Hunters flew into the eye of Mitch just as this Category 5 hurricane with winds of 155 mph smacked right into Central America.)

Who are the "Hurricane Hunters"?
Coastal beach erosion is the wearing away of land, the removal of beach or dune sediments by wave action, tidal currents, wave currents, or drainage. Waves are generated by storms, wind, or hurricanes and can cause coastal erosion. This may take the form of long-term losses of sediment and rocks, or merely the temporary redistribution of coastal sediments.

What is coastal beach erosion?

Hurricanes...Past and Present
Tropical Tracks: Click to see the tracks of this year's storms.

Know the Lingo Past Hurricane Info: Click to find all the data and information about a specific hurricane by just knowing the year.
HURRICANE WATCH - Hurricane conditions with sustained winds of 74 mph or greater are possible in your area within the next 36 hours. This WATCH should trigger your family's disaster plan, and protective measures should be initiated. Especially, those actions that require extra time such as securing a boat and leaving a barrier island.
TROPICAL STORM WATCH - Tropical Storm conditions with sustained winds from 39 -74 mph are possible in your area within the next 36 hours.
HURRICANE WARNING - Hurricane conditions are expected in your area within 24 hours. Once this WARNING has been issued, your family should be in the process of completing protective actions and deciding the safest location to be during the storm.

COASTAL FLOOD WATCH - The possibility exists for the inundation of land areas along the coast within the next 12 to 36 hours.

COASTAL FLOOD WARNING - Land areas along the coast are expected to become, or have become, inundated by sea water above the typical tide action.

SMALL CRAFT ADVISORY - A small craft advisory is a type of warning issued by the National Weather Service, most frequently in coastal areas. It is issued when winds have reached, or are expected to reach within 12 hours, a speed marginally less than that which is considered gale force, usually 25-38 mph.

Click Here to see if there are any active warnings in your area.
Hurricane Stages
Saffir-Simpson Hurricane Scale

Category

Winds (MPH)

Pressure (Millibars)

Pressure (Inches)

Storm Surge
(Feet)

Damage

3

111-130

964-945

28.47-27.91

9'-12'

Extensive

Click Here to watch a variety of videos teaching you the ins and outs of nature's fury. I would like to give a special thanks to Pinellas County Emergency management for putting these instructional videos together.
Hurricane Safety Tips

BEFORE A HURRICANE: Have a disaster plan and a pet plan ready. Before a storm threatens, contact your veterinarian or local humane society for information on preparing your pets for an emergency. Board up windows and b ring in outdoor objects that could blow away. Make sure you know which county or parish you live in and k now where all the evacuation routes are.

Stay away from low-lying and flood prone areas. Always stay indoors during a hurricane, because strong winds will blow things around. Leave mobile homes and to go to a shelter. DURING A HURRICANE: If your home isn't on higher ground, go to a shelter. If emergency managers say to evacuate, then do so immediately.
AFTER A HURRICANE: Stay indoors until it is safe to come out. Check for injured or trapped people, without putting yourself in danger. Watch out for flooding which can happen after a hurricane. Do not attempt to drive in flooding water. Stay away from standing water. It may be electrically charged from underground or downed power lines. Don't drink tap water until officials say its safe to do so.

Evacuation Tips: Here is a list of tips on what you should do, if you chose to evacuate as a hurricane approaches.
Hurricane Activities
Hurricane Supply Checklist: Here is a list of what what you will need if you chose to stay at your home during a hurricane.
Lesson Plan: Here is a great lesson plan on learning about hurricane safety. In this activity, Owlie teaches kids about hurricanes and quizzes them about what they learned. Note: This is a PDF file, so you need to have Adobe Acrobat Reader.
Lesson Plan: Here is a great lesson plan on understanding hurricanes. Students will learn how wind speed increases the height and why higher waves occur in shallower water. This activity is for grades 6-8.

Lesson Plan: Here is a great interactive lesson plan on allowing kids to create their very own hurricane. It lets you try to create ideal hurricane conditions by changing the winds, latitude, moisture, and sea temperature. It even keeps score! This is a great activity for 5-8th graders.Lesson Plan: Here is a great lesson plan on elementary reading comprehension. This lesson contains a broader vocabulary, more hurricane facts, and more thought-provoking writing prompts. It works well for grades 3-5. Note: This is a PDF file, so you need to have Adobe Acrobat Reader. Lesson Plan: Here is a great interactive lesson plan that lets kids aim a hurricane. It lets you move the hurricane symbol, high pressure symbol and low pressure symbol to track your very own hurricane. Sometimes your hurricane will make landfall, other times it misses the U.S. all together. It lets kid understand how important the setup of weather impacts where hurricanes go.

Pressure Experiment: Here is an experiment that shows how pressure is created in our atmosphere by sucking an egg in a bottle. This is a very cool experiment!
Science Fair Project Ideas: Here is a complete list of science fair project ideas. Discover the science behind the weather that impacts us every day.
Make A Barometer Experiment: Here is an experiment that allows the kids to make a barometer.

Evaporation Experiment: Here is an experiment that shows kids how evaporation takes place.

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Weather Wiz Kids weather information for kids

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Weather Wiz Kids is a fun and safe website for kids about all the weather info they need to know. It contains tools for weather education, including weather games, activities, experiments, photos, a glossary and educational teaching materials for the classroom.

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Severe Weather 101

Thunderstorm Basics

Read more about NSSL's thunderstorm research here.

What is a thunderstorm?

A thunderstorm is a rain shower during which you hear thunder. Since thunder comes from lightning, all thunderstorms have lightning.

Why do I sometimes hear meteorologists use the word "convection" when talking about thunderstorms?

Usually created by surface heating, convection is upward atmospheric motion that transports whatever is in the air along with it-especially any moisture available in the air. A thunderstorm is the result of convection.

What is a severe thunderstorm?

A thunderstorm is classified as "severe" when it contains one or more of the following: hail one inch or greater, winds gusting in excess of 50 knots (57.5 mph), or a tornado.

How many thunderstorms are there?

Worldwide, there are an estimated 16 million thunderstorms each year, and at any given moment, there are roughly 2,000 thunderstorms in progress. There are about 100,000 thunderstorms each year in the U.S. alone. About 10% of these reach severe levels.

When are thunderstorms most likely?

Thunderstorms are most likely in the spring and summer months and during the afternoon and evening hours, but they can occur year-round and at all hours.

Along the Gulf Coast and across the southeastern and western states, most thunderstorms occur during the afternoon. Thunderstorms frequently occur in the late afternoon and at night in the Plains states.

What kinds of damage can thunderstorms cause?

Many hazardous weather events are associated with thunderstorms. Under the right conditions, rainfall from thunderstorms causes flash flooding, killing more people each year than hurricanes, tornadoes or lightning. Lightning is responsible for many fires around the world each year, and causes fatalities. Hail up to the size of softballs damages cars and windows, and kills livestock caught out in the open. Strong (up to more than 120 mph) straight-line winds associated with thunderstorms knock down trees, power lines and mobile homes. Tornadoes (with winds up to about 300 mph) can destroy all but the best-built man-made structures.

Where are severe thunderstorms most common?

The greatest severe weather threat in the U.S. extends from Texas to southern Minnesota. But, no place in the United States is completely safe from the threat of severe weather.

What is the difference between a Severe Thunderstorm WATCH and a Severe Thunderstorm WARNING?

A Severe Thunderstorm WATCH is issued by the NOAA Storm Prediction Center meteorologists who are watching the weather 24/7 across the entire U.S. for weather conditions that are favorable for severe thunderstorms. 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 Severe Thunderstorm WARNING is issued by your localNOAA National Weather Service Forecast Office meteorologists who watch a designated area 24/7 for severe weather that has been reported by spotters or indicated by radar. Warnings mean there is a serious threat to life and property to those in the path of the storm. ACT now to find safe shelter! A warning can cover parts of counties or several counties in the path of danger.

How does a thunderstorm form?

Three basic ingredients are required for a thunderstorm to form: moisture, rising unstable air (air that keeps rising when given a nudge), and a lifting mechanism to provide the "nudge."

The sun heats the surface of the earth, which warms the air above it. If this warm surface air is forced to rise-hills or mountains, or areas where warm/cold or wet/dry air bump together can cause rising motion-it will continue to rise as long as it weighs less and stays warmer than the air around it.

As the air rises, it transfers heat from the surface of the earth to the upper levels of the atmosphere (the process of convection). The water vapor it contains begins to cool, releases the heat, condenses and forms a cloud. The cloud eventually grows upward into areas where the temperature is below freezing.

As a storm rises into freezing air, different types of ice particles can be created from freezing liquid drops. The ice particles can grow by condensing vapor (like frost) and by collecting smaller liquid drops that haven't frozen yet (a state called "supercooled"). When two ice particles collide, they usually bounce off each other, but one particle can rip off a little bit of ice from the other one and grab some electric charge. Lots of these collisions build up big regions of electric charges to cause a bolt of lightning, which creates the sound waves we hear as thunder.

The Thunderstorm Life Cycle

Thunderstorms have three stages in their life cycle: The developing stage, the mature stage, and the dissipating stage. The developing stage of a thunderstorm is marked by a cumulus cloud that is being pushed upward by a rising column of air (updraft). The cumulus cloud soon looks like a tower (called towering cumulus) as the updraft continues to develop. There is little to no rain during this stage but occasional lightning. The thunderstorm enters the mature stage when the updraft continues to feed the storm, but precipitation begins to fall out of the storm, creating a downdraft (a column of air pushing downward). When the downdraft and rain-cooled air spreads out along the ground it forms a gust front, or a line of gusty winds. The mature stage is the most likely time for hail, heavy rain, frequent lightning, strong winds, and tornadoes. Eventually, a large amount of precipitation is produced and the updraft is overcome by the downdraft beginning the dissipating stage. At the ground, the gust front moves out a long distance from the storm and cuts off the warm moist air that was feeding the thunderstorm. Rainfall decreases in intensity, but lightning remains a danger.

What does a thunderstorm look like?

Thunderstorms can look like tall heads of cauliflower or they can have "anvils." An anvil is the flat cloud formation at the top of the storm. An anvil forms when the updraft (warm air rising) has reached a point where the surrounding air is about the same temperature or even warmer. The cloud growth abruptly stops and flattens out to take the shape of an anvil.

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

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Hurricanes are among nature's most powerful and destructive phenomena. On average, 12 tropical storms, 6 of which become hurricanes form over the Atlantic Ocean, Caribbean Sea, or Gulf of Mexico during the hurricane season which runs from June 1 to November 30 each year. In the Central Pacific Ocean, an average of 3 tropical storms, 2 of which become hurricanes form or move over the area during the hurricane season, which runs from June 1 to November 30 each year. Guam, the Northern Marianas and Micronesia experience typhoons all year round but the main season in July through November with a peak from mid-August to mid-September. Over a typical 2-year period, the U.S. coastline is struck by an average of 3 hurricanes, 1 of which is classified as a major hurricane (winds of 111 mph or greater). By knowing what actions to take before the hurricane season begins, when a hurricane approaches, and when the storm is in your area, as well as what to do after a hurricane leaves your area, you can increase your chance of survival. If you, or someone you know, have been a victim of a hurricane, please share your story, including the town and state you were in and the year the event took place.. Please note that NS will then have permission to use your story for educational campaigns. Sharing this information may help save someone's life in the future. Read stories from survivors and learn how to stay safe.

Hurricane Hazards

While hurricanes pose the greatest threat to life and property, tropical storms and depression also can be devastating. The primary hazards from tropical cyclones (which include tropical depressions, tropical storms, and hurricanes) are storm surge flooding, inland flooding from heavy rains, destructive winds, tornadoes, and high surf and rip currents.

Storm surge is the abnormal rise of water generated by a storm's winds. This hazard is historically the leading cause of hurricane related deaths in the United States. Storm surge and large battering waves can result in large loss of life and cause massive destruction along the coast.

Storm surge can travel several miles inland, especially along bays, rivers, and estuaries.

Flooding from heavy rains is the second leading cause of fatalities from landfalling tropical cyclones. Widespread torrential rains associated with these storms often cause flooding hundreds of miles inland. This flooding can persist for several days after a storm has dissipated.

Winds from a hurricane can destroy buildings and manufactured homes. Signs, roofing material, and other items left outside can become flying missiles during hurricanes.

Tornadoes can accompany landfalling tropical cyclones. These tornadoes typically occur in rain bands well away from the center of the storm.

Dangerous waves produced by a tropical cyclone's strong winds can pose a significant hazard to coastal residents and mariners. These waves can cause deadly rip currents, significant beach erosion, and damage to structures along the coastline, even when the storm is more than a 1,000 miles offshore.

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Hurricanes are among nature's most powerful and destructive phenomena. On average, 12 tropical storms, 6 of which become hurricanes form over the Atlantic Ocean, Caribbean Sea, or Gulf of Mexico during the hurricane season which runs from June 1 to November 30 each year.

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Advancing Drought Science and Preparedness Across the Nation

The National Integrated Drought Information System (NIDIS) is a multi-agency partnership that coordinates drought monitoring, forecasting, planning, and information at national, tribal, state, and local levels.

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Advancing Drought Science and Preparedness Across the Nation The National Integrated Drought Information System (NIDIS) is a multi-agency partnership that coordinates drought monitoring, forecasting, planning, and information at national, tribal, state, and local levels.

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Info on every hurricane documented

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Please note that the following list is not exhaustive and does not include every notable storm in history.

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A drought is a prolonged period with less-than-average amounts of rain or snow in a particular region. The severity of the drought depends on the amount of time that a region receives below-average precipitation.

For example, a few weeks without rain could stress a farmer's crops during the growing season. This is called a flash drought. But it could take a much longer dry period to see a full drought that would affect a region's water supply.

What causes a drought?

A drought is caused by drier than normal conditions that can eventually lead to water supply problems. Really hot temperatures can make a drought worse by causing moisture to evaporate from the soil. Just because a region is hot and dry doesn't necessarily mean it is going through a drought. Droughts only occur when an area is abnormally dry. Here's why:

Rain and snow don't fall evenly across Earth. Some regions are routinely wet and others are routinely dry. From season to season - and from year to year - the amount of rain or snow in a location can vary.

However, over a period of many years, the average amount of precipitation in a region is fairly consistent. For example, in the deserts of the American Southwest, the average precipitation is less than 3 inches per year. But, the average yearly precipitation in Atlanta is about 50 inches.

When a particular area gets less rain than usual, the soil gets much less moisture, too. The soil starts drying out and plants die. When this pattern continues for several weeks, months or years, the flow of streams and rivers decreases and water levels in lakes, reservoirs and wells fall. Eventually, the unusual dry weather causes water supply issues, and the dry period becomes a drought.

Does a drought end when it rains?

Yes and no. Rain can help during a drought, but it doesn't make the drought go away. A light rain shower might cause the grass and plants to green up. But if that water quickly evaporates into the air or is soaked up by plants, the drought relief doesn't last long.

A thunderstorm can bring lots of rain into a region, but often the rain comes so quickly that it goes into sewers and ditches instead of soaking into the soil. In fact, if a thunderstorm suddenly hits a region experiencing drought, it can cause a flash flood.

The only way a drought can really end is with enough regular soaking rains or significant snow. Rains that soak into the soil can replenish the groundwater. Groundwater provides water to plants and can refill streams during non-rainy periods.

One soaking rain may help improve drought conditions. However, multiple soaking rains over several months are needed to truly return things to normal.

How do we monitor droughts?

One way to monitor droughts is from weather satellites in space. For example, satellite data were used to develop a tool that alerts farmers about upcoming flash droughts.

The National Oceanic and Atmospheric Administration, or NOAA, GOES-R (Geostationary Operational Environmental Satellites-R Series) and JPSS (Joint Polar Satellite System) series satellites can capture thermal infrared images of Earth. These images provide information about the amount of heat on Earth's surface. This information can be used to estimate evapotranspiration, which is a measure of how much water is being transferred from the land to the atmosphere through the soil and plants.

By comparing the weekly evapotranspiration data from satellites with the average for the region, scientists can predict whether or not a region is at risk for flash droughts - and give warnings to farmers and ranchers if crops will soon be under stress.

How do we compare droughts over time?

To get historical data about past dry years, scientists can use data from NOAA and a computer climate model called the North American Drought Atlas. Scientists can also look at tree rings from trees that are hundreds of years old.

You may have heard of using tree rings to learn about the past, but what are scientists looking for? There is information in the thickness of each ring. Thick rings mean the tree was able to grow faster due to plentiful water, indicating a wetter year. Scientists don't have to cut down a tree to see the rings. They can just insert a hollow drill into the trunk and pull out a cylindrical sample with the rings included.

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A drought is a prolonged period with less-than-average amounts of rain or snow in a particular region. The severity of the drought depends on the amount of time that a region receives below-average precipitation. For example, a few weeks without rain could stress a farmer's crops during the growing season.

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California's Extreme Drought, Explained | The New York Times

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What causes flooding? - National Geographic Kids

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Causes of floods: Why is it that some places flood and some don’t? Well, turns out its all in the geography. Post Boxing Day floods, one school decided to investigate

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A flood is an overflow of water onto land that is normally dry. Floods can happen almost anywhere. They can cover an area with just a few inches of water or they can bring enough water to cover the roof of a house. Floods can be dangerous for communities, lasting days, weeks or sometimes even longer.

Many different situations can cause a flood. Here are just a few:

Geography can also make an area more likely to flood. For example, areas near rivers are often at risk for floods. Urban areas (areas near cities) are also at higher risk for floods because rooftops funnel rainfall to the ground below, and paved surfaces such as highways and parking lots prevent the ground from absorbing the rain.

Mountains or steep hills can increase an area's flood risk, too. Rain or snowmelt running down a mountain can cause streams and rivers to rise quickly. In fact, if a thunderstorm lingers over a mountain, a creek only 6 inches deep can swell to a 10-foot-deep river in less than an hour.

What is a flash flood?

Flash floods are very dangerous floods that can happen with little or no warning. When there is more rain than the soil can absorb, the excess water quickly runs into rivers and creeks, overwhelming storm drains and ditches and causing a flash flood. Flash floods can cause water to rise significantly in a short amount of time.

Where does all of that rain come from?

Several different weather conditions can cause extreme rainfall in a region. Tropical cyclones form in some tropical and subtropical areas, usually in the summer and fall. When they appear in the Atlantic Ocean or the northwest part of the Pacific Ocean and reach a certain intensity, they are called hurricanes. Tropical cyclones can produce huge amounts of rain, causing flooding and flash flooding once the storm reaches land.

They can also send a rush of water from the ocean onto coastlines in an event called a storm surge, which floods low-lying areas. ( Learn more about how hurricanes form.)

Another phenomenon that can cause extreme rainfall is called an atmospheric river. Atmospheric rivers are long, narrow conveyor belts of moisture that move through the atmosphere.

Strong atmospheric rivers can deliver enormous amounts of rain and snow in California, the Pacific Northwest and Alaska, especially during the winter months. This can lead to serious flooding and mudslides.

How do satellites help during floods?

Weather satellites, like those in NOAA's Geostationary Operational Environmental Satellite-R ( GOES-R) series, keep an eye on atmospheric events that can lead to flooding.

For example, the GOES-R series satellites are equipped with an instrument called the Advanced Baseline Imager (ABI), which can detect and monitor the formation of atmospheric rivers. The ABI can also determine the amount of total moisture in the atmosphere from the ground all the way to the top of the atmosphere. This provides useful information for weather forecasters to improve predictions for heavy rains and flash flooding and help people stay safe.

After flooding occurs, weather satellites can also be used to spot heavily flooded regions from space. Information from GOES-R and the NOAA-NASA Suomi NPP satellite can be used to create "flood maps." These maps help officials quickly determine where to send help and resources during a flood.

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How do atmospheric conditions and an area's geography cause a flood?

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Why Do Floods Happen? | Weather Science | SciShow Kids

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Flooding is a coast-to-coast threat to some part of the United States and its territories nearly every day of the year. This site is designed to teach you how to stay safe in a flood event. If you know what to do before, during, and after a flood you can increase your chances of survival and better protect your property. For instance, it is vital to know what to do if you are driving and hit a flooded road. Here you will find an interactive flood map, information describing the different types of flooding and educational material. You will also learn how the National Weather Service keeps you aware of potentially dangerous flooding situations through alerts and warnings.

Learn how to better protect yourself and your family by reading our flood survivor stories. If you, or someone you know, have been a victim of a flood, please share your story so we can prevent others from becoming a victim. When you write, please note that NWS has permission to use your story and, if possible, let us know the town and state you were in and the year the event took place.

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Flooding is a coast-to-coast threat to some part of the United States and its territories nearly every day of the year. This site is designed to teach you how to stay safe in a flood event. If you know what to do before, during, and after a flood you can increase your chances of survival and better protect your property.

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Severe thunderstorms are officially defined as storms that are capable of producing hail that is an inch or larger or wind gusts over 58 mph. Hail this size can damage property such as plants, roofs and vehicles. Wind this strong is able to break off large branches, knock over trees or cause structural damage to trees. Some severe thunderstorms can produce hail larger than softballs or winds over 100 mph, so please pay attention to the weather so you know when severe storms are possible. Thunderstorms also produce tornadoes and dangerous lightning; heavy rain can cause flash flooding. These hazards are covered in more detail under the tornado, lightning safety and flood safety websites.

This website is designed to teach you how to stay safe in a severe thunderstorm. If you know what to do before, during, and after severe weather you can increase your chances of survival. You'll also find links to research, past events, other topics of interest, and downloadable materials about thunderstorms, lightning and tornadoes. If you, or someone you know, have been a victim of severe weather, please share your story so we can prevent others from becoming victims. When you write, please note that NWS has permission to use your story and, if possible, let us know the town and state you were in and the year the event took place.

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Severe thunderstorms are officially defined as storms that are capable of producing hail that is an inch or larger or wind gusts over 58 mph. Hail this size can damage property such as plants, roofs and vehicles. Wind this strong is able to break off large branches, knock over trees or cause structural damage to trees.

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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.