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Deadly wildfires threaten Northern and Southern California.

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Tornadoes are violently rotating columns of air that extend from a thunderstorm to the ground. Tornadoes can destroy buildings, flip cars, and create deadly flying debris.

A tornado can:

Happen anytime and anywhere.

Bring intense winds, over 200 miles per hour.

Look like funnels.

If you are under a tornado or severe weather warning:

Go to NOAA Weather Radio and your local news or official social media accounts for updated emergency information. Follow the instructions of state, local and tribal officials.

Go to a safe shelter immediately, such as a safe room, basement, storm cellar or a small interior room on the lowest level of a sturdy building.

Stay away from windows, doors, and outside walls.

Do not go under an overpass or bridge. You're safer in a low, flat location.

Watch out for flying debris that can cause injury or death.

Use your arms to protect your head and neck.

If you can't stay at home, make plans to go to a public shelter.

Preparing for a Tornado

Staying Safe During a Tornado

Staying Safe After a Tornado

Associated Content

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Learn what to do if you are under a tornado warning and how to stay safe when a tornado threatens. Prepare for Tornadoes Stay Safe During Stay Safe After Associated Content Tornadoes are violently rotating columns of air that extend from a thunderstorm to the ground.

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

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Explore National Geographic. A world leader in geography, cartography and exploration.

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National Geographic: Tornadoes, General Info.

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National Geographic: Tornadoes, General Info.

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Tornadoes are vertical funnels of rapidly spinning air. Their winds may top 250 miles an hour and can clear a pathway a mile wide and 50 miles long.

Also known as twisters, tornadoes are born in thunderstorms and are often accompanied by hail. Giant, persistent thunderstorms called supercells spawn the most destructive tornadoes.

These violent storms occur around the world, but the United States is a major hotspot with about a thousand tornadoes every year.

" Tornado Alley," a region that includes the area in the eastern state of South Dakota, Nebraska, Kansas, Oklahoma, northern Texas, and eastern Colorado, is often home to the most powerful and destructive of these storms. U.S. tornadoes cause 80 deaths and more than 1,500 injuries per year.

What is a tornado?

A tornado is a violently rotating column of air that extends from a thunderstorm to the ground. It's often portended by a dark, greenish sky. Black storm clouds gather. Baseball-size hail may fall. A funnel suddenly appears, as though descending from a cloud. The funnel hits the ground and roars forward with a sound like that of a freight train approaching. The tornado tears up everything in its path.

Every U.S. state has experienced twisters, but Texas holds the record: an annual average of 120. Tornadoes have been reported in Great Britain, India, Argentina, and other countries, but they are most often seen in the United States.

Related to tornadoes, waterspouts are weak twisters that form over warm water. They sometimes move inland and become tornadoes.

Dust devils are small, rapidly rotating columns of air that are made visible by the dust and dirt they pick up. Dust devils are not associated with thunderstorms. Either are fire tornadoes, which can spawn from wildfires.

What causes tornadoes?

The most violent tornadoes come from supercells, large thunderstorms that have winds already in rotation. About one in a thousand storms becomes a supercell, and one in five or six supercells spawns off a tornado.

Tornadoes can occur at any time of year, but they are more common during a distinct season that begins in early spring for the states along the Gulf of Mexico. The season follows the jet stream -as it swings farther north, so does tornado activity. May generally has more tornadoes than any other month, but April's twisters are sometimes more violent. Farther north, tornadoes tend to be more common later in summer.

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.

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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Learn more and understand better with BrainPOP's animated movies, games, playful assessments, and activities covering Science, Math, History, English, and more!

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Scientific America: Are Hurricanes Connected to Climate Change?

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Scientific America: Are Hurricanes Connected to Climate Change?

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Summer and fall 2017 saw an unusual string of record-breaking hurricanes pummel the U.S. Gulf Coast, eastern seaboard, Puerto Rico and the Caribbean.

Hurricane Harvey brought unbelievable floods to Houston. Irma, one of the two strongest hurricanes ever recorded in the northern Atlantic, wreaked havoc on Florida and many Caribbean islands. Maria devastated Puerto Rico and the Virgin Islands. The destruction begs the question: Has climate change influenced these extreme events? Hurricanes can be difficult to decipher, but experts are gaining a sense of what our warming world might mean for monster storms in the U.S. and worldwide.

Storm Intensity

Many experts are confident that a warmer world will create stronger storms-and already is doing so. Since 1981 the maximum wind speed of the most powerful hurricanes has risen, according to research ( pdf) by Jim Elsner, a climatologist at The Florida State University. That's because higher ocean heat provides more energy for storms, fueling their intensity. Hurricane Patricia, in 2015, set the record at the time for top wind speed-215 miles per hour-in the north Atlantic. The next year Winston shattered records as the most intense cyclone in the Southern Hemisphere.

The dynamic between storms and warming oceans occurs in part because of the role hurricanes play in our climate system: they rebalance Earth's heat. The storms remove heat from tropical oceans in the form of moisture and pump the heat up into the atmosphere, where heat is redistributed and radiated out into space. "In some sense, hurricanes are a relief valve," explains Kevin Trenberth, a senior scientist in the Climate Analysis Section at the National Center for Atmospheric Research. "From the climate standpoint, you need to have some hurricanes to come along and cool the ocean, to keep them at reasonable temperatures. No other phenomenon can play this role."

Frequency

How climate change will influence the frequency of hurricanes is less well understood. Some experts predict a potential drop in overall numbers. In rebalancing Earth's heat, Trenberth says, "one big hurricane can play the role of four smaller hurricanes." Tom Knutson, a climate scientist at the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory, says nearly all models predict this effect-theyshow a decrease in the total number of hurricanes in a warmer climate. Elsner notes, however, "There's still a lot of uncertainty on this point."

Both Trenberth and Knutson think the number of very intense storms may actually grow. Some studies show that globally, "we end up with more storms at the high end-at category 4 and 5," Trenberth says. The north Atlantic, in particular, may experience this trend.Elsner is more hesitant; he says the frequency of the strongest hurricanes is also controlled by the occurrence of ideal conditions-no wind shear high in the atmosphere to tear storms apart from above, no land in their path to break them up from below and no dry air to absorb some of their moisture. "It is unknown how or if these conditions will change," he says.

Size and Duration

Scientists are still unsure about how climate change will influence the physical breadth and duration of hurricanes. In their modeling Knutson and his team have found that the number of category 4 and 5 storm days could potentially increase slightly by the late 21st century. Trenberth thinks higher ocean temperatures may mean storms will become both bigger in size and last longer. He does note, though, that these traits depend on how storm details are defined: For a storm's breadth across the ocean, for example, where are the boundaries drawn? Does "duration" apply to its time as a specific hurricane category (4, 5) or as an overall tropical storm?

Storm Surge

Scientists do agree climate change means higher storm surges are hitting coastlines. This would happen even if hurricanes do not become stronger. "Once you have a higher baseline sea level," Knutson says, "that's going to add to the water level experienced during storm surges." If sea level is a half-meter higher, for example, then a storm surge will be a half-meter higher than it would have been otherwise.

Rainfall

Experts also expect that climate change may increase the intensity of hurricane rainfall. The unprecedented, deep flooding in the Houston area certainly bears witness to that idea. Warmer air holds more water vapor. For hurricanes, "that can lead to more efficiency; the rate at which rain falls out of the clouds increases," Elsner says. "We're seeing that in some of these storms." Knutson gives a number for this phenomenon: Hurricane rainfall rate is projected to rise 7 percent for every degree Celsius rise in tropical sea surface temperatures.

Expanding Reach

Regions of the world that have not experienced hurricanes may in the future. As oceans heat up, the cyclonic storms' territory could enlarge. "If a storm remains over warm water, it can maintain a high intensity," Elsner says. "If those warm waters are expanding, then you can find these strong storms in [new] places." Trenberth agrees: "Witness Ophelia," which surprised Ireland and the U.K. in October.

Even though scientists are investigating these potentially changing factors, they caution that uncertainty remains for many of them. Hurricanes are particularly challenging because they are such complex and relatively rare events. "Not only is there large variability but the reliable record is short-satellite [tracking] began about 1970," Trenberth says. Plus, many other forces ( pdf)-such as weak wind shear and low pressure at the sea surface-influence this type of storm.

Knutson is less certain than others that it is possible to already see global warming's influence on any of these factors in the record (except for higher storm surge due to sea level rise). It is too early "to say that we can detect this change already in the data, and it's clearly distinct from natural variability," Knutson says. "That limits our confidence in future projections."

But Elsner says the growing intensity of hurricanes is already evident in the record. "It has already been shown that the strongest hurricanes are getting stronger worldwide," Elsner wrote in an e-mail to Scientific American. Trenberth goes further. "The environment in which all such storms occur is warmer and moister, and we know that this has effects," he notes. "The evidence is that climate change is already with us."

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Was the Extreme 2017 Hurricane Season Driven by Climate Change?

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Summer and fall 2017 saw an unusual string of record-breaking hurricanes pummel the U.S. Gulf Coast, eastern seaboard, Puerto Rico and the Caribbean. Hurricane Harvey brought unbelievable floods to Houston. Irma, one of the two strongest hurricanes ever recorded in the northern Atlantic, wreaked havoc on Florida and many Caribbean islands.

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Story Highlights

The 2017 Atlantic hurricane season had numerous deadly and record-breaking tropical cyclones.

Hurricanes Harvey, Irma and Maria changed parts of Texas, Florida and the Caribbean forever.

September was the busiest month of hurricane activity on record.

The 2017 Atlantic hurricane season will long be remembered as one of the busiest and most destructive hurricane seasons on record.

( MORE: Hurricane Central)

Seventeen named storms, 10 hurricanes, and 6 major (Category 3 or stronger) hurricanes tore through the Atlantic Basin, well above the 30-year average of 12 storms, 6 hurricanes and 2 major hurricanes. This placed 2017 among the top 10 most active Atlantic seasons on record, according to Dr. Phil Klotzbach, tropical scientist at Colorado State University.

Five named storms and one tropical depression had already formed by the end of July with little fanfare or destruction, but that rapidly changed with ten consecutive hurricanes, most of which were in the news.

Hurricanes Harvey, Irma and Maria all have damage estimates in the tens of billions of dollars, if not more. One island was nearly made uninhabitable, two locations received more than 60 inches of rain, and three Category 4 hurricanes made landfall in the United States following twelve years without a major hurricane landfall in America.

Below is a look at what we will remember most about this devastating season.

1. Maria's Toll on Puerto Rico

Maria's Category 4 landfall on the island territory was a worst-case scenario for Puerto Rico. The hurricane knocked out power to more than 90 percent of the island, and largely disrupted cell and water service. At least 51 people were killed by Maria in Puerto Rico alone, although the actual number may be much higher.

Less than eight percent of Puerto Rico's roads were open and usable a month following Maria's passage over the island.

Puerto Rico may take years to fully recover from Maria's devastation.

2. Maria and Irma's Impact on the Virgin Islands

Hurricane Irma made two direct landfalls in the British Virgin Islands (BVI), both at peak intensity; one on Tortola and another on Ginger Island.

Damage in the BVI was extensive, and on some islands it was catastrophic. Many buildings and roads were left in ruins.

Winds gusted to more than 85 mph and powerful rainbands dropped more than a foot of rain in St. Thomas in the U.S. Virgin Islands (USVI) due to Irma's southern and western quadrants. Widespread structural damage was reported across the USVI. Many trees were heavily damaged or uprooted.

The island of St. John was also decimated and could be home to Irma's worst destruction on American soil, CBS News reports.

Irma delivered the first powerful punch to the electrical grid in both the United States and the British Virgin Islands.
(MORE: Irma's Deadly Impact in the Virgin Islands)
Hurricane Maria delivered the second punch to electricity. Some 80 percent of the USVI remained without power more than a month after Maria battered the U.S. territory, according to Buzzfeed News.

Numerous ports were closed following the pair of hurricanes due to structural damage, but have since reopened.

Hurricane Maria's eyewall passed near St Croix and damaged most structures on the island.
(MORE: 10 of the Most Dramatic Weather Images From Hurricane Maria)
While tropical storm watches were issued for the Virgin Islands due to the threat of Jose, the hurricane stayed far enough away to spare the island chain.

3. Hurricanes Harvey, Irma and Maria End U.S. Major Hurricane Drought

After a 12-year lull in U.S. major hurricane ( Category 3 or stronger) landfalls with sustained winds of at least 111 mph, Hurricane Harvey slammed into the central Texas Coast with winds of 130 mph near Rockport, Texas, on Aug. 25. The most recent major hurricane landfall prior to Harvey was in 2005 when Hurricane Wilma made landfall in southwest Florida.

Just when we were hoping to restart another long streak, Hurricane Irma made landfall a few weeks later in the Florida Keys, also as a Category 4 hurricane with winds of 130 mph. Irma made another major hurricane landfall a few hours later in southwest Florida with winds of 115 mph. Before arriving in Florida, Irma's eyewall also affected the U.S. Virgin Islands when it was a Category 5.

Reset the time clock and here comes Maria just over a week later. Maria made a stunning third American Category 4 landfall on the southeast shores of Puerto Rico with winds of 155 mph.
(MORE: Three Category 4 Hurricanes Have Made a U.S. Landfall in 2017)
4. Harvey's Record-smashing Rainfall

According to the NWS in Lake Charles, Louisiana, two gauges in southeastern Texas reported more than 60 inches of rain between Aug. 24 and Sep. 1 as Harvey trudged through the region. This is more than a foot higher than the previous record heaviest storm-total rainfall from any tropical cyclone in the U.S. in records dating back to 1950.

The areal extent to which the rainfall fell in such quantities was also impressive. The chunk of real estate where at least five inches of rain fell would cover most of the state of Florida or stretch from Boston to Washington, D.C. if the rain had fallen there instead.

( MORE: Normalcy Hard to Find Over Two Months After Harvey)

5. Harvey's Duration Over the U.S. as a Tropical Cyclone

Following landfall near Rockport, Texas, Harvey relentlessly swirled over portions of Texas, Louisiana and the Tennessee and Ohio Valleys for the better part of seven days. Not counting the time that Harvey spent over the Gulf of Mexico after its first landfall, the system spent 117 hours as a tropical cyclone over land, all the while dropping the tremendous amount of rainfall mentioned above.

6. Irma's Brute Strength Was Not Something We Have Seen Before

Hurricane Irma held on to Category 5 status for three consecutive days in the Atlantic, but even more impressive was that it held on to its peak intensity - 185 mph - for 37 hours which set a world record. Hurricanes of this intensity often undergo fluctuations in intensity, but Irma did not.

Irma's peak intensity based on wind speed was the second highest in Atlantic basin history only behind Hurricane Allen in 1980, which had winds of 190 mph.

Outside of winds, Irma also contained the 10th lowest pressure of any hurricane in the basin. At its peak, the hurricane's pressure bottomed out at 914 millibars.

7. Irma Depopulated an Entire Island

Hurricane Irma's direct strike on the island of Barbuda in the eastern Caribbean brought sustained winds of 185 mph with higher gusts, torrential rain and destructive waves. At least 95 percent of the island's structures - including hospitals, schools, homes and docks - were damaged or destroyed.

Following the catastrophic damage unleashed on the island, the government of Antigua and Barbuda forced a mandatory evacuation of Barbuda, with residents being brought to Antigua. Few residents have made the return trip to Barbuda even months after the evacuation order was lifted due to insecurity on the island, the inability to build new shelter, and local politics.

8. September was the Most Active Month on Record, with the Help of Ten Consecutive Hurricanes

September 2017, featuring Category 5 Hurricanes Irma and Maria and Category 4 Hurricane Jose, was the most active month of any Atlantic hurricane season on record in terms of Accumulated Cyclone Energy (ACE).

Generally long-lived, intense hurricanes have a high ACE index, while short-lived, weak tropical storms have a low value. The ACE of a month is the sum of the ACE for each storm and takes into account the number, strength and duration of all the tropical storms and hurricanes in the month.

September generated more ACE than any other calendar month on record, surpassing September 2004, according to Colorado State tropical meteorologist Dr. Phil Klotzbach.

This was about three and a half times the average activity from 1981-2010 in the Atlantic Basin, according to the National Hurricane Center.

Included in September 2017's ACE calculation were Irma, Jose, Katia, Lee and Maria. All of those except Katia were long-lived hurricanes. Lee and Katia were the only ones to not reach Category 4 or Category 5 intensity at their peaks.

By the time mid-October rolled around, Ophelia became the 10th consecutive Atlantic named storm to become a hurricane in 2017.

That tied the record for the most consecutive Atlantic named storms reaching hurricane strength, which also happened in three other years: 1878, 1886 and 1893, according to Dr. Phil Klotzbach.

9. Tropical-Storm-Force Winds Felt in Most of the Coastal United States and the Caribbean

More than 95 percent of the U.S. Gulf Coast was affected by at least tropical-storm-force winds, or winds of at least 39 mph, this hurricane season.

The numbers are slightly better along the East Coast. More than half of the East Coast saw sustained tropical-storm-force winds with higher gusts, mainly due to Hurricane Irma's large wind field and Jose's sideswipe of the Northeast and mid-Atlantic coasts.

In the Caribbean, nearly all of the Antilles saw sustained tropical-storm-force winds. Only a few islands completely escaped wind impacts from a tropical cyclone this year: Jamaica, the Cayman Islands, the ABC islands and the Grenadines.

Central America was far safer this hurricane season with respect to winds. Mexico bore the brunt of two hurricanes this year, while Nicaragua and Honduras dealt with Nate's early phase.

When we begin to talk about overall impacts in Central America, though, their luck runs out. An 800-plus mile wide area of rotation, the source of Nate's birth, brought very heavy rainfall to a large swath of Central America over a period of a few weeks, resulting in flooding. Earlier in the season, Franklin drenched parts of Mexico and even helped spawn a new tropical storm in the Pacific.

Even Ireland and the United Kingdom picked up tropical-storm-force winds from Ophelia's post-tropical cyclone phase.

10. Irma's Many Landfalls

Hurricane Irma made more than a half dozen landfalls along its devastating path. The list below shows the intensity of Irma at the time of each landfall.

Barbuda: Category 5, 185 mph

St. Martin: Category 5, 185 mph

BVI (2 landfalls): Category 5, 185 mph

Little Inagua, Bahamas: Category 5, 160 mph

Northern Cuba (possibly a few landfalls): Category 3-4, 125-160 mph

Florida Keys: Category 4, 130 mph

SW Florida: Category 3, 115 mph

11. Hurricane Maria Obliterates Puerto Rico's Radar

Maria's direct hit on the island territory took its full force out on the island's main radar. Wind gusts up to 160 mph battered the radar's protective dome and tore it away from the pedestal.

Perhaps more shockingly, the 30-foot wide radar dish went missing following the storm, likely blown off of its pedestal that is left behind. A second radar on the island was not as badly damaged and is now back up and running.

While it may take months to replace the main radar, two U.S. Marine Corps mobile radar units have been temporarily deployed to Puerto Rico while the Federal Aviation Administration radar is being assessed and repaired.
(MORE: The Latest Radar Situation in the U.S. Territories and How Long it Took to Find the Radome)
12. Ophelia Smacks Ireland and Britain

While not as impactful as a tropical cyclone, after Ophelia turned into a post-tropical cyclone it brought damaging high winds to parts of Ireland and the United Kingdom.

Numerous locations in Ireland, Wales and Scotland reported wind gusts over 75 mph, and one location in Ireland reported a gust of 119 mph.

Three people died in what was called Storm Ophelia in the U.K. while hundreds of thousands of customers lost power in Ireland alone.
(MORE: Ophelia's European Impacts)
Before Ophelia's post-tropical cyclone hammered Ireland and the U.K., it became the farthest east Category 3 hurricane on record for the Atlantic Basin.

13. Two Hurricanes Make Landfall Within 55 Miles of Each Other in Eastern Mexico

Hurricanes Franklin and Katia both made landfall in 2017 between Tampico and Veracruz, both in the state of Veracruz.

Katia made landfall as a weakening Category 1 with winds of 75 mph and Franklin also made landfall as a Category 1 hurricane, but with 85 mph winds.

In Franklin's final landfall, the hurricane brought heavy rainfall to eastern Mexico and soaked Mexico's interior before spinning off a new tropical storm in the Pacific. Damage in the Veracruz state was largely minimal, but sporadic power outages and a few landslides were reported.

Katia was more damaging in Mexico's eastern states. Many landslides were reported and 77,000 people lost power during the hurricane. Katia made landfall just days after a major 8.2 magnitude earthquake rattled the southern part of the country, and Katia's rain helped move loose ground over central and southern Mexico.

14. Rapid Intensification Keeps Us on Edge

Rapid intensification is noted by the National Hurricane Center as being one of the most difficult processes to forecast and understand in tropical forecasting. The highest errors in forecasting are often associated with phases of rapid changes in hurricanes.

The 2017 hurricane season produced six tropical cyclones that developed in ideal environmental conditions allowing rapid intensification to occur. This is defined as an increase of wind speeds by 35 mph in 24 hours.

Harvey: 35 mph to 130 mph; Late Aug. 23 - Aug. 25

Jose: 90 mph to 150 mph; Midday Sep. 7 - midday Sep. 8 [Explosive intensification]

Katia: 35 mph to 80 mph; Late Sep. 5 - late Sep. 6

Lee: 50 mph to 90 mph; Late Sep. 23 - late Sep. 24

Maria: 50 mph - 160 mph; Early Sep. 17 - evening Sep. 18 [Explosive intensification]

15. Tropical Systems Produce Most U.S. Tornadoes in Nearly a Decade

Five tropical cyclones were responsible for 119 reports of tornadoes in a dozen states, according to preliminary data provided by NOAA's Storm Prediction Center (SPC). This includes two tropical storms (Cindy and Philippe) and three hurricanes (Harvey, Irma and Nate).

Hurricane Harvey had the largest number of tornado reports with 57, mostly clustered near the southeastern Texas coast. Tornadoes were also spawned by Harvey as it weakened to a tropical storm and a tropical depression in Louisiana, Mississippi, Alabama and Tennessee.

The 2017 hurricane season produced the largest number of reported tornadoes spawned by tropical storms and hurricanes in the continental United States since 2008, and the fourth-most overall in 23 years.
(MORE: Tornadoes This Hurricane Season)
16. Hurricane Irma Pushes Water Out of Tampa Bay While Slamming the East Coast with Storm Surge

As Hurricane Irma entered the state through the Florida Keys, residents of Tampa Bay noted something peculiar: The bay was emptying.

Three to six feet of water was pushed out of Tampa Bay by northeast winds ahead of Irma's eyewall, surprising residents on Tampa's famous Bayshore Boulevard, a street that typically floods in hurricanes, heavy rain events and even strong cold fronts.

This go-around, however, residents were able to precariously walk out onto the dry seabeds of Tampa Bay. These residents had to rescue manatees on the muddy seabed after they were also surprised by the disappearance of Tampa Bay.
(MORE: Water in Tampa Bay Retreats from Tampa Bay During Irma)
On the flip side, numerous feet of storm surge piled up along much of Florida's Atlantic shoreline and surprised residents well inland on the First Coast.

A storm surge at Virgina Key, in Biscayne Bay, was close to 4 feet above normal tides while the middle keys experienced a storm surge of more than 10 feet. A storm surge of 2 to 4 feet was recorded in northeast Florida, including along much of the St. John's River near and south of Jacksonville well inland from the Atlantic.

More than 26 river gauges reached or exceeded major flood stage across northern Florida - and Jacksonville's St. Johns River reached its highest level ever. Gauges remained higher than normal for weeks following Hurricane Irma due to the river's slow movement.

Irma's large wind field also forced water inland along the coasts of Georgia and South Carolina. The combination of extremely high tides, storm surge and heavy rain resulted in dangerous flooding in Charleston, South Carolina, where inundation exceeded the levels brought by Hurricane Matthew just a year earlier.

Record flooding was also observed at the St. Simons Island Pier in Georgia due to storm surge and heavy rainfall.

17. Arlene Starts the Season Early

Arlene's early formation on Apr. 20 was not only more than a month before the beginning of hurricane season, but was also the second earliest-forming tropical cyclone in the Atlantic in the satellite era (or since 1966).

This was the third consecutive year a system has formed in the Atlantic before the official June 1 start of the hurricane season.
(MORE: Tropical Storm Arlene Recap)
On a long-term average, a tropical system forms prior to June about once every 10 years, and these storms tend to be relatively weak, due in part to cooler sea-surface temperatures.

There has been a recent trend in early starts to the Atlantic hurricane season, with 2012, 2015 and 2016 all reporting tropical cyclone formation before June 1.

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2017 Atlantic Hurricane Season Recap: 17 Moments We'll Never Forget

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The 2017 Atlantic hurricane season will long be remembered as one of the busiest and most destructive hurricane seasons on record. ( MORE: Hurricane Central ) Seventeen named storms, 10 hurricanes, and 6 major (Category 3 or stronger) hurricanes tore through the Atlantic Basin, well above the 30-year average of 12 storms, 6 hurricanes and 2 major hurricanes.

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This is a top 10 list no one wants to be on.

The three big hurricanes of 2017 - Harvey, Irma and Maria - are now three of the five costliest hurricanes in U.S. history, the National Oceanic and Atmospheric Administration (NOAA) announced this week.

The storms brought widespread death and destruction to Texas, Florida, Puerto Rico and the U.S. Virgin Islands.

While 2005's Katrina remains the costliest hurricane on record at $160 billion, last year's Hurricane Harvey ranks second, with damage costs of $125 billion.

Hurricane Maria ranks third at $90 billion, and Irma ranks fifth at $50 billion. With damages of $70 billion, 2012's Sandy has been pushed down to fourth place. Sandy was popularly labeled a "superstorm" as it neared landfall despite being a hurricane for nearly its entire life cycle.

(On this list, the damage costs of both Katrina and Sandy are adjusted for inflation.)

NOAA said the dollar amounts are "the estimated total costs of these events - that is, the costs in terms of dollars that would not have been incurred had the event not taken place. Insured and uninsured losses are included in damage estimates."

In all, 2017's hurricanes caused more than a quarter-trillion dollars in insured and uninsured losses, the National Hurricane Center reported.

That makes 2017 the costliest hurricane season in U.S. history, even topping the disastrous 2005 season, which included Rita and Wilma in addition to Katrina.

September was the true behemoth, the single-most powerful month ever recorded in the Atlantic in terms of hurricane intensity.

In a statistic known as "Accumulated Cyclone Energy," Hurricanes Irma, Jose, Katia, Lee and Maria that month generated the largest amount of energy for any month on record, according to Colorado State University meteorologist Phil Klotzbach.

Both Irma and Maria soared to Category 5 strength, the top of the Saffir-Simpson scale of hurricane intensity. Irma's landfall on Barbuda and Maria's landfall on Dominica makes 2017 only the second season on record to feature two hurricanes that hit at Category 5 strength.

Other records from the season included the 60.58 inches of rain that fell near Nederland, Texas, during Harvey. This broke the record for the greatest rainfall amount ever recorded in the 48 contiguous states from a single storm, WeatherBug reported.

Meanwhile, Irma was the strongest storm on record in the Atlantic - excluding the Caribbean and Gulf of Mexico - with maximum winds of 185 mph and an unofficial wind gust of 199 mph.

And Maria was the worst natural disaster in Puerto Rico's history, leaving potentially hundreds dead.

The 2017 Atlantic hurricane season was also the first season since records began in 1851 to have two Category 4 hurricanes make continental U.S. landfall in the same year (Harvey and Irma), according to Klotzbach.

Bumped out of the top 10 costliest hurricane list were Charley (2004), Irene (2011) and Hugo (1989), the Weather Channel reported.

No hurricanes will ever have the names Harvey, Irma and Maria again, as they will all certainly be retired by the World Meteorological Organization due to the storms' ferocity and impact.

That official designation will come at the organization's meeting in April, the hurricane center said.

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2017's three monster hurricanes - Harvey, Irma and Maria - among five costliest ever

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CLOSE This is a top 10 list no one wants to be on. The three big hurricanes of 2017 - Harvey, Irma and Maria - are now three of the five costliest hurricanes in U.S. history, the National Oceanic and Atmospheric Administration (NOAA) announced this week.

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National Geographic: What Causes Hurricanes?

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National Geographic: What Causes Hurricanes?

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Every year, coastal regions brace themselves for violent windstorms known as hurricanes. But how do these storms form and grow?

The oversimplified answer: Warm ocean water plus the Earth's eastward rotation.

"They're heat engines," said meteorologist Jeff Masters of the website Weather Underground in a previous interview. "They take heat from the oceans and convert it to the energy of their winds. They're taking thermal energy and making mechanical energy out of it."

The natural engine that is a hurricane is fueled by warm, moist air. The storms move heat from the ocean surface high into Earth's atmosphere. They can travel thousands of miles from the tropics toward the Earth's poles.

According to NOAA's National Hurricane Center, the average hurricane eye-the still center where pressure is lowest and air temperature is highest-stretches 20 to 30 miles across, with some even growing as large as 120 miles wide.

The strongest storms, equivalent to Category 5 on the Saffir-Simpson scale, have sustained winds that exceed 155 miles an hour.

Why Are Hurricanes Dangerous?

While hurricanes are categorized based on their wind speeds, wind isn't typically the most dangerous part of such storms. "It's the storm surge," said Kerry Emanuel, an atmospheric scientist at MIT, in an earlier interview. The storm surge is the bulge of water built up in front of a cyclone or hurricane courtesy of its winds.

It's the number one killer in hurricanes, Emanuel explained. "That's what killed people in Katrina, it's what killed people in Sandy and in Haiyan." (Read "Charts Show How Hurricane Katrina Changed New Orleans.")

Emanuel likened a storm surge to a tsunami. One just happens to be caused by earthquakes (tsunamis), while the other is generated by hurricanes.

Flash flooding caused by intense rains is also a major killer, Emanuel said. " Hurricane Mitch [in 1998] killed 12,000 people and it was all from flash flooding."

Then comes wind that blows around debris. Hurricane Andrew in 1992 is an example of this. "It didn't really cause too much of a storm surge," the atmospheric scientist said, "but boy did it blow a lot of buildings down."

Climate change will likely increase the frequency of "the high-end hurricanes," Emanuel said.

And those powerful storms have the potential to produce a lot of rain, flooding, and strong storm surges.

Is That a Hurricane or a Cyclone?

What's the difference between hurricanes, cyclones, and typhoons? Actually, they're all the same weather phenomenon. Scientists just call these storms different things depending on where they occur.

In the Atlantic and northern Pacific, the storms are called hurricanes, after the Caribbean god of evil, named Hurrican.

In the northwestern Pacific, the same powerful storms are called typhoons. In the southeastern Indian Ocean and southwestern Pacific, they are called severe tropical cyclones.

In the northern Indian Ocean, they're called severe cyclonic storms. In the southwestern Indian Ocean, they're just called tropical cyclones.

To be classified as a hurricane, typhoon, or cyclone, a storm must reach wind speeds of at least 74 miles an hour.

If a hurricane's winds reach speeds of 111 miles an hour, it is upgraded to an "intense hurricane."

If a typhoon hits 150 miles an hour-as Usagi did in 2013-then it becomes a "supertyphoon."

While the Atlantic hurricane season runs from June 1 through November 30, the typhoon and cyclone seasons follow slightly different patterns.

In the northeastern Pacific, the official season runs from May 15 to November 30. In the northwestern Pacific, typhoons are most common from late June through December. And the northern Indian Ocean sees cyclones from April to December.

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What Are Hurricanes, and How Do They Form?

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Every year, coastal regions brace themselves for violent windstorms known as hurricanes. But how do these storms form and grow? The oversimplified answer: Warm ocean water plus the Earth's eastward rotation. "They're heat engines," said meteorologist Jeff Masters of the website Weather Underground in a previous interview.

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The cause of most California wildfires? People

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The firefighters were assigned to a water tender by the California Governor's Office of Emergency Services and headed to the 12,353-acre wildfire at about 6:20 a.m., when the rollover crash occurred on the westbound California 246, said Capt. Mike Lindbery, spokesman for the Ventura County Fire Department.

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Wildfire Causes and Evaluations (U.S. National Park Service)

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Learn about the causes of wildland fires and how managers decide how to deal with wildland fires.

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Here's a look at the records obliterated by the 2017 wildfires:

The costliest in US history

Damage estimates from the 2017 wildfires topped $10 billion, said Adam Smith of the National Oceanic and Atmospheric Administration. And that was before devastating wildfires tore through Southern California in December.

The most acres burned in California in the last 5 years

For the past five years, wildfires scorched an average of 202,751 acres from January 1 through December 24, according to state fire protection agency Cal Fire. That number doubled in 2017. More than 505,900 acres have gone up in flames so far in California. That's triple the area of Chicago.

The worst for air pollution in the Bay Area

The October wildfires in Northern California caused "unprecedented levels of air pollution throughout the Bay Area," air quality officials said. To put things in perspective, air quality is considered "very unhealthy" when the index reaches 201. At one point, the air quality index in parts of Napa reached 486.

The biggest in modern California history

The Thomas Fire started on December 4 and hasn't stopped since. As of Tuesday, it's destroyed more than 281,000 acres and was 88% contained. This Southern California inferno is the biggest wildfire in California since it least 1932.The Thomas Fire is the seventh most destructive, with at least 1,063 homes, businesses and other structures destroyed. That number is sure to grow, since fire officials said the fire might not be fully controlled until January 7.

CNN's Judson Jones, Brandon Miller and Doug Criss contributed to this report.

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the wildfires in california in 2017 just keep shattering records - cnn

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The wildfires in California just keep shattering records this year

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Here's a look at the records obliterated by the 2017 wildfires: The costliest in US history Damage estimates from the 2017 wildfires topped $10 billion, said Adam Smith of the National Oceanic and Atmospheric Administration. And that was before devastating wildfires tore through Southern California in December.

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© 1996-2015 National Geographic Society. © 2015-2016 National Geographic Partners, LLC. All rights reserved. Learn about our nonprofit work at NationalGeographic.org

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Learn more about the causes and effects of natural disasters.