Tornadoes are the most violent of the storms the atmosphere produces, according to the National Severe Storms Laboratory (NSSL) in Norman, Okla.
A tornado is defined by the NSSL as "a narrow, violently rotating column of air that extends from the base of a thunderstorm to the ground."
Tornadoes have been observed in all 50 U.S. states.
Unlike for hurricanes, there is no defined tornado season and no official tornado season forecast.
The main time for tornado activity is during the spring, generally picking up in March and then hitting a peak between May and June.
The earliest tornado in the year in the modern record occurred at 12:02 a.m. CST on Jan. 1, 2011, in Attala County, Miss.
The latest that the first tornado of the year has ever been recorded was on Feb. 15, 2003, in Marengo County, Ala.
May is historically the most active month for tornadoes. The record holder for any May is May 2003, with 543 confirmed tornadoes.
The most active tornado month on record is April 2011, which had 758 tornadoes, according to National Oceanic and Atmospheric Administration records.
No direct measurements have been made of the winds in the eye of a tornado because the instruments that measure wind can't survive long enough.
It is thought that winds in the most violent tornadoes can reach speeds as high as 300 mph (480 km/h), according to the NSSL.
The strength of tornadoes is measured on the Enhanced Fujita scale, which rates tornadoes based on the damage they cause. The ratings on the scale, which run from EF-0 to EF-5, correspond roughly to the strength of the winds seen in the twister. The original Fujita scale was developed by meteorologist Ted Fujita and attempted to relate tornado damage to the wind speeds that caused the damage, as the present EF scale does, but it didn't take into account the fact that winds affect different structures in different ways. The new scale takes these factors into account and rates a tornado based on the most intense damage it caused over its whole path.
Tornadoes can be on the ground from anywhere between a brief moment to several hours. The NSSL says the average time on the ground is about five minutes.
On average, about 60 people are killed by tornadoes each year, but the exact year-to-year numbers vary widely, according to the Storm Prediction Center.
The deadliest single tornado was the Tri-State Tornado, which hit Missouri, Illinois and Indiana on March 18, 1925, and killed 695 people.
The deadliest tornado in the modern era (from 1950 onward) was the EF5 that struck Joplin, Mo., on May 22, 2011, killing 158 people. It ranks as the 7th deadliest tornado in recorded U.S. history.
The May 22, 2011, Joplin, Mo., tornado was also the costliest on record, causing an estimated $2.8 billion in damage (in 2011 dollars).
1925 is the deadliest tornado year on record in the United States, with 794 people killed by twisters. Nearly 700 were killed on March 18 of that year by the so-called Tri-State Tornado, which virtually wiped several towns off the map.
The deadliest day for tornadoes was also March 18, 1925, with a reported 747 people killed.
Since 1875, only six other years have seen more than 500 tornado-related deaths, according to data from the NSSL. (They are 1896, 1917, 1925, 1927, 1936 and 2011.)
The biggest tornado outbreak for the modern era was the April 27-28, 2011, outbreak across the Southeast, according to the definitions used by the Storm Prediction Center. That outbreak saw 59 EF0s, 65 EF1s, 20 EF2s, 16 EF3s, 11 EF4s and four EF5s, for a total of 175 tornadoes in a single 24-hour period. It killed more than 300 people and caused billions of dollars in damage. The 2011 outbreak took the place of the previous record holder, the so-called Super Outbreak of April 3-4, 1974, which produced 147 known tornadoes.
The 1974 "Super Outbreak" still retains the record for the most F5s (as the Fujita scale was still in use then) in a single day: seven.
The widest tornado on record was an EF5 that struck near El Reno, Okla., on May 31, 2013. At its peak, the tornado had a width of 2.6 miles (4 kilometers), according to the National Weather Service's Norman, Okla., office. This tornado replaced the previous record-holder, an F4 that struck Hallam, Neb., on May 22, 2004, and that was 2.5 miles wide.
No one knows what the strongest tornado on record is, according to the SPC, because ground-level wind speeds have never been measured in the strongest tornadoes.
With tornadoes, size is not necessarily an indication of strength.
Oklahoma City is the city that has been hit by the most tornadoes based on existing records, according to the Storm Prediction Center. Factoring in changes to the city limits over time and changes to tornado reporting, the city has been hit by more than 100 tornadoes.
Tornado Alley is not a technically defined area, but is the nickname for the region in the central part of the United States that typically sees the most tornado activity. Tornadoes do happen outside this region.
The highest twister ever recorded was photographed by a hiker at 12,000 feet (3,658 meters) in California's Sequoia National Park on July 7, 2004.
On July 21, 1987, there was an EF-4 tornado in Wyoming between 8,500 and 10,000 feet in elevation, the highest altitude ever recorded for a violent tornado, according to the National Weather Service.
While the United States is essentially the tornado capital of the world, tornadoes can happen anywhere where conditions are right and they have been observed around the world.
Secondary tornado-prone areas include Canada's prairie provinces, northeastern Mexico, northern Argentina, southern Brazil, Britain, Bangladesh and parts of southern Russia, according to the Storm Prediction Center.
The first recorded F5 in Canada occurred on June 22, 2007, near Elie, Manitoba.
Tornadoes in the Northern Hemisphere tend to spin counterclockwise, though some spin clockwise.
A so-called "second tornado season" comes in November in the United States, as fall is a transitional weather time like spring, when warm and cold air masses collide and can create the conditions for tornado formation.
November tornadoes are more common in the lower Mississippi Valley, where the warm air from the Gulf can easily reach.
Nighttime tornadoes are about twice as likely to kill people as those that occur in the daytime, mostly because people tend to be asleep and not hear sirens and weather radios or be located in unsafe structures like mobile homes.
A tornado watch means that weather in a particular area could produce tornadoes (often this means thunderstorms are in the area). But it doesn't mean tornadoes will definitely occur.
A tornado warning means that a tornado has been spotted on the ground in the area or that the storm circulation seen on Doppler radar could produce a tornado.
The first successful tornado forecast was made on March 25, 1948, when a tornado hit Tinker Air Force Base in Oklahoma just a few days after a previous tornado hit the base. Then Air Force Capt. (later Col.) Robert Miller and Major Ernest Fawbush noticed similarities between the two weather patterns (as well as other systems that had produced tornadoes), and alerted their superiors to the possibility of another tornado hit, which then happened just a few hours later.
Before 1950, the Weather Bureau (the precursor to the National Weather Service) at times banned or discouraged the use of the word "tornado" in forecasts for fear or causing a panic. The ban was revoked in 1950.
A supercell storm is a rotating thunderstorm with a well-defined circulation called a mesocyclone (the parent updraft circulation for a tornado). One of the most well-known features of a supercell is its anvil cloud, which is formed when the updraft (or upward moving air) hits the top of the troposphere (the lowermost layer of the atmosphere) and spreads out because it has nowhere else to go.
A gustnado is a small whirlwind that does not form from a so-called supercell thunderstorm and isn't a tornado because it doesn't connect to any rotation in the clouds above. It forms along the gust front of a storm, with a whirl of dust and debris that can cause minor damage.
'Landspout' is a term used by stormchasers to mean a tornado that didn't form from a supercell. They are typically smaller and weaker than supercell tornadoes.
A waterspout is basically a tornado over water. Like landspouts, they typically don't form from supercell tornadoes. They are most common around the U.S. Southeast and aren't counted in official tornado statistics unless they hit land, according to the Storm Prediction Center.
Wedge tornado is another storm observer slang term used to describe a particular shape and means that the tornado is at least as wide as it is tall, according to the Storm Prediction Center.
Rope tornado is yet another observer slang term that is used for narrow, sinuous tornadoes. Many tornadoes take this shape in their last stage of life, according to the Storm Prediction Center.
Hurricanes and tropical storms can produce tornadoes (including waterspouts). Tornadoes in tropical cyclones (the encompassing term for hurricanes and tropical storms) tend to occur in supercells in the outer bands of the cyclone. Whether or not a cyclone produces a tornado doesn't seem to have any relation to the cyclone's size or intensity, the Storm Prediction Center said.
The deadliest tornado spawned by a tropical cyclone was one spawned by Hurricane Hilda in October 1964, according to NOAA's Hurricane Research Division. It killed 22 people in Larose, La.