A succession of tornadoes ripped through Alabama's Lee County on Sunday with winds of 150 miles per hour (241 kph), killing at least 23 people including children in the deadliest such storms to strike the United States in almost six years.
What is a tornado?
Tornadoes, described as a violently rotating column of air touching the ground, are nature's most violent storms, capable of killing people and wiping out neighborhoods in a matter of seconds, according to the U.S. National Weather Service.
Tornadoes occur when dry cold air moving south from Canada meets warm moist air traveling north from the Gulf of Mexico, according to the University Corporation for Atmospheric Research.
They can strike with little warning, developing and dissipating quickly, the weather service said. Most tornadoes are on the ground for less than 15 minutes.
Can tornadoes be predicted?
?Meteorologists at the weather service's Storm Prediction Center can forecast general conditions that are favorable for tornadoes and will issue a severe thunderstorm or tornado watch that lasts four to six hours.
But a tornado warning is not issued until a tornado has been sighted or indicated by weather radar, the prediction center says.
Where and when do tornadoes form?
Most tornadoes are found in the Great Plains of the central United States, also known as Tornado Alley, according to the University Corporation for Atmospheric Research.
Tornadoes can form at any time of year, but most occur in the spring and summer months along with thunderstorms. May and June are usually the peak months for tornadoes.
Do tornadoes occur outside the United States
Tornadoes occur most frequently in the United States, where there is an average of more than 1,000 recorded each year, and Canada is a distant second with around 100 per year, according to the National Centers for Environmental Information.
But tornadoes can occur anywhere and have been documented on every continent except Antarctica and in all 50 states.
How are tornadoes measured?
The National Weather Service measures tornadoes on the Enhanced Fujita (EF) Scale ranging from 0 to 5. At the low end, an EF rating of 0 measures three-second gusts of wind from 65 to 85 mph (105 to 138 kph). At the high end, an EF rating of 5 measures gusts over 200 mph (322 kph). The Alabama tornadoes reached 150 mph (241 kph) for an EF rating of 3.
Does climate change make tornadoes more deadly?
Warmer oceans can create the conditions for more tornadoes, climate scientist Kevin Trenberth told Scientific American, and may have be responsible an increase in tornado damage.
But the link between tornadoes and climate change is unclear, according to the Center for Climate and Energy Solutions.
U.S. tornado records only date back to 1950, and much of Tornado Ally was so sparsely populated before then that tornadoes may have occurred without anyone seeing them. Improved technology enables scientists to detect tornadoes now that may not have been noticed decades ago.