What causes tornadoes? In Tornado Alley, where a tornado killed at least 24 people in Moore, Okla., Monday, the disasters are relatively frequent when moist air from the Gulf of Mexico meets dry, cool air from the Rocky Mountains.
Instability caused by such differences in air temperatures fuels even common thunderstorms. But in the case of the Moore tornado, the severity was increased because of a particularly strong blast of cool air from the jet stream mixing with warm Gulf air that built up over the weekend, explained AccuWeather.com's Henry Margusity.
The moist air moves in low to the ground, without any geographic obstructions over the flat Midwest. The cooler air moves above it off the mountains.
Because hot air rises and cold air sinks, these interactions create forceful updrafts and downdrafts. Massive storms known as supercells, which feature rotating updrafts of air, are known for producing tornadoes like those seen Monday.
This Scientific American video explains why that may be -- scientists believe differences in the speeds of the colder jet stream air high in the atmosphere and the warm Gulf air below creates rotation horizontally that can be shifted vertically, forming a funnel cloud. But there is still much scientists don't understand about how tornadoes form.
Tornado intensity is measured by a complex system known as the Enhanced Fujita Scale. The original Fujita Scale used wind estimates and general categorizations of damage to determine where a tornado fell on a range from F-0, the weakest, to F-5, the strongest. The enhanced scale also goes from EF-0 to EF-5 uses more complex and precise wind speed estimates as well as a detailed set of damage indicators to rate tornado intensity, according to the National Oceanic and Atmospheric Administration.
The Moore tornado received a preliminary ranking of EF-4, but was upgraded Tuesday afternoon to an EF-5. Such tornadoes packwinds in excess of 200 mph. The tornado was on the ground for 40 minutes and cut a path 17 miles long and 1.3 miles wide.
It was shown carrying a ball of debris measuring 2 miles wide on radar, according to the Weather Underground. The debris was tossed as far as 100 miles from Moore. Earlier reports suggested as many as 91 feared deaths, but officials revised that number downward.
Such tornadoes make up about 1 percent of all tornadoes but cause 70 percent of tornado deaths, according to Climate Central.
Tornadoes aren't just limited to Tornado Alley. They occur on all continents except Antarctica.
In Maryland, they occur frequently relative to the state's size -- a Weather Channel state-by-state ranking released last year put the state at No. 3 among top tornado states, with about 10 tornadoes per 10,000 square miles each year from 1950 to 2010. Florida ranked first, Kansas was second, and Illinois and Mississippi rounded out the top five.