What is a Tornado?
Why Do They Happen?
The simplest explanation for what causes tornadoes is the rapid convection of air. A good analogy to explain how convection works would be water in a pot on the stove; the water at the bottom heats first since it's closer to the heat source, causing the water molecules in the hotter area of the pan near the bottom to accelerate. Then they bombard the other, cooler water molecules and set them in motion upward through the water column. This causes the differing water temperatures in the pot to circulate around inside (without ever having to stir it with a spoon!).
The same conditions happen in the air in the lower and upper atmosphere. When very warm, humid air that is close to the ground continues to heat up from the heat rising off the ground it rises upward rapidly towards superheated, dry air that is forcing that air down towards the ground. Think of the warm, humid air layer above the ground as starting to "boil" and push its way upward. When it rapidly breaks through the mid-to-upper layer of stable, dry air into the cooler, moist air of the upper atmosphere, those rapidly moving air molecules pick up speed, forcing the air currents to flow downward rapidly. This reaction happens very quickly and gains momentum as the wind shear increases. Typically, the rotating column of air begins parallel to the horizon, but then swings down and touches the ground. This is when it becomes painfully visible to all because the dirt, dust and debris the twister is picking up, colors the column black. It's kind of hard to imagine that something so small and invisible to the human eye - air molecules - could move with such tremendous energy and cause such a gigantic wind phenomenon.
Where Do they Occur?
The incidence of tornadoes is very well documented in the U.S. and in Canada. The frequent incidence of very large, dangerous tornadoes in the U.S. in the Great Plains area between the Rocky Mountains and Appalachians has earned it the nickname, "tornado alley". They happen here most frequently because of the favorable conditions. They need lots and lots of warm, humid air. This usually comes in from the Gulf of Mexico. And the rotating thunderstorms, called supercells, that spawn the biggest tornadoes need low-level winds that shift direction and grow stronger just above the ground.
The higher and drier elevations of the Rockies allow a hot, dry layer of air to blow over the region from the southwest. Above 10,000 feet, cooler air races east over the region. These wind flows stack up over the center of the nation, creating low and mid level wind shear, which spawns the violent twisters.
Record Tornado Event
On April 3-4, 1974 a phenomenal weather event, known as the "Super Tornado Outbreak" occurred in the U.S. According to the NOAA, "It was the worst tornado outbreak in U.S. history with 148 twisters touching down in 13 states. Before it was over 16 hours later, 330 people were dead and 5,484 were injured in a damage path covering more than 2,500 miles."
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