Tornadoes: How Do They Form?
Tornadoes are one of the most destructive and unpredictable weather phenomena. Characterized by strong winds that spiral in a vertical column, they often cause widespread damage and loss of life.
The first step in tornado formation is the development of a thunderstorm. Thunderstorms are formed when warm, moist air rises and cools, causing water vapor to condense into clouds. As the warm air continues to rise, it forms an updraft. The updraft can become stronger and more organized, forming a supercell thunderstorm.
Supercells are thunderstorms with a rotating updraft, creating wind shear and causing the storm to begin to similarly rotate and forming a mesocyclone. A mesocyclone, which is a large area of rotating air, can be several miles wide and may last for several hours. This rotation is caused by the wind shear, forcing the air to spin horizontally. As the updraft continues to rise, it begins to tilt the rotating air, creating a vertical, rotating column of air.
Once the column of air is formed, it begins to stretch and intensify under a process known as vorticity stretching. The stretching causes the column of air to become thinner and longer, increasing its rotation speed. Meanwhile, the updraft within the storm continues to feed the rotating column of air, causing it to grow stronger and more organized. As the rotating column of air strengthens, it may begin to reach the ground. When it does, it is officially classified as a tornado.
Tornadoes have touched down in every U.S. state, but they are most commonly found in the Great Plains and Mississippi Valley, a region known as Tornado Alley, due to the unique meteorological conditions in the region. The warm, moist air from the Gulf of Mexico meets the dry, cool air from the Rockies, creating an excellent environment for the formation of supercell thunderstorms.
Tornadoes can be incredibly destructive and dangerous. If you spot a rotating storm above you, take shelter and check the messaging from your local NWS office. Make sure you know the difference between and watch and a warning; a watch indicates that hazardous weather has a chance to occur in your area, while a warning mean threatening weather is imminent and action should be taken immediately.
Source: NWS, UCAR
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