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What Causes Tornadoes to Form?

Tornadoes are the most violent storms that nature can produce. In a matter of seconds, entire towns can be destroyed as vicious winds level everything in their path. Scientists have struggled for decades to understand the dynamics of these deadly but short-lived storms. Many of the factors that contribute to tornado formation are now known, but the finer details of how all the variables come together are still murky.

Colliding Air Masses
- In the United States, the battle between cool Canadian air and warm, moist air from the south often leads to tornadic storm development. Especially during spring and early summer, the Central Plains are the site for this clash of air masses. The region from northern Texas to Nebraska and across to Illinois has become known as Tornado Alley. The interaction of these different air masses charges the atmosphere with a lot of energy. Dry air descending from the Rocky Mountains across the High Plains is frequently the trigger for large, powerful storms, which can generate tornadoes at any time.
Wind Shear
- The collision of air masses alone is not enough to cause a tornado. Another factor called wind shear must be present. Wind shear occurs when air at different altitudes is moving at different speeds in different directions. This causes a horizontal tube of air to begin rotating. A stronger shear leads to a stronger rotation. In the Central Plains, cool air generally invades from the north and west. Warm air moving in from the south rides up and over the cooler air, leading to strong wind shear.
Supercell Thunderstorms
- The flat landscape of the central United States provides a fertile atmosphere for supercell thunderstorms.
  
  Meteorologists define a "supercell thunderstorm" as one that sustains itself for a long period of time. Supercells are responsible for all the strongest tornadoes on record. When this kind of storm forms in an area of high wind shear, the rotating air can sometimes be tilted vertically. When conditions are perfect, this vertical rotation can descend to ground level and form a tornado. The exact mechanism for this is still unclear. Why one supercell can generate a tornado while another does not is also an open question.
Updrafts
- Updrafts and downdrafts are present in all thunderstorms, but especially potent in a supercell storm. Rising warm air provides the fuel for a storm's rain, wind and hail. As rain cools the surrounding air, it can descend rapidly in a downdraft. As long as the warm air supply remains constant and isn't disrupted by the falling air, the storm can sustain itself. While the process is not completely understood, meteorologists know that updrafts are an essential ingredient in tornado formation.
Other Causes of Tornadoes
- Waterspouts and landspouts are similar, but not identical, to tornadoes.
  
  Waterspouts, landspouts and gustnadoes are different kinds of tornadoes that don't form during a supercell thunderstorm. Waterspouts typically form when cool air moves over warm water. Landspouts are most frequent in dry, desert areas and, like waterspouts, are less powerful than ordinary tornadoes. Gustnadoes form at the edge of a gust front: the rain-cooled air that gushes down and out from a large thunderstorm. Landfalling hurricanes often produce brief tornadoes in their northeastern quadrants, but meteorologists don't yet understand how.

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