WeatherEye
Tornado DiagramThe Nuts and Bolts of a Tornado
Tornadoes can form in a variety of ways. The thunderstorms that create twisters can be caused by hurricanes, drylines, low pressure systems and more.

The really big tornadoes, however, frequently form like this:

  1. Inversion (cap): A small layer of warmer air gets above the warm and humid air at the surface. This is like an atmospheric sandwich, with warm and humid air at the surface, the capping layer of warmer air in the middle and cold and dry air above. This 'cap' keeps storms from developing.
  2. Break the cap: A weather force arrives to help the warm/humid surface air break free. The force is frequently an upper-level air disturbance or the arrival of a front (defined). This extra force helps storms crack through the cap.
  3. Shooting through the break: The lower level air is now free and starts shooting upward. Storms shooting up through these breaks can end up towering upward to 50,000 feet and may become 'supercell' thunderstorms that can support huge tornadoes.
  4. Rain/Hail Form: As the lower-level air rises, it expands in the reduced air pressure aloft (air pressure drops as altitude increases). This expansion cools the air and causes the moisture to condense (defined).
  5. Condensation Helps the Storm Grow: As the raindrops & hail form, they release heat. (Called 'latent heat.') This extra heat helps keep the storm's air warmer than the surrounding air. This heating keeps the air rising, even at speeds up to 150 mph!
  6. Anvil Top: As the storm reaches high altitudes, it runs into fast upper level winds. These winds tilt the thunderhead to create the anvil at the top.
  7. Live or Die: The thunderstorm may die out in intense rain and/or hail. If the storm survives long enough, however, it may spawn a mesocyclone...then a tornado.
  8. Mesocyclone: As the storm's air rushes upward, it interacts with air at various altitudes. This can cause the updraft to rotate, creating a "mesocyclone." Low-level wind helps cause this rotation, which is almost always counter-clockwise (seen from above) in the Northern Hemisphere.
  9. Tornado: If the spinning air of the mesocyclone moves downward, it narrows and rotates faster. When it touches the ground, we have a tornado...and the damage begins.

Time to let you predict today's tornadoes!

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This lesson created by "The Why Files," a NISE project funded by the NSF. Lesson used by permission. Original lesson and graphics copyright NISE. Modifications copyright BPNM.