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Tornadoes are usually the extreme result of a very large thunderstorm called a supercell. During the storm cold air and warm air combine. The cold air goes drops as the warm air rises. The warm air eventually twists into a spiral and forms a funnel cloud. The sky turns a very dark green color and the tornado begins its destruction.
There is a clear set of steps in the formation of a tornado. First, just before the thunderstorm develops, a change in wind direction and an increase in wind speed, at an increasing altitude, creates an invisible horizontal spinning effect in the lower atmosphere. Second, rising air within the thunderstorm’s updraft tilts the rotating air from horizontal to vertical. Next, An area of rotation, 2-6 miles wide is contained within a vast majority of the storm. The strongest, most violent tornadoes form within this area of rotation. After that, a lower cloud base in the center of the storm becomes a rotating wall cloud. This area is often nearly rain-free. Lastly, just a very few minutes later, a tornado develops and starts to wreak its destruction.
Tornadoes follow a definite life cycle.
As the mesocyclone(area of organized rotation) approaches the ground, a visible condensation funnel appears to descend from the base of the storm. As the funnel descends, the rear flank downdraft (RFD) also reaches the ground. This creates a gust front that can cause damage a good distance from the tornado. The funnel cloud becomes a tornado within minutes of the RFD reaching the ground.
Early on, the tornado has a good source of warm, moist in-flowing air to power it, so it grows to maturity. This can be a few minutes or more than an hour. This is the most destructive phase of the tornado and can be more than 1.6 km across. The RFD has become an area of cool surface winds and begins to wrap around the tornado, cutting off the inflow of warm air, effectively choking the tornado.
As the RFD chokes off the tornado’s air supply, the vortex begins to weaken. This dissipating stage only lasts a few minutes then the tornado fizzles. The tornado is still capable of causing damage. The storm is contracting, but the winds can increase in speed.
If the initial supercell thunderstorm is strong enough it can breed multiple tornadoes. One dissipates, creating a new mesocyclone, and a new tornado can form.
If you’d like more info on tornadoes, check out the National Oceanic & Atmospheric Administration (NOAA) Homepage. And here’s a link to NASA’s Earth Observatory.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.