How do tornadoes form? That is pretty easy to answer since there has been a large amount of study into the subject. They are usually the extreme result of a supercell thunderstorm. During the storm cold air and warm air combine in a set pattern: the cold air drops as the warm air rises. The warm air eventually twists into a spiral and forms the funnel cloud that we all associate with a tornado.
The formation of a tornado follows a clear set of steps. First there a change in wind direction and an increase in wind speed. This change occurs at an increasing altitude and creates an invisible horizontal spinning effect in the lower atmosphere. Next, rising air within the thunderstorm’s updraft tilts the rotating air from horizontal to vertical. Third, an area of rotation, 3-10 km wide is contained within a vast majority of the storm. This is where the strongest tornadoes form. Then a lower cloud base in the center of the storm becomes a rotating wall cloud. This area can be nearly rain-free. Lastly, a tornado develops and starts to wreak its destruction.
Once a tornado has formed, it follows a predictable life cycle. First, the mesocyclone(rotating air), along with the rear flank downdraft( RFD), starts moving towards the ground. A small funnel appears to build up at the bottom of a wall cloud. As the RFD reaches the ground, the surrounding dirt rises up, causing damage even to heavy objects. The funnel touches the ground immediately after the RFD, forming a tornado.
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During the next stage the tornado’s main source of energy, the RFD, begins to cool. The distance the tornado covers, depends on the rate at which the RFD cools. If the RFD cannot further provide any more warm air to the tornado, it begins to die.
Lastly, with the tornado’s warm air supply cut, the vortex begins to weaken and shrivel away. As the tornado weakens, the mesocyclone also starts to dissipate, but a new mesocyclone can start very close to the dying one. Those are the basics of tornado formation and life.
If you’d like more info on tornado, check out the National Oceanic and Atmospheric Administration 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.