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Name: stephanie
Status: student
Grade: 9-12
Location: HI
Country: N/A
Date: 3/24/2005


Question:
What makes a tornado?


Replies:
Stephanie,

You may already find an answer to your question on the Newton web site, but here is a brief explanation.

The initial root of a tornado is in wind shear, where the wind speed high in the sky is much faster in one location than it is somewhere nearby. This is often caused by a jet stream tens of thousands of feet up in the air, moving near an air mass that has much less horizontal motion. Along the boundary between the faster and slower moving air (such as a squall line of thunderstorms or along a cold front) the air begins to rotate tens of thousands of feet up in the air. At the same time, a thunderstorm may be forming in the same area. If the thunderstorm becomes tall enough, it will grow into the area of rotating air; the upward moving air inside the thunderstorm can actually add to the speed of the rotation through physical forces that can change vertical motions into horizontal motions. Sometimes the entire thunderstorm begins to rotate, but a much more compact tornado vortex with very high rotation speed can form as the rotation is carried all the way to the ground in the form of a tornado funnel.



Dr. Cook
Atmospheric Physics and Chemistry Section
Environmental Research Division
Argonne National Laboratory


The detailed answer is complicated, but the simple answer is that there is the potential for a tornado arises when moist warm air (which is less dense than cold dry air) is overrun by denser cold dry air. This is an inversion because the less dense atmosphere is trapped under a "lid" of much heavier air. If this inversion inverts and the temperature differences are large enough, this re-adjustment in atmospheric densities can occur explosively (a tornado). Keep in mind the real mechanism is much more complicated, but the above is the simple process.

Vince Calder


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