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Weather Whys: Tornadoes

Document Number: 1101
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Reprinted with the permission of Davis Instruments Corp. and the Davis Weather Club e-newsletter.

After the devastating effect of the latest tornadoes in Oklahoma and Kansas, we decided to do a little digging and learn more about them. This is what we found:

About 800-1000 tornadoes touch down each year in the United States. Most of these tornadoes cause little or no damage whatsoever, it's just the few F4 and F5-intensity whoppers that wreak havoc. Which state is the worst hit? Well, it depends on what criteria you use. Oklahoma has the highest number of significant and violent tornadoes per square mile, while Kansas has the highest number of F5-intensity tornadoes recorded since 1880. Oklahoma City, in particular, has been struck by tornadoes at least 33 times in the past 90 years.

While three out of four twisters that do touch down occur in the US, many countries around the world are hit regularly. The worst tornado disaster in history may have been when a twister struck about 40 miles north of Dhaka, Bangladesh on April 26, 1989. At least 1,109 people were killed, 15,000 injured, and 100,000 left homeless. Other twisters have struck that region and may have taken even greater tolls.

Tornadoes are still not fully understood, though each year scientists learn more. We asked Jason, our resident meteorologist, to spotlight what science does know about how tornadoes form:

When cold and/or dry air overlays warm moist air, you have an unstable, thunderstorm-prone atmosphere. For example, in the spring, cold air frequently blankets the surface air of the Central Plains. Meanwhile, below this blanket, cold air coming in from the north induces warm moist air to flow from the south. A layer of warm dry air from the southwest then caps the moist air along a boundary called the dryline, creating in effect a pressure cooker. Where these air masses regularly collide, you have "Tornado Alley."

The advance of either cold or dry air into warm moist air (as when a cold front advances) forces the warm moist surface air upward where it cools, condenses and forms clouds. As more warm air rises and condenses, the storm matures and the updraft forces parts of the thunderstorm upwards into the shear environment (that is, where each air layer is moving at a different speed and/or in a different direction).

The shear environment causes the air to spin about an axis parallel to the ground, like rolling a cigar. This axis is then pushed into a "standing" position by the thunderstorm's updraft, creating a "mesocyclone" (a small scale low pressure system). As air rushes in toward the low pressure of the mesocyclone, the rotational wind speeds increase (like an ice skater bringing her arms closer to her body as she spins). Eventually, for reasons not yet understood, this rotation is translated downward and a tornado is formed.

There are two main theories about why this translation might occur: one, that the boundary between the thunderstorm's updraft and its downdraft creates more spin; two, that local topography induces certain wind flows that create more spin (thunderstorms and tornadoes have been observed to favor formation in certain areas over others).

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Please Note: The information contained in this publication is intended for general information purposes only. This publication is not a substitute for review of the applicable government regulations and standards, and should not be construed as legal advice or opinion. Readers with specific questions should refer to the cited regulation or consult with an attorney.