Download Weather Whys: Twisters

Weather Whys: Twisters
Document Number: 1101
Page: 1 of 1
Reprinted with the permission of Davis Instruments Corp.
and the Davis Weather Club e-newsletter.
After the devastating effect of 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.
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).
While three out of four twisters that do touch down occur
in the U.S., 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).
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general information purposes only. This publication is not a substitute for
review of the applicable government regulations and standards, and should
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