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Figure 6.17b. U-shaped funnel underneath the anvil of a tornadic supercell in eastern Okla-
homa on May 27, 1997 (photograph by the author)
6.2 TORNADO CLIMATOLOGY
Tornadoes occur all over the world, but are most common in the Great Plains of
the U. S. and just to the east during the spring ( Figures 6.18 -6.20 ) , when there is a
ready supply of moisture from the relatively warm Gulf of Mexico and climatolo-
gically there is a jet of air aloft that enables there to be vertical wind shear strong
enough to support supercells, the most prolific parent storms of the strongest
tornadoes. As air subsides in the lee of the Rocky Mountains, it warms and pro-
duces a trough of low pressure (see textbooks on synoptic meteorology for more
details). In response to this surface trough, surface geostrophic winds become
southerly and, under the influence of surface friction, the winds become southeast-
erly over the states of the Great Plains. This southeasterly flow advects moisture
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