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Figure 6.19. Tornado climatology information (worldwide). (Top) Distribution of number of
tornadoes/(100 F2 tornadoes) for selected countries (from Brooks and Doswell, 2001); (bot-
tom) days/year with ''favorable'' tornado parameters (from Brooks et al., 2003a). In lieu of a
good, consistent worldwide climatology of tornadoes, it is useful to estimate this climatology
from environmental data.
from the Gulf of Mexico, which is situated upstream, into the Great Plains and
nearby environs, an area sometimes colloquially referred to as ''Tornado Alley'',
and also adds to the vertical shear associated with strong westerlies, south-
westerlies, or northwesterlies aloft. Periodic incursions of cold air behind frontal
zones, however, act to remove the potential instability needed for convective
storm development. The frequency of occurrence of tornadoes varies from decade
to decade ( Figure 6.18 ), but the location of Tornado Alley is consistently from
Texas, northward through Oklahoma, Kansas, Nebraska, and into Iowa. The
dryline, a boundary separating moist, relatively cool Gulf of Mexico air from dry,
relatively warm continental air that has had contact with higher terrain to the
west, is a frequent locus of storm initiation, since just east of it the equivalent
potential temperature is greatest; thus, the potential for reaching convective tem-
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