severe thunderstorms as those that have winds in excess of 58 miles per hour,^3 ⁄ 4 -inch
diameter hail, or frequent lightning. Rainfall rates in excess of 4 inches per hour are
designated “downbursts” that can lead to flash flooding events in which streams
overflow their banks with little warning. Flash flooding kills about twice as many
U.S. citizens as does lightning.
Severe thunderstorms tend to be larger than air mass thunderstorms and can last
as long as several hours. Sometimes large thunderstorms will align parallel to the
passage of cold fronts and drylines. These linear zones can have significant surface
wind converging into them and make maritime Tropical air rise violently. “Wind
shear” is vital to the mechanics of most severe thunderstorms. If low-level winds
into a thunderstorm are moist and very unstable, and if the winds in the middle
and upper levels of the troposphere are much faster, then rising air starts to rotate
with its ascension. This wind shear makes for supercell storms that have mesoscale
cyclones with diameters of 10 km and capable of producing very high winds and
tornadoes. Additionally, wind shear helps prolong the storm’s duration by tilting
the storm so that the updrafts and downdrafts do not interfere with each other.
Squall lines of thunderstorms produced along fronts and drylines can exist for
hours; downdrafts from one storm act like a miniature cold front to focus lifting
and cause another storm nearby. Mesoscale convective complexes are large storms
without a focus along fronts. These storms develop in warm, moist surface air in
regionsof weak winds aloft. Storms are created and the outflow from one
becomes the inflow to the next. These convective complexes last for hours and
can cover thousands of square kilometers. They move very slowly and train along
the same paths thus making flooding rain the major hazard.
There is a well-known geography of thunderstorms. Mountains can lift air
enough to become unstable (the orographic effect) and mountain ridges usually
have more thunderstorms than their lowland surroundings.Oceans, at best, have
moderately warm surfaces that do not engender extreme instability and, so, have
modest thunderstorm occurrences. Thunderstorms occur year round in the warm,
moist air of the Intertropical Convergence. Some rainforest locations receive
thunderstorm precipitation almost 300 days per year. These locations experience
increased cloudiness as instability builds during the day and then a brief thunder-
storm.Monsoonclimates experience large, long-lived thunderstorm deluges during
the season of high sun angles. The middle latitudes tend to have most thunderstorms
in the summer season because the winter brings much greater stability of air masses.
Within the continental United States, thunderstorms are most numerous on the
Florida peninsula and least numerous on the West Coast because of the stabilizing
dual impacts of the influence of the Pacific subtropical high and the cold California
current. Notable exceptions are the middle latitude Mediterranean climate regions
where summers have stable air caused by subtropical highs and winter cold fronts
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