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105
Geographic features (such as mountain ranges) or linear boundaries (such as warm or
cold fronts) may create lines of thunderstorms which move across the landscape. A special
case of this is the squall line, which usually occurs in the warm sector of a cyclone. The
squall line is propelled by its own outflow, which reinforces continuous development of
updrafts along the leading edge.
Multicell or squall line systems may form a meteorologically-important feature known
as mesoscale convective system (MCS) stretching for hundreds of miles. They are large
enough to have a pronounced effect on the upper-level and surface weather pattern, and may
influence forecasts over half of a continent. MCS systems are common in the Midwest
region of the United States during the summer months and produce much of the region's
important agricultural rainfall.
Text 5. Wind
Wind, in the most general sense, is the movement of air. It occurs at all scales, from
local breezes generated by heating of land surfaces and lasting tens of minutes to global
winds resulting from solar heating of the Earth, lasting eons. The two major influences on
the atmospheric circulation are the differential heating between the equator and the poles,
and the rotation of the planet (Coriolis effect). Because of differential heating and the fact
that warm air rises and cool air falls, there arise circulations that (on a non-rotating planet)
would lead to an equator-to-pole flow in the upper atmosphere and an pole-to-equator flow
at lower levels. Because of the Earth's rotation, this simple situation is vastly modified in the
real atmosphere. In almost all circumstances the horizontal component of the wind is much
larger than the vertical - the exception being violent convection.
Given a difference in barometric press ure between two air mas s es, a wind will aris e
between the two which tends to flow from the area of high pressure to the area of low
pressure until the two air mass es are at the s ame pressure, although this will be strongly
modified by the Coriolis effect.
While all winds are the movement of air more or less parallel to the Earth's surface,
they come in a variety of forms. There are global winds, such as the wind belts which exist
between the atmospheric circulation cells. There are upper-level winds, such as the jet
streams. There are synoptic winds that result from pressure differences in surface airmasses
at the middle latitudes, and there are winds that come about as a consequence of geographic
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