Air movements result from unbalanced forces acting on the ATMOSPHERE. One
basic cause of these imbalances are the large temperature variations found
in the atmosphere, with the equatorial and polar regions differing most
widely. Temperature variations, together with resulting pressure gradients
and the CORIOLIS EFFECT, combine to form the basic wind fields that exists
around the Earth. The wind flows thus produced are an attempt by the
atmosphere to return to equilibrium. Because differences in global heating
are continually maintained by the Sun, however, equilibrium is never
attained. A steady state is instead reached in which relatively constant
planetary winds carry heat from the equator toward the poles.
Three basic physical effects govern the distribution of
wind fields. The first includes the forces already indicated. The second
is the effect of dissipative processes such as friction, which reduce a
wind's kinetic energy. The third is, the Coriolis effect. All sizes of
wind, from planetary to regional in scope, are governed by this process.
The effects of friction create eddies in the turbulent boundary layer of
the atmosphere. Higher than 1 kilometer (0.62mi) above ground, winds are
generally smooth. (The atmosphere, however, may be quite turbulent near
jet streams.) As energy is dissipated by friction at the lower boundary,
the winds throughout the entire lower atmosphere , or troposphere, adjust
to replenish the lost energy. In this manner, the entire atmosphere is
affected by the dissipative processes at the lower boundary.
In middle and high latitudes, the frictional forces
that act in the opposite direction to the wind create a balance of forces.
This produces a change in wind direction in the vertical vector as the
wind speed decreases to zero at the ground. In low attitudes the Coriolis
force is insignificant, and the wind decreases to zero at the ground with
little directional change in its movement.
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