Types of
Wind
Three groups of winds may be described. First are the
planetary winds, which include the TRADE WINDS and middle latitude WESTERLIES. Second are the secondary winds, sea breezes, MONSOON winds,
and cyclonic winds. The last group includes the regional anabatic and
katabatic winds, as described below:
 Planetary
winds. In planetary winds, which are a major component of the
general circulation, a rising motion occurs in the low latitudes where low
pressure is created by maximum solar heating. A compensating inflow occurs
at low levels in each hemisphere, producing the northeast trade winds of
the Northern Hemisphere. The outflow of air from the tropics in the upper
troposphere is diverted by the Coriolis force to produce strong
westerlies.
The westerlies may become unstable and produce such
mid-latitude disturbances as jet streams, cyclones, and anticyclones as
well as associated secondary wind systems. They remain essentially
westerly, however, if stable. The westerlies penetrate the atmosphere to
the Earth's surface, where they are dissipated by friction.
Secondary
winds. Secondary winds develop to minimize local imbalances.
Monsoons, for example, minimize the temperature gradient between warm
landmasses and the adjacent cooler by the Coriolis force, the summer
monsoon winds approach Asia from the southwest. In winter, air flows off
the extremely cold Asian landmasses toward the warmer oceans as the
northeast monsoons.
An example of middle-latitude secondary wind are the
winds associated with transient WEATHER systems. Besides possessing
specific directions relative to the pressure centers, the winds associated
with these systems are characterized by gales. A more benign system is the
sea breeze, which is usually confined to within 20 kilometer
(12.5 mi) of a coastline and to a vertical column of a few kilometers. If
daytime heating is strong enough, as in the tropics, the moist onshore
flow, which is forced to rise by surface heating, may form large
convective clouds that yield brief, heavy showers. The cooling of the land
at night reverses the sea-breeze circulation, resulting in a weak offshore
surface low.
Regional
winds. Regional winds are usually the effects of such
geographic features as mountain ranges. A moist wind upon a mountain
is forced to rise and may produce significant climatological effects.
Cooling adiabatically, the rising air may produce rainfall on the windward
side of mountains. As the air descends on the lee side, the adiabatic
heating, plus the heat added to the system from condensation on the
windward side, causes a very warm wind. Such MOUNTAIN and VALLEY WINDS may
be very turbulent and can inflict severe damage. Ascending winds are
called katabatic. A potentially destructive katabatic wind called a
microburst - an intense localized downdraft that spreads along the ground
- causes the condition known as wind shear. Often associated with
THUNDERSTORMS and rain showers, wind shears are characterized by sudden
shifts in wind speed and direction. |
|
