1959 US Army Vietnam War Terrain Intelligence ... - Survival Books

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with fair, dry weather; low-pressure systems, with unsettled, cloudy
conditions.
49. Winds
a. Wind is air in motion and results from differences in atmospheric
pressure. A wind is described by its direction and speed.
b. The direction of a wind is the direction from which it is blowing.
A wind coming from the north, for example, is termed a north
wind. Wind direction is reported in air weather observations to the
nearest 16 points of the compass.
c. Wind velocities are reported by the air weather service in knots.
A weather map scale of wind velocity (fig. 7) is used as an aid in
estimating wind speed.
d. Over irregular terrain, a wind does not move with a steady force
or direction, but as a succession of gusts and lulls of variable speed
and direction. These eddy currents, caused by friction between air
and terrain, are called gusts or turbulence. Turbulence also results
from unequal heating of the earth's surface, the cooler air of adjacent
areas rushing in to replace the rising warm air from heated areas.
Usually the. turbulence produced by surface friction is intensified on
a sunny afternoon.
e. Local pressure and wind systems are created by valleys, mountains,
and land masses that change the weather characteristics of
areas. Since land masses absorb and radiate heat more rapidly than
water masses, the land is heated more than the sea during the day and
cools more at night. In coastal areas, warm air over the land rises
to a higher altitude and then moves horizontally out to sea. To replace
this warnm air, the colder air over the water moves on to the land,
creating the so-called sea breeze. The circulation is reversed at night,
so that the surface air moves from the land to the sea, resulting in a
land breeze.
f. Heated by the daytime sun, the air in contact with a mountain
slope becomes lighter than the surrounding air and rises up the slope,
being replaced by denser, colder air. This air movement is called a
valley wind because it appears to be flowing up from the valley. At
night the air in contact with the slope becomes colder and more dense,
sinking down along the slope to create a mountain breeze that seems to
flow out of the mountain. Mountain breezes generally are stronger
than valley winds, especially in the winter.
g. A chinook (North America) or foehn. (Europe) (fig. 8) is a
phenomenon that occurs in winter and spring on the lee or downwind
side of moutain ranges over which there flows a steady crosswind of
moisture-laden air. As this air rises over the windward side of the
mountains it expands and cools rapidly, producing clouds and precipitation.
As the air moves down the lee side of the mountain range
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