Chapter 1

temperature, humidity (moisture content), air pressure (barometric pressure), how it is being moved, and if
it is being lifted or not.
1-47. Air pressure is the weight of the atmosphere at any given place. The higher the pressure, the better
the weather will be. With lower air pressure, the weather will more than likely be worse. In order to
understand this, imagine that the air in the atmosphere acts like a liquid. Areas with a high level of this
liquid exert more pressure on an area and are called high-pressure areas. Areas with a lower level are called
low-pressure areas. The average air pressure at sea level is 29.92 inches of mercury (hg) or 1,013 millibars
(mb). The higher in altitude, the lower the pressure.

HIGH PRESSURE
1-48. The characteristics of a high-pressure area are as follows:

The airflow is clockwise and out.

Otherwise known as an anticyclone.

Associated with clear skies.

Generally the winds will be mild.

Depicted as a blue “H” on weather maps.

LOW PRESSURE
1-49. The characteristics of a low-pressure area are as follows:

The airflow is counterclockwise and in.

Otherwise known as a cyclone.

Associated with bad weather.

Depicted as a red “L” on weather maps.
1-50. Air from a high-pressure area is basically trying to flow out and equalize its pressure with the
surrounding air. Low pressure, on the other hand, is building up vertically by pulling air in from outside
itself, which causes atmospheric instability resulting in bad weather.
1-51. On a weather map, these differences in pressure are shown as isobars. Isobars resemble contour lines
and are measured in either millibars or inches of mercury. The areas of high pressure are called “ridges”
and lows are called troughs.

WIND
1-52. In high mountains, the ridges and passes are seldom calm; however, strong winds in protected
valleys are rare. Normally, wind speed increases with altitude since the earth’s frictional drag is strongest
near the ground. This effect is intensified by mountainous terrain. Winds are accelerated when they
converge through mountain passes and canyons. Because of these funneling effects, the wind may blast
with great force on an exposed mountainside or summit. Usually, the local wind direction is controlled by
topography.
1-53. The force exerted by wind quadruples each time the wind speed doubles; that is, wind blowing at 40
knots pushes four times harder than a wind blowing at 20 knots. With increasing wind strength, gusts
become more important and may be 50 percent higher than the average wind speed. When wind strength
increases to a hurricane force of 64 knots or more, Soldiers should lay on the ground during gusts and
continue moving during lulls. If a hurricane- force wind blows where there is sand or snow, dense clouds
fill the air. The rocky debris or chunks of snow crust are hurled near the surface. During the winter season,
or at high altitudes, commanders must be constantly aware of the wind-chill factor and associated
cold-weather injuries (see Chapter 2).
1-54. Winds are formed due to the uneven heating of the air by the sun and rotation of the earth. Much of
the world’s weather depends on a system of winds that blow in a set direction.

1-12

TC 3-97.61

26 July 2012