Mountain Terrain, Weather, and Hazards

1-55. Above hot surfaces, air expands and moves to colder areas where it cools and becomes denser, and
sinks to the earth’s surface. The results are a circulation of air from the poles along the surface of the earth
to the equator, where it rises and moves to the poles again.
1-56. Heating and cooling together with the rotation of the earth causes surface winds. In the Northern
Hemisphere, there are three prevailing winds:

POLAR EASTERLIES
1-57. These are winds from the polar region moving from the east. This is air that has cooled and settled at
the poles.

PREVAILING WESTERLIES
1-58. These winds originate from about 30 degrees north latitude from the west. This is an area where
prematurely cooled air, due to the earth’s rotation, has settled to the surface.

NORTHEAST TRADEWINDS
1-59. These are winds that originate from about 30o north from the northeast. The jet stream is a long
meandering current of high-speed winds often exceeding 250 miles per hour near the transition zone
between the troposphere and the stratosphere known as the tropopause. These winds blow from a generally
westerly direction dipping down and picking up air masses from the tropical regions and going north and
bringing down air masses from the polar regions. The patterns of wind mentioned above move air. This air
comes in parcels called air masses. These air masses can vary from the size of a small town to as large as a
country. These air masses are named from where they originate:

Maritime―over water.

Continental―over land.

Polar―north of 60o north latitude.

Tropical―south of 60o north latitude.
1-60. Combining these parcels of air provides the names and description of the four types of air masses:

Continental Polar―cold, dry air mass.

Maritime Polar―cold, wet air mass.

Maritime Tropical―warm, wet air mass.

Continental Tropical―warm, dry air mass.
1-61. Two types of winds are peculiar to mountain environments, but do not necessarily affect the weather.

ANABATIC WIND (VALLEY WINDS)
1-62. These winds blow up mountain valleys to replace warm rising air and are usually light winds.

KATABATIC WIND (MOUNTAIN WIND)
1-63. These winds blow down mountain valley slopes caused by the cooling of air and are occasionally
strong winds.

HUMIDITY
1-64. Humidity is the amount of moisture in the air. All air holds water vapor even if it cannot be seen. Air
can hold only so much water vapor; however, the warmer the air, the more moisture it can hold. When air
can hold all that it can the air is “saturated” or has 100 percent relative humidity.

If air is cooled beyond its saturation point, the air will release its moisture in one form or another
(clouds, fog, dew, rain, snow, and so on). The temperature at which this happens is called the
condensation point. The condensation point varies depending on the amount of water vapor
contained in the air and the temperature of the air. If the air contains a great deal of water,

26 July 2012

TC 3-97.61

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