Chapter 1



condensation can occur at a temperature of 68 degrees Fahrenheit, but if the air is dry and does
not hold much moisture, condensation may not form until the temperature drops to 32 degrees
Fahrenheit or even below freezing.
The adiabatic lapse rate is the rate at which air cools as it rises or warms as it descends. This rate
varies depending on the moisture content of the air. Saturated (moist) air will warm and cool
about 3.2 degrees Fahrenheit per 1,000 feet of elevation gained or lost. Dry air will warm and
cool about 5.5 degrees Fahrenheit per 1,000 feet of elevation gained or lost.

CLOUD FORMATION
1-65. Clouds are indicators of weather conditions. By reading cloud shapes and patterns, observers can
forecast weather with little need for additional equipment such as a barometer, wind meter, and
thermometer. Any time air is lifted or cooled beyond its saturation point (100 percent relative humidity),
clouds are formed. The four ways air gets lifted and cooled beyond its saturation point are as follows:

CONVECTIVE LIFTING
1-66. This effect happens due to the sun’s heat radiating off the Earth’s surface causing air currents
(thermals) to rise straight up and lift air to a point of saturation.

FRONTAL LIFTING
1-67. A front is formed when two air masses of different moisture content and temperature collide. Since
air masses will not mix, warmer air is forced aloft over the colder air mass. From there it is cooled and then
reaches its saturation point. Frontal lifting creates the majority of precipitation.

CYCLONIC LIFTING
1-68. An area of low pressure pulls air into its center from all over in a counterclockwise direction. Once
this air reaches the center of the low pressure, it has nowhere to go but up. Air continues to lift until it
reaches the saturation point.

OROGRAPHIC LIFTING
1-69. This happens when an air mass is pushed up and over a mass of higher ground such as a mountain.
Air is cooled due to the adiabatic lapse rate until the air’s saturation point is reached.

TYPES OF CLOUDS
1-70. Clouds are one of the signposts to what is happening with the weather. Clouds can be described in
many ways. They can be classified by height or appearance, or even by the amount of area covered
vertically or horizontally. Clouds are classified into five categories: low-, mid-, and high-level clouds;
vertically developed clouds; and less common clouds.

LOW-LEVEL CLOUDS
1-71. Low-level clouds (0 to 6,500 feet) are either cumulus or stratus (Figures 1-1 and 1-2). Low-level
clouds are mostly composed of water droplets since their bases lie below 6,500 feet. When temperatures are
cold enough, these clouds may also contain ice particles and snow.

The two types of precipitating low-level clouds are nimbostratus and stratocumulus (Figure 1-3
and Figure 1-4).
—
Nimbostratus clouds are dark, low-level clouds accompanied by light to moderately falling
precipitation. The sun or moon is not visible through nimbostratus clouds, which
distinguishes them from mid-level altostratus clouds. Due to the fog and falling
precipitation commonly found beneath and around nimbostratus clouds, their bases are
typically diffuse and difficult to accurately determine.

1-14

TC 3-97.61

26 July 2012