Chapter 1

WARM FRONT
1-87. A warm front occurs when warm air moves into and over a slower or stationary cold air mass.
Because warm air is less dense, it will rise up and over the cooler air. The cloud types seen when a warm
front approaches are cirrus, cirrostratus, nimbostratus (producing rain), and fog. Occasionally,
cumulonimbus clouds will be seen during the summer months.

COLD FRONT
1-88. A cold front occurs when a cold air mass overtakes a slower or stationary warm air mass. Cold air,
being more dense than warm air, will force the warm air up. Clouds observed will be cirrus, cumulus, and
then cumulonimbus producing a short period of showers.

OCCLUDED FRONT
1-89. Cold fronts generally move faster than warm fronts. The cold fronts eventually overtake warm fronts
and the warm air becomes progressively lifted from the surface. The zone of division between cold air
ahead and cold air behind is called a cold occlusion. If the air behind the front is warmer than the air ahead,
it is a warm occlusion. Most land areas experience more occlusions than other types of fronts. The cloud
progression observed will be cirrus, cirrostratus, altostratus, and nimbostratus. Precipitation can be from
light to heavy.

STATIONARY FRONT
1-90. A stationary front is a zone with no significant air movement. When a warm or cold front stops
moving, it becomes a stationary front. Once this boundary begins forward motion, it once again becomes a
warm or cold front. When crossing from one side of a stationary front to another, there is typically a
noticeable temperature change and shift in wind direction. The weather is usually clear to partly cloudy
along the stationary front.

TEMPERATURE
1-91. Normally, a temperature drop of 3 to 5 degrees Fahrenheit for every 1,000 feet gain in altitude is
encountered in motionless air. For air moving up a mountain with condensation occurring (clouds, fog, and
precipitation), the temperature of the air drops 3.2 degrees Fahrenheit with every 1,000 feet of elevation
gain. For air moving up a mountain with no clouds forming, the temperature of the air drops 5.5 degrees
Fahrenheit for every 1,000 feet of elevation gain.
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An expedient to this often occurs on cold, clear, calm mornings. During a troop movement or
climb started in a valley, higher temperatures may often be encountered as altitude is gained.
This reversal of the normal cooling with elevation is called temperature inversion. Temperature
inversions are caused when mountain air is cooled by ice, snow, and heat loss through thermal
radiation. This cooler, denser air settles into the valleys and low areas. The inversion continues
until the sun warms the surface of the earth or a moderate wind causes a mixing of the warm and
cold layers. Temperature inversions are common in the mountainous regions of the arctic,
subarctic, and mid-latitudes.

At high altitudes, solar heating is responsible for the greatest temperature contrasts. More
sunshine and solar heat are received above the clouds than below. The important effect of
altitude is that the sun’s rays pass through less of the atmosphere and more direct heat is
received than at lower levels, where solar radiation is absorbed and reflected by dust and water
vapor. Differences of 40 to 50 degrees Fahrenheit may occur between surface temperatures in
the shade and surface temperatures in the sun. This is particularly true for dark metallic objects.
The difference in temperature felt on the skin between the sun and shade is normally 7 degrees
Fahrenheit. Special care must be taken to avoid sunburn and snow blindness. Besides permitting
rapid heating, the clear air at high altitudes also favors rapid cooling at night. Consequently, the
temperature rises fast after sunrise and drops quickly after sunset. Much of the chilled air drains
downward, due to convection currents, so that the differences between day and night
temperatures are greater in valleys than on slopes.

1-20

TC 3-97.61

26 July 2012