Chapter 1

Cumulonimbus
1-80. These are much larger and more vertically developed than fair weather cumulus (see Figure 1-9).
They can exist as individual towers or form a line of towers called a squall line. Fueled by vigorous
convective updrafts, the tops of cumulonimbus clouds can reach 39,000 feet or higher. Lower levels of
cumulonimbus clouds consist mostly of water droplets while at higher elevations, where the temperatures
are well below freezing, ice crystals dominate the composition. Under favorable conditions, harmless fair
weather cumulus clouds can quickly develop into large cumulonimbus associated with powerful
thunderstorms known as super cells. Supercells are large thunderstorms with deep rotating updrafts and can
have a lifetime of several hours. Super cells produce frequent lightning, large hail, damaging winds, and
tornadoes. These storms tend to develop during the afternoon and early evening when the effects of heating
from the sun are the strongest.

OTHER CLOUD TYPES
1-81. These clouds are a collection of miscellaneous types that do not fit into the previous four groups.
They are orographic clouds, lenticulars, and contrails.

Orographic
1-82. These develop in response to the forced lifting of air by the earth’s topography. Air passing over a
mountain oscillates up and down as it moves downstream. Initially, stable air encounters a mountain, is
lifted upward, and cools. If the air cools to its saturation temperature during this process, the water vapor
condenses and becomes visible as a cloud. Upon reaching the mountain top, the air is heavier than the
environment and will sink down the other side, warming as it descends. Once the air returns to its original
height, it has the same buoyancy as the surrounding air. However, the air does not stop immediately
because it still has momentum carrying it downward. With continued descent, the air becomes warmer then
the surrounding air and accelerates back upwards towards its original height. Another name for this type of
cloud is the lenticular cloud.

Lenticular
1-83. These are cloud caps that often form above pinnacles and peaks, and usually indicate higher winds
aloft (Figure 1-10). Cloud caps with a lens shape, similar to a “flying saucer,” indicate extremely high
winds (over 40 knots). Lenticulars should always be watched for changes. If they grow and descend, bad
weather can be expected.

Contrails
1-84. These are clouds that are made by water vapor being inserted into the upper atmosphere by the
exhaust of jet engines (Figure 1-11). Contrails evaporate rapidly in fair weather. If it takes longer than two
hours for contrails to evaporate, then there is impending bad weather (usually about 24 hours prior
to a front).

1-18

TC 3-97.61

26 July 2012