General Theory of Clouds
Clouds are condensed water vapor, consisting of water droplets or ice crystals. They form when
the air becomes saturated either by being cooled to the dew point or through the addition of
moisture. Most clouds are the result of cooling from some lifting process, such as surface
heating. The excess moisture condenses on minute particles in the atmosphere, thus forming
Water vapor requires a surface on which to condense. An abundance of microscopic solid
particles, called condensation nuclei, are suspended in the air and provide condensation surfaces.
Condensation nuclei consist of dust, salt crystals from the sea, acid salts from industrial waste,
ash and soot from volcanoes and forest fires, rock particles from wind erosion, and organic
matter from forests and grass lands. The most effective condensation nuclei are the various salts
since they can induce condensation even when air is almost, but not completely, saturated.
TYPES OF CLOUDS
Clouds provide visible evidence of the atmosphere's motions, water content, and degree of
stability and are therefore weather signposts in the sky. They can be numerous, widespread,
form at very low levels, or show extensive vertical development.
Knowledge of principal cloud types helps the aircrew member when being briefed to visualize
expected weather conditions. Additionally, knowledge of cloud types helps the pilot recognize
potential weather hazards in flight. Clouds are classified according to their appearance, form,
and altitude of their bases, and may be divided into four groups:
Low clouds, ranging from just above the surface to 6500 feet AGL.
Middle clouds with bases between 6500 and 20,000 feet AGL.
High clouds with bases usually above 16,000 feet AGL.
Special clouds with extensive vertical development.
The height of the cloud base, not the top, determines the classification. A cloud with a base at
5000 feet AGL and a top at 8000 feet AGL is classified as a low cloud. Each group is
subdivided by appearance. There are two principal cloud forms:
Cumuliform A lumpy, billowy cloud with a base showing a definite pattern or structure.
Stratiform A cloud with a uniform base, formed in horizontal, sheet-like layers.
2-16 Atmospheric Mechanics of Winds, Clouds and Moisture, and Atmospheric Stability