Clouds are classified into twelve types. The names used for the clouds are based on three factors: the altitude at which the cloud occurs, the shape of the cloud, and whether the cloud is producing precipitation.
There are three altitude ranges, or cloud levels. The height of the cloud base determines a cloud’s level. Clouds with a base below 2,000 meters are considered low-level clouds. Clouds with a base between 2,000 and 6,000 meters are mid-level clouds. Those with a base above 6,000 meters are considered high-level clouds.
In 1803, Luke Howard classified four main cloud types with Latin terms. Cumulus means “pile” and describes heaped, lumpy clouds. Cirrus means “curl of hair” and is used to name clouds that look like wispy locks of hair. Featureless clouds that form sheets are named stratus, meaning “layer.” Howard used the term nimbus, which means “cloud,” to name low, gray rain clouds.
A good way to determine the level of cumulus clouds is to assess the size of the individual cloud elements. Low-level cumulus clouds are about the same size, or larger than, your fist held at arms’ length. One exception to this rule is when a small cumulus cloud is developing or evaporating. In that case, its direction or speed of motion may indicate that it is in the same layer as nearby larger cumulus clouds. Mid-level cumulus clouds are farther away and the individual cloud pieces appear substantially smaller, about the size of your thumb at arms’ length. High-level cumulus clouds are smaller still, with individual cloud pieces about the size of the nail on your little finger at arms’ length.
Stratus clouds have no distinct cloud pieces to measure. For these clouds, a general rule is that cloud opacity tends to decrease with height. Thus, low-level clouds are generally thicker than mid-level clouds, and a high-level cirrostratus is very thin. Thus, by observing how much the cloud obscures the Sun, you can estimate the level of a stratus cloud.
If there is precipitation, the chances are very good that you are dealing with a low-level cloud. Mid-level clouds occasionally precipitate, but this is a rare occurrence.
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TEXT CREDIT: NASA Langley Science Directorate