Cloud descriptions include the form of the cloud as well as the height. These descriptions are grouped into five categories: four with Latin root names that describe their shape or form and a fifth group that includes all the rare or specialized forms. The four major categories are cirrus, cumulus, stratus and nimbus. Cirrus means “curl of hair”’; cumulus means “heap”; stratus means “layer” and nimbus means “rain”. Cirrus are among the most easily recognized forms. They are high, wispy clouds that look like feathers or hair. In fact, another name for cirrus is “mare’s tail.” They are a signal of good weather. Nimbostratus clouds, on the other hand, signal exactly what you would expect from the combination of “nimbus” or rain and “stratus” or layer. They are a dark, low layer of clouds accompanied by light to moderate precipitation. And cumulonimbus are those towering thunderheads that tend to bring severe weather.
Mary Knapp, Weather Data Library