Almost everyone watches clouds. Clouds are among the most fascinating of all weather phenomenon. They are formed through the process of condensation when water vapor rises into the atmosphere where it cools and condenses into cloud forms. Different types of clouds form under different atmospheric conditions. Some clouds look like fluffy cotton balls, some warn us of approaching storms, and others bring rain.
Rain clouds are called nimbus clouds. Nimbus is an ancient Latin word meaning "rain storm." Nimbus clouds carry an immense amount of moisture, giving them a gray color. The gray color can sometimes be very dark. Besides rain, nimbus clouds are responsible for other precipitation such as hail and snow.
The prefix "nimbo" or suffix "nimbus" designate a rain cloud. The common nimbus clouds are "nimbostratus" and "cumulonimbus."
A cumulonimbus cloud is a precipitating cumulus cloud. Cumulonimbus clouds, sometimes called "thunderheads," are associated with thunderstorms, lightning and intense, heavy rains. Cumulonimbus clouds grow vertically and are recognized by their anvil shape, with a low, dark base often only 1,000 feet above ground and tops reaching up to 50,000 feet into the atmosphere.
Cumulonimbus clouds carry a mass of unstable air and often produce unpredictable high winds and downdrafts. These clouds are capable of generating violent supercell storms, tornadoes and dangerous wind shear conditions.
When you look to the sky on a rainy day and see nothing but a dense blanket of featureless, low gray clouds, you are looking at nimbostratus clouds. These are clouds that form at low altitudes and block sunlight. In contrast with the short-lived intense rains associated with unstable cumulonimbus clouds, nimbostratus clouds typically produce light or moderate rainfall of longer duration.
Because nimbostratus clouds form with their bases below 6,500 feet, they normally contain water droplets, but can contain snow or ice if temperatures are low enough.
Most cloud types do not contain rain and are often identified by the altitudes where they form.
Low-altitude clouds include stratus, cumulus and stratocumulus clouds. Mid-level clouds are designated with a prefix of alto and include altocumulus and altostratus clouds. The highest altitude clouds are called cirrus clouds and include cirrocumulus, cirrostratus and cirrus clouds. These are the clouds that look like wispy feathers.
- "Understanding Weather and Climate (2nd Edition)"; Edward Aguado, James E. Burt, James Burt; 2000
- University of Wisconsin: Introduction to Weather and Climate
- Weather Questions: How Do Clouds Form?