Clouds (continued)

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Clouds often indicate an upward movement of air. As air rises, it becomes less dense and the air temperature lowers from 2 to 5 degrees Fahrenheit (1 to 3 degrees Celsius) for each 1,000 feet of height. This lowering of the air temperature increases the humidity. At 100% humidity, the air cannot hold any additional water vapor. At that point, water droplets (or if the air temperature is below 32 degrees F, or 0 degrees C, ice crystals) are produced, forming clouds. Fog is simply a cloud that is at the surface of the earth.

Types of Clouds

Each cloud has unique properties that meteorologists use to classify clouds. They have identified three basic types of clouds: cumulus, stratus, and cirrus.

Cumulus clouds are low-altitude, billowy clouds that are usually dark on the bottom while the top resembles a giant white cotton ball. Small cumulus clouds can indicate “fair skies.” A cumulus cloud may be relatively tall, extending from a base at around 2,000 feet to a top near 10,000 feet above the ground. Large cumulus clouds cast dark shadows and may be a source of moisture, but generally produce no more than a summer shower.

Stratus clouds are wispy foglike clouds that hover a few hundred feet above the ground, sometimes obscuring hills or tall buildings. They may begin as ground fog and can be a source of drizzle. They tend to cover most, if not all, of the sky.

Cirrus clouds are generally the highest clouds, forming “mares’ tails” at altitudes between 20,000 and 40,000 feet. The clouds may appear as delicate white filaments, feather-like tufts, or fibrous bands of ice crystals. Cirrus clouds are not a source of precipitation.

Because clouds actually are much more diverse than just the three basic types discussed above, meteorologists also classify clouds by identifying characteristics that include properties of two of the basic forms of clouds. For example, cirrocumulus clouds have properties of both cirrus and cumulus clouds. They have the loosely packed sheets of small white cloud segments often associated with cumulus clouds, but these segments can dominate and cover the sky, as is typically seen with stratus clouds. Cirrostratus clouds show the characteristics of both cirrus and stratus clouds.

Clouds that are delivering rain are often given the suffix -nimbus. For instance, a cumulonimbus cloud is a large cumulus cloud associated with thunderstorm clouds. These clouds can vary considerably in altitude from ominously dark lower portions below 5,000 feet to white anvil-shaped tops that may reach upward to 50,000 feet. They contain large amounts of moisture, some of which may be in the form of hail. The cumulonimbus cloud may appear alone or as part of a wall of advancing storm clouds.