At first glance, most clouds probably seem to all be at roughly the elevation. They're high, and that's the end of the discussion, right?
Not necessarily. Indeed, some clouds are surprisingly low to the ground - fog, for example, is a cloud that's right in your face - and some clouds can be high enough that some airplanes would be hard pressed to penetrate their fluffy white exteriors. This latter class is known, quite simply, as high clouds.
High clouds form like any other cloud, though they do so at a much higher elevation: typically over 16,000 feet. Consequently these clouds typically look rather light and wispy compared to other types of clouds. Because they're found at such incredible heights high clouds are subjected to low temperatures regardless of the year, and consist exclusively of ice crystals, as the water vapor is too far from a heat source to become water droplets.
High clouds come in three different broad varieties: cirrus, cirrostratus and cirrocumulus.
- Cirrus clouds, informally known as mare's tails, are characterized by their thin strands accompanied by thick tufts. Their thin, insubstantial quality comes from the lack of moisture found in the upper portions of the troposphere required for larger clouds. It's possible for jet streams in the sky to become cirrus clouds as the exhaust leaves condensation behind.
- Cirrostratus clouds are a form of cirrus clouds that appear as thicker bands high in the sky. Overlapping one another, these clouds create a more even appearance that nevertheless retains more of a wispy, light quality than clouds in the lower levels of the troposphere. They can coat the sky entirely or appear in large patches.
- Cirrocumulus clouds appear as a series of small puffs in the sky, the cloud's structure mounting upon itself and resembling a cotton ball. They often appear in long, rippling lines, creating a wave-like effect that appears almost fuzzy.
Cirrus clouds are often the result of two different atmospheric developments. The first is that of an upper atmosphere disturbance, including the development of a storm or the approach of a weather front. Cirrus clouds often appear before, or in the wake of, major disturbances like storms, hurricanes and typhoons. The second, mentioned above, is that of contrails left behind by passing jet airplanes. This latter contribution of cirrus clouds is a continuing subject of environmental debate, as contrails are arguably bad for the environment.
Lloyd, Julie. A Pocket Guide to Weather. Parragon Books, 2007.
Wikipedia: Cirrus Clouds