Clouds and Cloud Types
Clouds and cloud types
Clouds are condensations of water and other particles in the atmosphere. Cloud shapes—and the dynamics of their formation—are accurate indicators of important atmospheric properties, including air stability, moisture content, and motion.
Clouds are divided into families of high level, middle level, low level, and vertically developing clouds, and are classified again, in accord with their general shape (e.g., cumuliform or stratoform)
High level clouds include cirrus, cirrostratus, and cirrocumulus clouds that occur at altitudes between 16,000 and 45,000 feet. Middle level clouds include altostratus, altocumulus, and nimbostratus clouds that occur between 6500 and 22,000 feet. Low-level clouds include stratus and stratocumulus clouds that occur between the surface and 6,500 feet. Vertical development clouds include cumulus and cumulonimbus clouds, and range in their development from the surface to 45,000 feet. The heights of the bases of the clouds used to designate cloud families can vary with latitude . At extreme northern or southern latitudes, high altitude family clouds can be observed at much lower altitudes.
In general, cloud shape is determined by the method of cooling to reach condensation and the forces of winds that can shear or tear the cloud. Cloud opacity (i.e., whether it is light or dark) is a function of cloud thickness.
Cirrus clouds occur at high levels and are generally wispy and elongate in form. Vertically rising air is unstable and gives rise to cumulus cloud formation. Cumulus clouds are billowy. Stratus clouds (i.e., stratified clouds) are heavily layered and often appear in sheet-like formations. With regard to cloud nomenclature, nimbus clouds (e.g., clouds with the prefix nimbo or the suffix nimbus) are rain-producing clouds. The use of "fracto" designates broken cloud formations.
High clouds—cirrus, cirrostratus, and cirrocumulus—are composed of ice crystals and dust or pollution particles. The particles often serve as centers of crystallization or condensation nuclei. Cirrus clouds often produce "mares' tails" that are tail-like wisps of ice crystals. Cirrostratus clouds, because they are thin and the ice crystals act to both reflect and refract sunlight, are often associated with halos of ice crystals that appear to encircle the Sun or Moon . Cirrocumulus clouds often appear as patch-like thin clouds.
Middle level cloudsmdash; altostratus, altocumulus, and nimbostratus—are composed of water with some ice crystal formation near cloud tops. Both middle level and low level clouds may be composed of super-cooled water (water below freezing ) that has not yet crystallized around a condensation nucleus. Altostratus clouds often present a bluish-layered appearance. Depending on thickness, altocumulus clouds often have white or gray layers that appear in washboard or wave-like formations. Atmospheric instability and convective air currents can result in the formation of altocumulus castellanus clouds, a form of altocumulus that often appear as isolated cumulous clouds with billowing tops. Another form of altocumulus cloud, a standing lenticular altocumulus clouds, is formed by turbulent updrafts of air uplifted by terrain barriers (e.g., mountains, ridges, etc.). Although dynamic, the standing lenticular altocumulus cloud formations appear static or "standing" over the terrain feature leading to their formation. Nimbostratus clouds often appear as heavy, gray, moistureladen cloud layers
Low-level stratus clouds are usually gray clouds associated with precipitation and fog . Stratocumulus clouds present the familiar, cotton ball-like cumulus shapes in an elongate form (a cumulus shape drawn out by shearing winds).
Clouds with extensive vertical development—cumulus and cumulonimbus clouds—often present a gradient of ice and water. Rapid updrafts and downdrafts allow ice crystals to appear at much lower levels than would be expected by atmospheric temperature . Although arising from convective currents, cumulus clouds often form in fair weather and do not show extensive vertical development. Cumulus clouds present flat bases and curved or domed tops. More extensive vertical development occurs as atmospheric instability increases. Highly developed cumulus clouds often present mushroomed or cauliflower-like tops, and can ultimately produce rain. Under the most unstable of atmospheric conditions, cumulonimbus clouds form. Cumulonimbus clouds are dark clouds with anvil like tops sheared by very high altitude winds. Heavy turbulence, violent rains, lightning , and thunder often accompany cumulonimbus clouds. Particularly unstable and violent clouds can occur in cells capable of spawning tornadoes.
The identification of cloud types is an important skill for aviators and aviation meteorologists because clouds present variable icing hazards. Ice formation can drastically reduce the effectiveness of airfoils (wings, flaps, rudder, ailerons, elevators) and destroy lift and/or interfere with the ability to control aircraft.
See also Atmospheric circulation; Atmospheric composition and structure; Atmospheric inversion layers; Atmospheric lapse rate; Atmospheric pressure; Phase state changes; Troposphere and tropopause; Weather forecasting methods; Weather forecasting; Weather radar; Wind shear