Cloud A cloud is any concentration of gas, liquid droplets, or solid particles suspended as a distinct body in a gas or liquid. Generally, however, the term cloud is used to refer to the suspension of small ice or water particles in the Atmosphere. Cloud Formation – Clouds in the atmosphere form whenever the relative Humidity of an air mass, or parcel, reaches slightly more than 100 percent. This can occur for a number of reasons: the upward motion of air, which causes expansion and cooling, input of water from outside the parcel, or loss of heat by radiation. Among the major producers of the upward motion that results in clouds are the Low-Pressure systems with their cold, warm, and occluded Fronts, tropical disturbances such as Hurricanes, Cyclones, or Typhoons, and the lifting of air as it flows over hilly and mountainous terrain. The size of cloud droplets and ice crystals ranges from about 1 to 100 micrometers (4/100,000 to 4/1,000 in). Particles this small fall to the ground so slowly that they appear suspended in air, tending to move with the wind. The fall of larger particles, at much greater speeds, is called Precipitation. About 1 million cloud droplets, with an average radius of 10 micrometers (4/10,000 inch), are required to make a typical raindrop of 1 mm (4/100 inch). Cloud droplets can exist at temperatures below 0°C (32°F) and are then referred to as supercooled. When supercooled water and ice crystals occur at the same location, the ice grows at the expense of the water, and an ice cloud forms. This occurs because at a given temperature ice has a greater affinity than liquid water for water vapor. Cloud droplets and ice crystals first form on certain types of small particles of dust or other airborne materials. They are called condensation nuclei when water droplets are formed and ice nuclei when ice crystals result. The nuclei generally range in size from as small as 0.01 micrometer to about 1 micrometer (4/10,000,000 to 4/100,000 inch). The number of nuclei vary widely, depending on the source of the air mass in which the parcel is imbedded. The atmosphere over the ocean generally has the lowest number of nuclei, whereas polluted air has the highest. The more nuclei, and therefore the more water droplets or ice crystals, the slower the process of formation of precipitation-sized particles, because the competition for the available water is greater. Thus, although Rain often falls shortly after a cloud forms over the ocean, a much longer time is required over continental areas. Cloud Classification – Clouds are classed as warm if their temperature throughout is above 0°C (32°F) and cold if they extend to heights where temperatures are less than 0° C. Cold clouds containing both supercooled water and ice are defined as mixed clouds, clouds composed entirely of ice are said to be glaciated. Some cold clouds contain only supercooled water. These clouds are hazardous to aviation because the water, freezing on impact with an airplane, can cause ice to build up on the fuselage and wings. Clouds, defined in terms of their gross physical characteristics, can be classified as Stratiform or Cumuliform. Stratiform, or layered, clouds form when the upward motion is relatively uniform over an area, and cumuliform, or cottony, billowing clouds develop when upward and downward air currents are separated by fairly short distances. When clouds form at ground surface they are called Fog. Clouds that form in the middle Troposphere are called Altostratus and Altocumulus, and those in the upper troposphere are referred to as Cirrocumulus, Cirrostratus, or Cirrus. For those with bases in the lower troposphere, the terms Stratus and Cumulus are used. When precipitation is falling from these clouds, they are referred to with such terms as Nimbostratus or Cumulonimbus. Nimbostratus are the gray, leaden-sky clouds often produced by large-scale winter Cyclones in which precipitation is fairly steady and long-lasting. Cumulonimbus clouds, on the other hand, are associated with typical summertime Thunderstorms, in which rainfall is generally brief but heavy. A system of classifying clouds according to their physical characteristics has been devised by the World Meteorological Organization. Some of the more common cloud types are listed below: 1 Cirrus – A high-altitude cloud composed of narrow bands or patches of thin, generally white, fleecy parts, typically at an average height of 7 miles (11.3 kilometers), 2 Cirrocumulus – A high-altitude cloud composed of a series of small, regularly arranged cloudlets in the form of ripples or grains, typically at an average height of 5 miles (8 kilometers), 3 Cirrostratus – A high-altitude, thin hazy cloud, usually covering the sky and often producing a halo effect, typically at an average height of 6 miles (9.7 kilometers), 4 Altostratus – A somewhat high level, blue to grayish blue cloud that forms a sheet or layer, typically at an average height of 3.5 miles (5.6 kilometers), 5 Altocumulus – A fleecy cloud, usually a rounded mass, but which can change radically and unexpectedly, producing intermediate forms, typically at an average height of 2.5 miles (4 kilometers), 6 Cumulonimbus – An extremely dense, vertically developed cumulus with a relatively hazy outline and a glaciated top extending to great heights, usually producing heavy rains, thunderstorms, or hailstorms, typically at an average height of 4 miles (6.4 kilometers), 7 Cumulus – A dense, white, fluffy, flat-based cloud with a multiple rounded top and a well-defined outline, usually formed by the ascent of thermally unstable air masses, typically at an average height of 2 miles (3.2 kilometers), 8 Nimbus/Nimbostratus – A rain cloud, especially a low dark layer of clouds precipitating continuous rain or snow, typically at an average height of .25 mile (.4 kilometer), 9 Stratus – A low-altitude cloud formation consisting of a horizontal layer of gray clouds, typically at an average height of .25 mile (.4 kilometer), 10 Stratocumulus – A low-lying cloud formation occurring in extensive horizontal layers with rounded summits, typically at an average height of 1 mile (1.6 kilometers).