Cirrus is a genus of high cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. Cirrus are usually formed when warm, dry air rises, causing water vapor deposition onto rocky or metallic dust particles at high altitudes. Globally, they form anywhere between 4,000 and 20,000 meters above sea level, with the higher elevations usually in the tropics and the lower elevations in more polar regions.
In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may compose the droplets and crystals. On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture from an adjacent source to raise the dew point to the ambient temperature.
Cumulonimbus is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents. Above the lower portions of the cumulonimbus the water vapor becomes ice crystals, such as snow and graupel, the interaction of which can lead to hail and to lightning formation, respectively. When occurring as a thunderstorm these clouds may be referred to as thunderheads. Cumulonimbus can form alone, in clusters, or along squall lines. These clouds are capable of producing lightning and other dangerous severe weather, such as tornadoes, hazardous winds, and large hailstones. Cumulonimbus progress from overdeveloped cumulus congestus clouds and may further develop as part of a supercell. Cumulonimbus is abbreviated Cb.
Altostratus is a middle-altitude cloud genus made up of water droplets, ice crystals, or a mixture of the two. Altostratus clouds are formed when large masses of warm, moist air rise, causing water vapor to condense. Altostratus clouds are usually gray or blueish featureless sheets, although some variants have wavy or banded bases. The sun can be seen through thinner altostratus clouds, but thicker layers can be quite opaque.
A stratocumulus cloud, occasionally called a cumulostratus, belongs to a genus-type of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumulus, and the whole being at a lower height, usually below 2,000 metres (6,600 ft). Weak convective currents create shallow cloud layers because of drier, stable air above preventing continued vertical development. Historically, in English, this type of cloud has been referred to as a twain cloud for being a combination of two types of clouds.
A nimbostratus cloud is a multi-level, amorphous, nearly uniform and often dark grey cloud that usually produces continuous rain, snow or sleet but no lightning or thunder.
Cirrocumulus is one of the three main genus-types of high-altitude tropospheric clouds, the other two being cirrus and cirrostratus. They usually occur at an altitude of 5 to 12 km. Like lower-altitude cumuliform and stratocumuliform clouds, cirrocumulus signifies convection. Unlike other high-altitude tropospheric clouds like cirrus and cirrostratus, cirrocumulus includes a small amount of liquid water droplets, although these are in a supercooled state. Ice crystals are the predominant component, and typically, the ice crystals cause the supercooled water drops in the cloud to rapidly freeze, transforming the cirrocumulus into cirrostratus. This process can also produce precipitation in the form of a virga consisting of ice or snow. Thus, cirrocumulus clouds are usually short-lived. They usually only form as part of a short-lived transitional phase within an area of cirrus clouds and can also form briefly as a result of the breaking up of part of a cumulonimbus anvil.
Cirrostratus is a high-level, very thin, generally uniform stratiform genus-type of cloud. It is made out of ice-crystals, which are pieces of frozen water. It is difficult to detect and it can make halos. These are made when the cloud takes the form of thin cirrostratus nebulosus. The cloud has a fibrous texture with no halos if it is thicker cirrostratus fibratus. On the approach of a frontal system, the cirrostratus often begins as nebulous and turns to fibratus. If the cirrostratus begins as fragmented of clouds in the sky it often means the front is weak. Cirrostratus is usually located above 5.5 km (18,000 ft). Its presence indicates a large amount of moisture in the upper troposphere. Clouds resembling cirrostratus occasionally form in polar regions of the lower stratosphere. Polar stratospheric clouds can take on this appearance when composed of tiny supercooled droplets of water or nitric acid.
Stratus clouds are low-level clouds characterized by horizontal layering with a uniform base, as opposed to convective or cumuliform clouds that are formed by rising thermals. More specifically, the term stratus is used to describe flat, hazy, featureless clouds at low altitudes varying in color from dark gray to nearly white. The word stratus comes from the Latin prefix strato-, meaning "layer". Stratus clouds may produce a light drizzle or a small amount of snow. These clouds are essentially above-ground fog formed either through the lifting of morning fog or through cold air moving at low altitudes over a region. Some call these clouds "high fog" for their fog-like form. While light rain may fall, this cloud does not indicate much meteorological precipitation.
Cirrus castellanus or Cirrus castellatus is a species of cirrus cloud. Its name comes from the word castellanus, which means of a fort, of a castle in Latin. Like all cirrus, this species occurs at high altitudes. It appears as separate turrets rising from a lower-level cloud base. Often these cloud turrets form in lines, and they can be taller than they are wide. This cloud species is usually dense in formation.
Cirrus intortus is a variety of cirrus cloud. The name cirrus intortus is derived from Latin, meaning "twisted, wound". The variety of intortus clouds is specific to cirrus clouds, and they appear as interwound strands of cirrus clouds with a purely random pattern. The filaments are often curved in a very irregular pattern.
Cirrus fibratus or also called Cirrus filosus is a species of cirrus cloud. The name cirrus fibratus is derived from Latin, meaning "fibrous". These clouds are similar to cirrus uncinus, commonly known as "mares' tails"; however, fibratus clouds do not have tufts or hooks at the end. The filaments are usually separate from one another.
Cirrus floccus is a type of cirrus cloud. The name cirrus floccus is derived from Latin, meaning "a lock of wool". Cirrus floccus occurs as small tufts of cloud, usually with a ragged base. The cloud can have virga falling from it, but the precipitation does not reach the ground. The individual tufts are usually isolated from each other. At formation, the cirrus floccus clouds are bright white and can be mistaken for altocumulus clouds; however, after a few minutes, the brightness begins to fade, indicating they are made up of pure ice, and are therefore at a higher level.
Cirrus duplicatus is a variety of cirrus cloud. The name cirrus duplicatus is derived from Latin, meaning "double". The duplicatus variety of cirrus clouds occurs when there are at least two layers of cirrus clouds. Most of the time, occurrences of cirrus fibratus and cirrus uncinus are in the duplicatus form. Like stratus clouds, cirrus clouds are often seen in the duplicatus form.
Cirrus radiatus is a variety of cirrus cloud. The name cirrus radiatus is derived from Latin, meaning "rayed, striped". This variety of cirrus clouds occurs in parallel bands that often cover the entire sky and appear to converge at a single point or two opposite points on the horizon. Cirrus radiatus is often partly made up of cirrocumulus or cirrostratus.
Cirrus vertebratus is a type of cirrus cloud. The name cirrus vertebratus is derived from Latin, meaning "jointed, articulated, vertebrated". Like cirrus intortus, the vertebratus species is exclusive to the cirrus genus. Cirrus vertebratus gives the impression of vertebrae in a spinal column, ribs, or a fish skeleton.
Cirrocumulus floccus is a type of cirrocumulus cloud. The name cirrocumulus floccus is derived from Latin, meaning "a lock of wool". Cirrocumulus floccus appears as small tufts of cloud with rounded heads, but ragged bottoms. The cloud can produce virga, precipitation that evaporates before reaching the ground. Like cirrocumulus castellanus, cirrocumulus floccus is an indicator of atmospheric instability at the level of the cloud. In fact, cirrocumulus floccus can form from cirrocumulus castellanus, being the evolutionary state after the base of the original cloud has dissipated.
Cirrostratus nebulosus is a type of high-level cirrostratus cloud. The name cirrostratus nebulosus is derived from Latin, the adjective nebulosus meaning "full of vapor, foggy, cloudy, dark". Cirrostratus nebulosus is one of the two most common forms that cirrostratus often takes, with the other being cirrostratus fibratus. The nebulosus species is featureless and uniform, while the fibratus species has a fibrous appearance. Cirrostratus nebulosus are formed by gently rising air. The cloud is often hard to see unless the sun shines through it at the correct angle, forming a halo. While usually very light, the cloud may also be very dense, and the exact appearance of the cloud can vary from one formation to another. In the winter, precipitation often follows behind these clouds; however, they are not a precipitation-producing cloud.
A homogenitus, anthropogenic or artificial cloud is a cloud induced by human activity. Although most clouds covering the sky have a purely natural origin, since the beginning of the Industrial Revolution, the use of fossil fuels and water vapor and other gases emitted by nuclear, thermal and geothermal power plants yield significant alterations of the local weather conditions. These new atmospheric conditions can thus enhance cloud formation.
