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.
Cumulus clouds are clouds that have flat bases and are often described as puffy, cotton-like, or fluffy in appearance. Their name derives from the Latin cumulus, meaning "heap" or "pile". Cumulus clouds are low-level clouds, generally less than 2,000 m (6,600 ft) in altitude unless they are the more vertical cumulus congestus form. Cumulus clouds may appear by themselves, in lines, or in clusters.
Altocumulus is a middle-altitude cloud genus that belongs mainly to the stratocumuliform physical category characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. However, if the layers become tufted in appearance due to increased airmass instability, then the altocumulus clouds become more purely cumuliform in structure. Like other cumuliform and stratocumuliform clouds, altocumulus signifies convection. A sheet of partially conjoined altocumulus perlucidus is sometimes found preceding a weakening warm front, where the altostratus is starting to fragment, resulting in patches of altocumulus perlucidus between the areas of altostratus. Altocumulus is also commonly found between the warm and cold fronts in a depression, although this is often hidden by lower clouds.
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 multilevel, 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-altitude, 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 formed by rising thermals. The term stratus describes 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. Some call these clouds "high fog" for their fog-like form.
The Morning Glory cloud is a rare meteorological phenomenon consisting of a low-level atmospheric solitary wave and associated cloud, occasionally observed in different locations around the world. The wave often occurs as an amplitude-ordered series of waves forming bands of roll clouds.
Cirrocumulus undulatus is a variety of cirrocumulus cloud. The name cirrocumulus undulatus is derived from Latin, meaning "diversified as with waves". They have a rippled appearance due to wind shear and usually cover only a small portion of the sky. They appear in bands as small patches or layers. Occasionally, they comprise two or more wave forms superposed upon one another. The individual cloudlets can either be circular or elongated in the direction of the rows.
The altostratus undulatus is a type of altostratus cloud with signature undulations within it. These undulations may be visible, but frequently they are indiscernible to the naked eye. These formations will generally appear in the early stages of destabilizing return flows, especially over the southern plains of the United States, when the surface temperature is still relatively cool. The wavy strips of clouds are generally near an inversion surface.
The altocumulus undulatus is a mid-level cloud, usually white or grey with layers or patches containing undulations that resemble "waves" or "ripples" in water. Elements within the cloud are generally darker than those in cirrocumulus and smaller than those in stratocumulus. These clouds may appear both as patches or as covering the sky. The width of these clouds is generally less than 300 feet thick. The presence of altocumulus undulatus may indicate precipitation within the next 20 hours or simply an overcast day.
Gavin Edmund Pretor-Pinney is a British author, known for his books The Cloudspotter's Guide and The Wavewatcher's Companion.
The cumulonimbus flammagenitus cloud (CbFg), also known as the pyrocumulonimbus cloud, is a type of cumulonimbus cloud that forms above a source of heat, such as a wildfire or volcanic eruption, and may sometimes even extinguish the fire that formed it. It is the most extreme manifestation of a flammagenitus cloud. According to the American Meteorological Society’s Glossary of Meteorology, a flammagenitus is "a cumulus cloud formed by a rising thermal from a fire, or enhanced by buoyant plume emissions from an industrial combustion process."
The Cloud Appreciation Society is a society founded by Gavin Pretor-Pinney from the United Kingdom in January 2005. The society aims to foster understanding and appreciation of clouds, and has over 60,000 members from 120 countries, as of September 2023.
A fallstreak hole is a large gap, usually circular or elliptical, that can appear in cirrocumulus or altocumulus clouds. The holes are caused by supercooled water in the clouds suddenly evaporating or freezing, and may be triggered by passing aircraft.
Lieutenant Colonel William Henry Rankin was one of only two known persons to survive a fall from the top of a cumulonimbus thunderstorm cloud; the other was Ewa Wiśnierska. He was a pilot in the United States Marine Corps and a World War II and Korean War veteran. He was flying an F-8 Crusader jet fighter over a cumulonimbus cloud when the engine failed, forcing him to eject and parachute into the cloud. Rankin wrote a book about his experience, The Man Who Rode the Thunder.
The International Cloud Atlas or simply the Cloud Atlas, is a cloud atlas that was first published in 1896 and has remained in print since. Its initial purposes included aiding the training of meteorologists and promoting more consistent use of vocabulary describing clouds, which were both important for early weather forecasting. The first edition featured color plates of color photographs, then still a very new technology, but noted for being expensive. Numerous later editions have been published.
Stratocumulus lenticularis is an uncommon cloud type that belongs to the stratocumulus cloud type, its appearance is that of a flat lens or almond. It forms as a result of gravity waves caused by wind passing over obstacles; for example, a mountain or a building. They are more common in locations that are hilly, or places where foehn winds are common They tend to look more well-defined when a foehn wind is causing their formation. They also sometimes show iridescence. It is a type of lenticular cloud.
