clouds In 1803 Luke Howard developed a classification system for naming different cloud types. Many of the names that he gave to clouds are still in use by surface observers today. There are ten distinguishable types, called genera, which have a Latin name and a recognized abbreviation. The identification of a cloud as of a particular type is based on a subtle blend of its appearance and altitude. The basic cloud forms are cumulus, which are heaped clouds; stratus, which are layer clouds; and cirrus, which are wispy. If the cloud is raining then the term ‘nimbus’ is used, as in cumulonimbus and nimbostratus. Further extensions to the genera names, called species, provide a fuller description. Apart from some notable exceptions in the stratosphere, clouds form in the lowest layer of the atmosphere, that is, in the troposphere. Although clouds can form at any altitude in the troposphere, they are usually assigned to one of three levels, low, middle, or high, which further aids classification. The exact height of these groups varies with latitude and season. Some clouds, such as cumulonimbus, develop vertically beyond one level. This often leads to a fourth group being defined: those clouds with vertical extent. Table 1 lists the ten genera together with their abbreviation and height of the cloud bases above sea level.

The appearance that clouds take is determined by the mechanism which forms them. Clouds form when water vapour, which is always present in the atmosphere, condenses out. Condensation occurs when the air is cooled to such a point that the air is saturated with respect to water vapour. Further cooling results in liquid water or ice. Condensation in clean air is very difficult to achieve, and air can easily become supersaturated under such conditions. Most condensation takes place on aerosols which are hygroscopic (water-absorbing). Naturally occurring aerosols are abundant in the Earth's atmosphere, for example in the form of dust and salt from sea spray. Pollution from human activity also contributes suitable aerosols for cloud formation. Cooling of the air to produce clouds can occur in a variety of ways. The most common is when air rises; the pressure decreases, and so the air expands, doing work and expending energy and therefore cooling. Small parcels of air rising because the atmosphere is unstable result in cumulus clouds. Clouds can result when air is forced to rise over mountains. Large-scale uplift, as seen at a front (the intersection of two air masses with different characteristics) creates predominantly stratus clouds. Other cooling mechanisms, such as warm moist air being cooled when it comes into contact with a cold surface, can result in fog. Lower clouds are predominantly composed of liquid water droplets, but high clouds such as cirrus are composed entirely of ice crystals.

1. Cloud genera

Frances Drake

Bibliography

Bohren, C. F. (1987) Clouds in a glass of beer: simple experiments in atmospheric physics. John Wiley and Sons, New York.
Scorer, R. (1986) Cloud investigation by satellite. Ellis Horwood, Chichester.

Clouds

Clouds are made up of minute water droplets or ice crystals that condense in the atmosphere. The creation of a cloud begins at ground level. As the Sun heats Earth's surface, the warmed ground heats the surrounding air, which then rises. This air contains variable amounts of water vapor that has evaporated from bodies of water and plants on Earth's surface. As the warmed ground-level air rises, it expands, cooling in the process. When the cooled air reaches a certain temperature, called the dew point, the water vapor in the air condenses into tiny microscopic droplets, forming a cloud. If condensation occurs below the freezing point (32°F; 0°C), ice crystals form the cloud. Clouds appear white because sunlight reflects off the water droplets. Thick clouds appear darker at the bottom because sunlight is partially blocked.

Classification

English scientist Luke Howard (1772–1864) developed a system to classify clouds in 1803. He grouped clouds into three major types: cumulus (piled up heaps and puffs), cirrus (fibrous and curly), and stratus (stretched out and layered). To further describe clouds, he combined these terms and added descriptive prefixes, such as alto (high) and nimbus (rain).

The International Cloud Classification presently recognizes ten forms of clouds, which are grouped into four height categories. Low-level clouds range from ground level to 6,500 feet (2,000 meters); mid-level from 6,500 to 20,000 feet (2,000 to 6,100 meters); high-level from 20,000 to 40,000 feet (6,100 to 12,200 meters); and vertical from 1,600 to 20,000 feet (490 to 6,100 meters).

Low-level clouds: Stratus, nimbostratus, stratocumulus. There are three forms of low-level clouds. Stratus clouds, the lowest, blanket the sky and usually appear gray. They form when a large moist air mass slowly rises and condenses. Fog is a stratus cloud at ground level. Nimbostratus clouds are thick, darker versions of stratus clouds. They usually produce continuous rain or snow. Stratocumulus clouds are large, grayish masses, spread out in a puffy layer. Sometimes they appear as rolls. If they are thick enough, stratocumulus will produce light precipitation.

Middle-level clouds: Altostratus, altocumulus. The two forms of mid-level clouds have the prefix "alto" added to their names. Altostratus clouds appear as a uniform blue or gray sheet covering all or almost all areas of the sky. The Sun or the Moon may be totally covered or shine through very weakly. These clouds are usually layered, with ice crystals at the top, ice and snow in the middle, and water droplets at the bottom. Altostratus clouds yield very light precipitation. Altocumulus are dense, fluffy white or grey balls or masses. When closely bunched together, they appear like fish scales across the sky: this effect is called a mackerel sky.

High-level clouds: Cirrus, cirrostratus, cirrocumulus. The three forms of high-level clouds are called cirrus or have the prefix "cirro" added to their names. Cirrus clouds, the highest, are made completely of ice crystals (or needles of ice) because they form where freezing temperatures prevail. Cirrus clouds are often called mares' tails because of their white, feathery or wispy appearance. Cirrostratus clouds are also made completely of ice crystals. They usually cover the sky as a thin veil or sheet of white. These clouds are responsible for the halos that occur around the Sun or the Moon. Cirrocumulus clouds, the least common clouds, are small roundish masses, often having a rippled appearance. These clouds usually cover a large area. They are made of either ice crystals or supercooled water droplets (droplets that stay in liquid form below the freezing point).

Vertical clouds: Cumulus, cumulonimbus. Two forms of clouds can extend thousands of feet in height. Flat-based cumulus clouds are vertically thick and appear puffy, like heaps of mashed potatoes or heads of cauliflower. They form when a column of warm air rises, expands, cools, and condenses. Low-level cumulus clouds generally indicate fair weather, but taller cumulus can produce moderate to heavy showers. Cumulonimbus clouds are thunderstorm clouds, rising in the air like a tower or mountain. The peak of a mature cumulonimbus resembles the flattened shape of an anvil. Because they often contain powerful updrafts and downdrafts, cumulonimbus can create violent storms of rain, hail, or snow.

[See also Precipitation; Weather forecasting ]

cloud / kloud/ • n. 1. a visible mass of condensed water vapor floating in the atmosphere, typically high above the ground. ∎  an indistinct or billowing mass, esp. of smoke or dust: a cloud of dust. ∎  a large number of insects or birds moving together: clouds of orange butterflies. ∎  a vague patch of color in or on a liquid or transparent surface. 2. fig. a state or cause of gloom, suspicion, trouble, or worry: a black cloud hung over their lives. ∎  a frowning or depressed look. • v. 1. [intr.] (of the sky) become overcast with clouds: the blue skies clouded over abruptly. ∎  [tr.] (usu. be clouded) darken (the sky) with clouds. ∎  make or become less clear or transparent: [tr.] blood pumped out, clouding the water | [intr.] her eyes clouded with tears. 2. fig. make or become darkened or overshadowed, in particular: ∎  [intr.] (of someone's face or eyes) show worry, sorrow, or anger: his expression clouded over. ∎  [tr.] (of such an emotion) show in (someone's face). ∎  [tr.] make (a matter or mental process) unclear or uncertain; confuse: don't allow your personal feelings to cloud your judgment. ∎  [tr.] spoil or mar (something). PHRASES: every cloud has a silver liningsee silver. have one's head in the clouds (of a person) be out of touch with reality; be daydreaming. in the clouds out of touch with reality: this clergyman was in the clouds. on cloud nine extremely happy. under a cloud under suspicion; discredited: he left under something of a cloud, accused of misappropriating funds.DERIVATIVES: cloud·less adj. cloud·less·ly adv. cloud·let / -lət/ n.

cloud Masses of water particles or ice crystals suspended in the lower atmosphere. Clouds are formed when water from the Earth's surface becomes vapour through evaporation. As the water vapour rises, it cools and condenses around microscopic salt and dust particles, forming droplets. There are ten different classifications of clouds: cirrus are high (above 6000m/20,000ft), white and thread-like. Cirrocumulus are also high clouds, but are often thin sheets. Cirrostratus are white, almost transparent, sheets. Altocumulus are greyish-white globular clouds found between 2400m (8000ft) and 6000m (20,000ft). Altostratus are grey/blue and streaky, and often cover the whole sky. Nimbostratus are low, thick and dark, and shed rain or snow. Stratocumulus are masses of white, grey or dark cloud. Stratus are low-lying and grey. Cumulus are white and fluffy-looking. Cumulonimbus are towering, dark clouds that often produce thunderstorms. Their bases almost touch the ground and extend upward to 23,000m (75,000ft). By day, clouds reflect the rays of the Sun back into the atmosphere, keeping the ground cool. At night, clouds trap and re-radiate heat rising from the Earth, keeping surface temperatures warm. See also fog; hydrological cycle

87. Clouds

See also 27. ATMOSPHERE ; 345. RAIN ; 375. SNOW ; 417. WEATHER .

ceilometer

an instrument for measuring by triangulation and recording the distance between the earth and the cloud ceiling.

nephelognosy

divination by the observation of clouds.

nepheloscope

an apparatus for expanding moist air to demonstrate the process of cloud formation.

nephogram

a photograph of clouds, taken with a nephograph.

nephograph

an instrument for photographing clouds and producing nephrograms.

nephology

the branch of meteorology that studies clouds. —nephologic, nephological , adj. —nephologist , n.

nephophobia

an abnormal fear of clouds.

noctilucence

the condition of being visible during the short summer nights, especially high-altitude clouds. —noctilucent adj.

nubilation

1. the formation or arrangement of clouds.

2. the obscuration caused by clouds.

cloud cloud cuckoo land a state of unrealistic or absurdly over-optimistic fantasy. The phrase is recorded in English from the late 19th century, and is a translation of Greek Nephelokokkugia, the name of the city built by the birds in Aristophanes' comedy Birds, from nephelē ‘cloud’ + kokkux ‘cuckoo’.
every cloud has a silver lining even the gloomiest circumstance has some hopeful element in it. The saying is recorded from the mid 19th century, but the idea is found earlier in Milton's Comus (1634), ‘Was I deceiv'd, or did a sable cloud Turn forth her silver lining on the night?’
on cloud nine extremely happy, with reference to a notional ten-part classification of clouds in which ‘nine’ was next to the highest.

See also Land of the Long White Cloud at land.

Cloud

a mass or volume of smoke, flying dust, etc.; a body of insects or birds; a mass of rock; a great crowd; a vast collection. See also drift, plague, swarm.

Examples: cloud of arrows, 1776; of disdain, 1591; of dust; of flies, 1855; of foxes, 1883; of gnats, 1590; of grasshoppers; of incense; of information, 1705; of insects; of locust, 1667; of rain; of rock; of sails, 1748; of seafowl, 1885; of smoke; of starlings, 1882; of witches; of witnesses, 1382.

cloud droplet The liquid component of clouds, occurring as water droplets with an average size of 10 μm. In a non-rain cloud the droplets are suspended at a near-constant level, because air friction approximately balances the gravitational force. See also RAINDROP.

cloud droplet The liquid component of clouds, occurring as water droplets with an average size of 10 μm. In a non-rain cloud the droplets are suspended at a near-constant level, because air friction approximately balances the gravitational force.

cloud formations are a useful guide in weather prediction. There are four main types, cirrus, cumulus, stratus, and nimbus, but combinations of these main formations can also be indications at sea of the type of weather ahead. See marine meteorology.

cloud †hill, rock OE.; visible mass of watery vapour in the air XIII. OE. clūd, prob. rel. to CLOD.
Hence cloud vb. XVI.

cloud •aloud, becloud, cloud, crowd, enshroud, loud, Macleod, proud, shroud, Stroud, unavowed, unbowed, unendowed, unploughed (US unplowed) •thundercloud