Clouds

Clouds provide an indication of the current weather and a forecast of weather to come as well as information regarding climate and other aspects of the atmosphere. They are also a resource for the investigation of the dynamic interactions of solid, liquid, and gaseous substances.

Principal Terms

cirrus: trailing or streaky clouds, at altitudes ranging from 5 to 13 kilometers, that are feathery or fibrous in appearance

condensation: the transformation of a substance from the vapor state to the liquid state; atmospheric condensation occurs when droplets of liquid form (or condense) around small particles in the atmosphere

convection: the transmission of heat by cyclic mass transport within a fluid substance; the movement of warmer, less dense material that rises as cooler, denser material sinks

cumulus: clouds with vertical development rising from a seemingly flat base, often appearing as fluffy masses, at altitudes ranging from ground level to 6 kilometers above the ground; sometimes called heap clouds

radiation: the transfer of energy emitted from one body through a transparent medium to another body, as occurs when light and heat energy from the sun impinge on Earth

stratus: sheet or layer clouds, at altitudes ranging from 2 to 6 kilometers above the ground (altostratus, or middle) or from 0 to 2 kilometers above the ground (stratocumulus, or low)

supersaturation: a state in which the air's relative humidity exceeds 100 percent, the condition necessary for vapor to begin transformation to a liquid state

Cloud Formation and Level

Clouds are the single most obvious feature of the atmosphere. The formation of clouds is essentially a two-part process. The heat provided by the radiation from the sun warms liquid water on Earth's surface. This moisture evaporates and the vapor rises, cooling as it reaches higher altitudes. Numerous particles in the atmosphere provide surfaces to which the water molecules adhere and become nuclei for the formation of water droplets or ice crystals, depending on the temperature of the atmosphere. The particles are usually composed of combustion products, meteoritic dust, volcanic material, soil, or salt. When a sufficient number of droplets or crystals have formed, a visible cloud has come into existence. The size, shape, and growth patterns of the cloud will depend upon the moisture and the atmospheric energy available during its formation.

Although clouds can appear to be relatively stable, they are always in motion—rising or falling, expanding or shrinking—depending on the temperature and humidity of the surrounding air and the direction of the winds. Eventually, clouds either precipitate as the water droplets in the form of rain, snow, or some type of ice fall back to the surface, or evaporate as the water droplets return to vapor that remains in the atmosphere. Clouds have been divided into three stratigraphic levels: high, middle, and low. The upper limits of the high stage, at the tropopause, range from 8 kilometers in the polar regions to 18 kilometers in the tropics. In the temperate zones, which include most of the world's landmasses, the high stage ranges up to approximately 13 kilometers.

Cloud Types

Cirrus clouds, the highest clouds, are composed of ice crystals and some supercooled liquid water. The air temperature of these clouds is usually below -25 degrees Celsius. Cirrus clouds form when air rises slowly and steadily over a wide area. They are usually delicate in appearance, white in color, and tend to occur in narrow bands. They are thin enough to permit stars or blue sky to be seen through them, and are responsible for the ring or halo effect that often appears to surround the sun. They typically signal large, slow-moving warm fronts and rain. Cirrus clouds sometimes seem to be gathered in branching plumes or arcs with bristling ends. Because the color of a cloud depends on the location of the light source that illuminates it, the position of the sun at sunset or sunrise may cast these clouds in bright reds or yellow-oranges. A variant of this classification is the cirrocumulus, a cloud composed of ice crystals gathered in columns or prismatic shapes. The cirrocumulus usually is arranged in ripples or waves; it is the cloud that sailors describe when they speak of a "mackerel sky" because of its similarity to the coloration of the mackerel fish. A close relation is the cirrostratus, which resembles a transparent white veil. Suffusing the blue of the sky with a milky tone, it is composed of ice crystals shaped like cubes. Because cirrus clouds often form in long, wispy streamers under the influence of strong winds, herding peoples traditionally described such clouds as "mares' tails."

On the middle atmospheric level are found the altocumulus clouds. They signal an approaching cyclonic front. The altocumulus clouds reach an altitude of 6 kilometers, where they intersect the lower level of the cirrocumulus, and tend to resemble the cirrocumulus at their uppermost reaches. Below this point, they tend to be larger and to have dark shadings on their lower boundaries. They are composed primarily of water drops and are formed by a slow lifting of an unstable layer of air. Typically, they look like large, somewhat flattened globules and are often arranged evenly in rows or waves. In the summer, they may appear as a group of small, turretlike shapes, and in that form they often precede thunderstorms. The other basic middle-level cloud is the altostratus. These clouds are formed in air that is ascending slowly over a wide area, and they often occur in complex systems between the higher cirrostratus and the low-level nimbostratus. They are composed of both ice crystals and water droplets, with raindrops or snowflakes in the lower levels of the cloud. They are usually gray or bluish-gray, resembling a thick veil that is uniform in appearance, and often cover a substantial portion of the sky. There is a reciprocal relationship between altocumulus and altostratus clouds, in that a sheet of altocumulus clouds—particularly high, scaly ones—may be forced down and become transformed into altostratus. Conversely, as weather improves, altostratus may be altered toward altocumulus.

Clouds on the lowest levels generally are associated with the onset of precipitation. They often produce what is frequently called a leaden sky—one that is flat and "dirty" in appearance. These clouds range in altitude from 2 kilometers to the air just above the surface. Stratocumulus clouds may appear in sheets, patches, or layers close together, in which case the undersurface has a wavelike appearance. These clouds are formed by an irregular mixing of air currents over a broad area and suggest instability in the atmosphere. They are composed primarily of water droplets and, because they often have a marked vertical development, they can be confused with small cumulus clouds; however, they have a softer, less regular shape than cumulus clouds. Stratus clouds tend to be the lowest level of cloud formation. Sometimes resembling fog, they are usually an amorphous, gray layer, and while they do not produce steady rain, drizzle is not uncommon. They are formed by the lifting of a shallow, moist layer of air close to the ground and are composed almost exclusively of water droplets. They produce the dullest of visible sky conditions.

Cumulus and Nimbus Clouds

The cumulus cloud is probably the cloud form most frequently depicted in artistic illustrations of the sky. In fair weather, cumulus clouds are separated into cottonlike puffs with distinctive outlines against a deep blue background. When they are few in number and almost pure white in color, they are a prominent feature of the most pleasant weather during the spring and summer; however, these clouds also have the potential to change into storm-producing systems. They are initially formed by the ascent of warm air in separate masses or bubbles, the air rising with increasing velocity as the cloud takes shape. Their development is driven by convection currents, and while their base is usually about 1 kilometer above the land surface (or somewhat less over the ocean), they may grow vertically to an altitude of more than 7 kilometers; in the case of the cumulonimbus, the classic anvil-shaped thunderhead, they may reach an altitude of more than 15 kilometers. In the transition to cumulonimbus, the warm air rises rapidly into a cooler layer, and the convergence of radically varying temperatures produces a "boiling" motion at the tops of the clouds. When the rising air encounters strong winds at its upper reaches, it becomes flattened against the stable, colder layer of air that resists the convection from below. This shearing effect is what creates the classic anvil shape of the thunderhead.

The cumulonimbus is the great thundercloud of the summer sky. It tends to be white or gray, dense, and massive, with a dark base. Generally growing out of cumulus clouds, these are the tallest of conventional cloud forms, created by strong convection currents with updrafts of high velocity within the clouds. The thunderstorm is a product of considerable turmoil within a cumulonimbus cloud. Violent updrafts in the center of the cloud result in an increasing size of water droplets that alternately descend and rise within the cloud until they fall as some form of precipitation. The size of the drops that eventually fall is an indication of the altitude of the cloud as well as the severity of the storm; in their lower regions, cumulonimbus are composed of water droplets, but as they rise, ice crystals, snowflakes, and hail may form. The development of the cumulonimbus is generally cellular (or compartmentalized) so that there is usually some space between clouds of this type, although they may occur in a long "squall line" that is often the boundary between warm and cold airflows. Cumulonimbus clouds, because of the great energy involved in their production, often generate subsidiary clouds as well. On lower levels, the sky seems to take on a convoluted, disorganized appearance, while upper levels often feature extensions that are like cirrus clouds. The forward edge of a cumulonimbus cloud often produces what looks like the front of a wave or rolling scud just under the base of the main cloud.

Although original attempts at cloud classification included the word "nimbus" to describe a separate order of low, rain-producing clouds, the term is now applied as a modifier. In addition to its application to the storm-producing version of the cumulus cloud, there are nimbostratus clouds, or low, gray, dark clouds with a base close to the ground, which generally carry rain. They are formed by the steady ascent of air over a wide area, usually associated with the arrival of a frontal system. They differ from stratus clouds in that they are much darker and are composed of a mixture of ice crystals and water droplets. These clouds have a ragged base, highly variable in shape, that is often hard to discern in steady rain. Depending on the temperature, the base will be primarily snowflakes or raindrops.

Other Cloud-Related Phenomena

In addition to the standard forms, other cloud-related phenomena may occur in the atmosphere. A cloud is essentially composed of tiny droplets of liquid water or of ice particles suspended in the air, or both; however, other substances that may not technically be considered the components of clouds may be suspended in the atmosphere and gathered in cloudlike masses. Haze, for example, is a suspension of extremely fine particles invisible to the naked eye but sufficiently numerous to produce an opalescent effect. Clouds of smoke are usually composed of the products of combustion gathered in a dense, swirling mass that rises from its source. Particles of sand may be gathered by strong, turbulent winds into clouds that move near the ground surface.

One other type of cloud, the noctilucent, is a high-altitude form found in the mesosphere at 8 to 90 kilometers above the surface in northern latitudes. Its formation is somewhat mysterious, but one theory maintains that it is created by turbulence that carries water vapor to unusual heights during the arctic summer. The nuclei for the formation of these clouds may be deposited in the atmosphere by the residue of meteors. Noctilucent clouds are either pure white or blue-white and have pronounced band or wave structures. One theory maintains that this wave configuration is a result of gravity waves in the atmosphere, but the source of these gravity waves is unknown. Noctilucent clouds resemble high cirrus clouds more than any other form.

On rare occasions, clouds have also been observed in the stratosphere. Their composition has not been determined, but at a range of 20 to 30 kilometers above Earth's surface, nacreous or "mother-of-pearl" clouds, so designated because they may display a full spectrum of colors, have been seen in the sky over Scandinavia and Alaska during the winter.

Study of Clouds

Recorded descriptions of the sky and weather-related phenomena date at least as far back as Aristotle's Meteorologica (c. 350 BCE). The first formal system of cloud classification owes its development to the work of Luke Howard in London in the early nineteenth century. Howard proposed the use of the Latin names later used to identify clouds. In 1897, C. T. R. Wilson's cloud chamber experiments simulated the cloud formation process in the laboratory. By 1925, radio wave observations were beginning to augment free balloon ascensions as a method of examining the atmosphere. In 1928, the first radiosonde apparatus—a kind of electrical thermometer that transmits temperature, pressure, and humidity information to a receiver—was placed into operation. Immediately after World War II, high levels of atmospheric exploration became possible with rockets.

In 1960, the first meteorological satellite, Tiros 1, was launched by the United States. It transmitted pictures of cloud patterns back to Earth, and whereas previously the motion of cloud systems could be developed only from separate ground sightings, the satellite provided an overview of areas more than 1,000 kilometers wide. The use of time-lapse imaging permitted an observer to follow the birth and decay of cloud systems and, in conjunction with computer data storage, made it possible to develop a history of the entire range of cloud formation in the atmosphere.

There are seven basic factors that must be considered to determine the fundamental properties of cloud systems. They are, according to Horace Byers, the amount of sky covered, the direction from which the clouds are moving, their speed, the height of the cloud base, the height of the cloud top, the form of the cloud, and the constitution of the cloud. Each of these categories depends upon the location of the observing system. The amount of sky covered will vary considerably, depending on whether a ground observer is reporting or satellite data are being analyzed. The direction of cloud motion also depends on the point of observation because relatively specific motion with regard to a fixed point is much easier to chart than the complex series of measurements necessary to describe the motion of a large-scale cloud formation. The upper and lower reaches of a cloud may be measured from balloons, from airplanes, by satellites, and in ground stations. The refinement of cloud investigation by satellite imaging has led to the consideration of what the eminent meteorologist Richard Scorer calls "messages," which consider the fact that cyclonic patterns are more varied than previously realized, requiring the assimilation of much more data into an explanation of how large-scale cloud systems are formed and why they exist for a particular duration in time.

In addition to measurements that are essentially external, the inner mechanics of a cloud must be studied to determine its basic properties and potential behavior. The inner cloud physics that influence its development and its potential for precipitation involve the supercooling and freezing of water, the origin and specific form of the ice nuclei that lead to the growth of a cloud and that depend on the rise and descent of particles within the cloud, the rate of collisions between particles in a cloud that influence the growth of droplets and crystals, and the reflecting and refracting properties of the components of a cloud that determine the color of a cloud in terms of both visual observation and radiophotometric measurement. Aside from spectographic observations, most cloud physics depend on an understanding of thermodynamics, or the effects of heat changes on the motion and elemental properties of particles in the atmosphere.

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