Name and describe the 10 basic cloud types, based on form and height. Contrast nimbostratus and cumulonimbus clouds and their associated weather.

In 1803, English naturalist Luke Howard published a cloud classification scheme that serves as the basis of our present-day system. Howard’s system classifies clouds on the basis of two criteria: form and height (Figure 5.1). We will look at the basic cloud forms or shapes first and then examine cloud height.

Cloud Forms

Clouds are classified based on how they appear when viewed from Earth’s surface. There are three basic forms or shapes:

• Cirrus (cirriform) clouds are high, white, and thin. They form delicate veil-like patches or wisplike strands and often have a feathery appearance. (Cirrus is Latin for “curl” or “filament.”)

• Cumulus (cumuliform) clouds consist of globular cloud masses that look like cotton balls or sheep in the sky. Normally cumulus clouds exhibit a flat base and appear as rising domes or towers. (Cumulus means “heap” or “pile” in Latin.) Cumulus clouds form within a layer of the atmosphere where there is some instability leading to convection and rising air.

• Stratus (stratiform) clouds consist of sheets or layers (strata) that cover much or the entire sky. Although there may be minor breaks in the layers, there are no distinct individual cloud units. Stratus clouds form when the atmosphere is stable.

All clouds have at least one of these three basic forms, and some are a combination of two of them; for example, stratocumulus clouds are mostly sheetlike structures composed of long parallel rolls or broken globular patches. In addition, the term nimbus (Latin for “violent rain”) is used in the name of a cloud that is a major producer of precipitation. Thus, nimbostratus denotes a low, relatively flat rain cloud, whereas cumulonimbus describes a puffy, tall rain cloud.

Cloud Height

The second aspect of cloud classification—height—recognizes three levels: high, middle, and low (see Figure 5.1). High clouds form in the highest and coldest region of the troposphere and normally have bases above 6000 meters (20,000 feet). Temperatures at these altitudes are usually below freezing, so the high clouds are generally composed of ice crystals or supercooled water droplets. Middle clouds occupy heights from 2000 to 6000 meters (6500 to 20,000 feet) and may be composed of water droplets or ice crystals, depending on the time of year and temperature profile of the atmosphere. Low clouds form nearer to Earth’s surface—up to an altitude of about 2000 meters (6500 feet)—and are generally composed of water droplets, except in winter. These altitudes may vary somewhat depending on season of the year and latitude. For example, at high (poleward) latitudes and during cold winter months, high clouds generally occur at lower altitudes. Further, some clouds extend upward to span more than one height range and are called clouds of vertical development.

The 10 internationally recognized cloud types are summarized in Table 5.1 and described in the sections that follow.

High Clouds

The family of high clouds (above 6000 meters [20,000 feet]) include cirrus, cirrostratus, and cirrocumulus. Low temperatures and small quantities of water vapor present at high altitudes result in high clouds that are thin, white, and made up primarily of ice crystals.

Cirrus (Ci) clouds are composed of delicate, icy filaments. Winds aloft often cause these fibrous ice trails to bend or curl. Cirrus clouds with hooked filaments are called “mares’ tails” (Figure 5.2A).

Cirrocumulus (Cc) clouds appear as white patches composed of small cells or ripples (Figure 5.2B). These small globules, which may be merged or separate, are often arranged in a pattern that resembles fish scales. When this occurs, it is commonly called “mackerel sky.”

Although high clouds are not precipitation makers, when cirrus clouds give way to cirrocumulus clouds, they may warn of impending stormy weather. This observation has given rise to an old mariners’ phrase: Mackerel scales and mares’ tails make lofty ships carry low sails.

Cirrostratus (Cs) are transparent, whitish cloud veils with a fibrous or sometimes smooth appearance that may cover much or all of the sky. These clouds are easily recognized when they produce optical effects such as halos around the Sun or Moon (Figure 5.2C). (Optical effects in the atmosphere are discussed in Chapter 16.) Occasionally, cirrostratus clouds are so thin and transparent that they are barely discernible.

Middle Clouds

Clouds that form in the middle altitude range (2000–6000 meters [6500–20,000 feet]) are described with the prefix alto- (meaning “middle”) and include two types: altocumulus and altostratus.

Altocumulus (Ac) clouds tend to form in large patches composed of rounded masses or rolls that may or may not merge (Figure 5.3A). Because they are generally composed of water droplets rather than ice crystals, the individual cells usually have a more distinct outline. Altocumulus are sometimes confused with cirrocumulus (which are smaller and less dense) and stratocumulus (which are thicker).

Altostratus (As) is the name given to a formless layer of grayish clouds that cover all or large portions of the sky. Generally, the Sun is visible through altostratus clouds as a bright spot but with the edge of its disc not discernible (Figure 5.3B). However, unlike cirrostratus clouds, altostratus do not produce halos. Infrequent precipitation in the form of light snow or drizzle may accompany these clouds. Altostratus clouds, which are commonly associated with approaching warm fronts, may thicken into a dark gray layer of nimbostratus clouds capable of producing steady, continuous rain or snow.

Low Clouds

There are three members of the family of low clouds (below 2000 meters [6500 feet]): stratus, stratocumulus, and nimbostratus.

Stratus (St) clouds form in low, horizontal layers that on occasion may produce light drizzle or mist. White to light gray in color, stratus clouds have very uniform bases and appear to blanket the entire sky.

Stratus-like clouds that develop a scalloped bottom that appear as long parallel rolls or broken globular patches are called stratocumulus (Sc) (Figure 5.4). Although stratocumulus clouds are similar in appearance to altocumulus, they are located lower in the sky and consist of broken patches that are generally much larger than those of altostratus. A simple way to distinguish between these is to point your hand in the direction of an individual cloud mass, and if the cloud is about the size of your thumbnail, it is an altocumulus; if it is the size of your fist, it is a stratocumulus cloud.

Stratocumulus clouds often cover vast stretches of the subtropical oceans, which provide a ready supply of surface moisture. Because stratocumulus clouds cover such large areas, they are extremely important for Earth’s energy balance, primarily because they reflect considerable amounts of incoming solar radiation.

Nimbostratus (Ns) clouds derive their name from the Latin nimbus, “rain cloud,” and stratus, “to cover with a layer” (Figure 5.5). Nimbostratus clouds tend to produce constant precipitation and low visibility. These clouds normally form under stable conditions when air is forced to rise, as along a front (discussed in Chapter 9). Such forced ascent of stable air leads to the formation of a stratified cloud deck that is widespread and that may grow into the middle level of the troposphere. Precipitation associated with nimbostratus clouds is generally light to moderate (but can be heavy), is usually of long duration, and covers a large area.

Clouds of Vertical Development

Clouds having their bases in the low height range and extending upward into the middle or high altitudes are called clouds of vertical development (see Figure 5.1). The most familiar type, cumulus (Cu) clouds, are individual masses that develop into vertical domes or towers having tops that resemble a head of cauliflower. Cumulus clouds most often form on clear days when unequal surface heating causes parcels of air to rise convectively above the lifting condensation level (Figure 5.6).

When cumulus clouds are present early in the day, we can expect an increase in cloudiness in the afternoon as solar heating intensifies. Furthermore, because small cumulus clouds (cumulus humilis) form on “sunny” days and rarely produce appreciable precipitation, they are often called “fair-weather clouds.” However, when the air is unstable, cumulus clouds can grow dramatically in height. As such a cloud grows, its top enters the middle height range, and it is called a cumulus congestus. Finally, if the cloud continues to grow and rain begins to fall, it is called a cumulonimbus.

Cumulonimbus (Cb) are large, dense, billowy clouds of considerable vertical extent in the form of huge towers (Figure 5.7). In late stages of development, the upper part of a cumulonimbus turns to ice, appears fibrous, and frequently spreads out in the shape of an anvil as rising air spreads out at the tropopause. Cumulonimbus towers extend from a few hundred meters above the surface upward to 12 kilometers (7 miles) or, in the tropics, 20 kilometers (12 miles). These huge towers, commonly known as thunderstorm clouds, are capable of producing heavy precipitation with accompanying lightning, hail, and occasionally tornadoes. We consider the development of these important weather producers in Chapter 10.

Cloud Varieties

The 10 basic cloud types can be further subdivided into varieties that are named using adjectives that describe particular cloud characteristics. For example, the term uncinus, meaning “hook shaped,” is applied to streaks of cirrus clouds that are shaped like a comma resting on its side. These cirrus uncinus are often precursors of bad weather.

When stratus or cumulus clouds appear broken (or fractured), the adjective fractus is used in their description. In addition, some clouds have rounded protuberances on their bottom surface, similar to a cow udder. When these structures are present, the term mammatus is applied. This configuration is usually associated with stormy weather and cumulonimbus clouds.

Stationary lens-shaped clouds, called lenticular clouds (formal name altocumulus lenticularis), are common in rugged or mountainous topographies (Figure 5.8A). Although lenticular clouds can develop whenever the airflow develops a wavy pattern, they most frequently form on the leeward side of mountains. As moist stable air passes over mountainous terrain, a series of standing waves form on the downwind side, as shown in Figure 5.8B. As the air ascends the wave crest, it cools adiabatically. If the air reaches its dew point temperature, moisture in the air will condense to form a lenticular cloud. As the moist air moves down into the trough of the wave, the cloud droplets evaporate, leaving areas with descending air cloud-free.

• altocumulus (Ac): Middle-level clouds that are white to gray, often made up of separate globules.

• altostratus (As): A middle-level stratified veil of clouds that is generally thin and may produce very light precipitation.

• cirrocumulus (Cc): High, thin, ice-crystal clouds that form ripples or waves, or globular masses in rows.

• cirrostratus (Cs): A high thin sheet of ice-crystal clouds that may give the sky a milky look and sometimes form optical effects, such as halos, sundogs, and sun pillars.

• cirrus (Ci): One of three basic cloud forms; also one of the three high cloud types. They are thin, delicate ice-crystal clouds often appearing as veil-like patches or thin, wispy fibers.

• clouds of vertical development: A cloud that has its base in the low height range and extends upward into the middle or high altitudes.

• cumulonimbus (Cb): A towering thunderstorm cloud associated with heavy rainfall, thunder, lightning, hail, and tornadoes.

• cumulus (Cu): One of three basic cloud forms; also the name of one of the clouds of vertical development. Cumulus are billowy, individual cloud masses that often have flat bases.

• high cloud: A cloud that normally has its base above 6000 meters; the base may be lower in winter and at high-latitude locations.

• lenticular clouds: A lens-shaped cloud that forms at the crest of air oscillations downstream of a mountain.

• low clouds: A cloud that forms below a height of about 2000 meters.

• middle cloud: A cloud occupying the height range from 2000 to 6000 meters.

• nimbostratus (Ns): A low layer of dark gray clouds that is a primary precipitation producer.

• nimbus: A term used in the name of cloud types that are major precipitation producers, such as nimbostratus and cumulonimbus.

• stratocumulus (Sc): Low-level soft, gray clouds in globular patches or rolls.

• stratus (St): One of three basic cloud forms; also the name of a type of low cloud. Stratus clouds are sheets or layers that cover much or all of the sky.

• Clouds are classified based on two criteria: form and height. The three basic cloud forms are cirrus (high, white, and thin), cumulus (globular, individual cloud masses), and stratus (sheets or layers).

• Cloud heights can be high, with bases above 6000 meters (20,000 feet); middle, from 2000 to 6000 meters; or low, below 2000 meters (6500 feet). Clouds of vertical development have bases in the low height range and extend upward into the middle or high range.

• Considering both form and height, 10 basic cloud types can be classified.