To review briefly, condensation occurs when water vapor in the air changes to a liquid. The result of this process may be dew, fog, or clouds. For any of these forms of condensation to occur, the air must be saturated. Saturation occurs most commonly when air is cooled to its dew point, or, less often, when water vapor is added to the air.

Condensation Nuclei and Cloud Formation

Under normal atmospheric conditions, condensation occurs only when a surface exists on which the water vapor can condense. When dew forms, objects at or near the ground, such as grass and car windows, serve this purpose. But when condensation occurs high above the ground, tiny bits of particulate matter, known as cloud condensation nuclei, serve as surfaces for water-vapor condensation. These nuclei are very important, for in their absence, a relative humidity well in excess of 100 percent is needed to produce clouds.

Condensation nuclei, such as microscopic dust, smoke, pollen, and salt particles (from the ocean), are profuse in the lower atmosphere. Because of this abundance of particles, relative humidity rarely exceeds  percent. Some particles, such as ocean salt, are particularly good nuclei because they absorb water. These particles are termed hygroscopic nuclei. When condensation takes place, the tiny cloud droplets grow quickly at first, but their growth slows as they use up the excess water vapor. The result is a cloud consisting of millions upon millions of tiny water droplets, all so fine that they remain suspended in air. When cloud formation occurs at below-freezing temperatures, tiny ice crystals form. Thus, a cloud might consist of water droplets, ice crystals, or both.

Clouds are among the most conspicuous and observable aspects of the atmosphere and its weather. Clouds are a form of condensation best described as visible aggregates of small droplets of water or tiny crystals of ice. In addition to being prominent and sometimes spectacular features in the sky, clouds are of continual interest to meteorologists because they indicate what is going on in the atmosphere.

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

Cloud Forms

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

• 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.”)

• Stratus (stratiform) clouds consist of sheets or layers (strata) that cover much or all of the sky.

• Cumulus (cumuliform) clouds consist of globular cloud masses that are often described as cottonlike in appearance. 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 convection and rising air.

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 “raincloud”) is added to the name of a cloud that is a major producer of precipitation. Thus, nimbostratus denotes a flat-lying rain cloud.

Cloud Heights

In terms of their height, clouds are classified as high, middle, low, and clouds of vertical development (▲). 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 Earth’s surface—up to an altitude of about 2000 meters (6500 feet)—and are generally composed of water droplets. Clouds that extend upward to span more than one height range are called clouds of vertical development. The altitudes at which high, middle, and low clouds occur can vary somewhat with season and latitude. For example, at high (poleward) latitudes and during cold winter months, high clouds generally occur at lower altitudes. The internationally recognized cloud types are described in the sections that follow.

High Clouds

Three cloud types make up the family of high clouds (above 6000 meters [20,000 feet]): cirrus, cirrostratus, and cirrocumulus. Cirrus (Ci) clouds are thin and delicate and sometimes appear as hooked filaments called “mares’ tails” (▼A). As the names suggest, cirrocumulus (Cc) clouds consist of fluffy masses (▼B), whereas cirrostratus (Cs) clouds are flat layers (▼C). Because of the low temperatures and small quantities of water vapor present at high altitudes, all high clouds are thin and white and are made up of ice crystals. Furthermore, high clouds do not produce precipitation that reaches the surface. However, when cirrus clouds are followed by cirrocumulus clouds and increased sky coverage, they may warn of impending stormy weather.

Middle Clouds

Clouds that appear in the middle range (2000-6000 meters [6500-20,000 feet]) have the prefix alto as part of their name. Altocumulus (Ac) clouds are composed of globular masses that differ from cirrocumulus clouds in being thicker and denser (▲D). Altostratus (As) clouds create a uniform white to grayish sheet covering the sky, with the Sun or Moon visible as a bright spot (▲E). Infrequent light snow or drizzle may accompany these clouds.

Low Clouds

There are three members in the family of low clouds (below 2000 meters [6500 feet]): stratus, stratocumulus, and nimbostratus. Stratus (St) is a uniform foglike layer of clouds that frequently covers much of the sky. These clouds sometimes produce light precipitation. When stratus clouds develop a scalloped bottom that appears as long parallel rolls or broken globular patches, they are called stratocumulus (Sc) clouds.

Nimbostratus (Ns) clouds derive their name from the Latin nimbus, which means “raincloud,” and stratus, “to cover with a layer” (▲F). 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. 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) and is usually of long duration, covering a large area.

Clouds of Vertical Development

Some clouds have their bases in the low height range but extend upward into the middle or high altitudes. Consequently, clouds in this category are called clouds of vertical development. They tend to be associated with unstable air, which rises because of its buoyancy. Although cumulus clouds are usually connected with fair weather (▲G), they may grow dramatically under the proper circumstances. Once upward movement is triggered, acceleration can be powerful, and clouds with great vertical extent may form. The end result is often a towering cloud, called a cumulonimbus (Cb), which usually produces moderate to heavy rain showers or a thunderstorm (▲H).

Definite weather patterns can often be associated with particular clouds or certain combinations of cloud types, so it is important to become familiar with cloud descriptions and characteristics. The table below lists the  basic cloud types that are recognized internationally and gives some characteristics of each.

• For water vapor to condense into cloud droplets, the air must reach saturation, and there must be a surface on which the water vapor can condense. The resulting cloud droplets are tiny and are held aloft by the slightest updrafts.

• Clouds are classified on the basis of their form and height. The three basic cloud forms are cirrus (high, white, thin wisps or sheets), 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 (6500 feet) to 6000 meters; or low, below 2000 meters. Clouds of vertical development have bases in the low height range and extend upward into the middle or high range.

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