stratus clouds are those clouds that resemble a sheet across the atmosphere. These clouds typically rest at a low altitude, found below 6,000 ft. (2,000 m.). Their color can vary between white to dark gray. A stratus cloud that rests at ground level is known more commonly as fog. Stratus clouds a bit higher than fog block the sun from view and cause a cloudy day' The name stratus is the Latin word to spread out.
The formation of stratus clouds occurs when a sheet of cool air passes under a sheet of moist, warm air. At the layer where these two sheets meet, the warm upper air is cooled to condensation and forms a stratus cloud. The cloud will extend as far as the overlap between the sheets of air.
Because stratus clouds are typically fog that has been elevated, they usually do not bode precipitation. At the most, stratus clouds might bring a drizzle. A type of stratus cloud, nimbostratus, do bring precipitation. Weather associated with nimbostratus clouds might be rain or snow. These clouds are so named because nimbus means rain in Latin. They are dark gray and typically rest in lower altitudes, no higher than 8,000 ft. (2,400 m.).
Yet another form of stratus cloud is the combination of a stratus cloud and a cumulus cloud, called a stratocumulus cloud. Stratocumulus clouds bring light precipitation, often a drizzle, and are found at the same elevation as nimbostratus clouds. They are somewhat fluffy, due to their cumulus nature, but darker than typical cumulus clouds. Generally, stratocumulus clouds do not bring much in terms of weather; they are used to predict dull weather.
There are many variations of stratocumulus clouds. These variations are classified as one of two types: stratocumulus undulatus (undulated, or waved) or stratocumulus cumuliformis (cumulus-shaped). The types of clouds in the stratocumulus undulatus category are tratocumulus lenticularis (lens-shaped, elongated and flat), stratocumulus opacus (dark and thick), stratocumulus perlucidus (occasionally exhibiting pockets of inconsistency that allow sunlight through), and stratocumulus translucidus (sheets of stratocumulus clouds, between which a clear sky can be seen). Stratocumulus cumuliformis clouds include stratocumulus castellanus (towers of clouds billowing from a common base), stratocumulus diurnalis (low altitude clouds resulting from spreading of cumulus or cumulonimbus clouds), stratocumulus mammatus (from mamma, the Latin word for breast; having rounded clouds hanging underneath the stratus layer), and stratocumulus vesperalis (generated by air cooling patterns that occur in the evening).
High stratus clouds are called altostratus. Alto-stratus clouds are also translucent for sunlight. They are formed from great patches of air that are elevated and condensed, due to the cold temperature at higher altitudes. Altostratus clouds are composed of ice crystals and, therefore, threaten to deposit layers of ice on airplanes passing through. Altostratus undula-tus clouds are similar to altostratus clouds, but with undulations, or waves, and therefore are often called billow or wave clouds.
A type of cloud, called cirrostratus, combines features of cirrus and stratus clouds. They are a sheet of wispy clouds made of ice crystals and tend to form at a higher altitude than regular stratus clouds. Because of their ice-crystal composition and high altitude, cirrostratus clouds are translucent; that is, sunlight and moonlight can be seen through them. Due to the ice crystals and their light refractive properties, cirrostra-tus clouds often cause a halo effect around the moon or sun when viewed from below.
Clouds can be found in the atmospheric layer called the troposphere. The troposphere is the lowest atmospheric region and is where all weather takes place. At the equator, it reaches up to 11 mi. (18 km.) from the Earth's surface. The next atmospheric layer is the stratosphere, extending to 31 mi. (50 km.) from the Earth's surface. A cloud forms when water vapor reaches its dewpoint and, thus, condenses to form a water droplet. These droplets condense around cloud condensation nuclei (CCN), which are often particles of aerosol providing a scaffold around which the cloud can form. Each CCN is approximately one one-hundredth the size of the cloud droplet, which is itself approximately one one-hundredth the size of a rain droplet (usually about 2 mm. in diameter). The nature of the clouds allows them to reflect sunlight away from the earth, known as the albedo effect, but also to trap infrared light beneath them on the Earth's surface. This latter phenomenon adds to the greenhouse effect.
SEE ALSO: Aerosols; Albedo; Clouds, Cirrus; Clouds, Cumulus; Precipitation; Troposphere; Weather.
BIBLIOGRAPHY. J.A. Day, V.J. Schaefer, and R.T. Peterson, eds., Peterson First Guide to Clouds and Weather (Houghton Mifflin, 1998); M. de Villiers, Windswept: The Story of Wind and Weather (Walker and Company, 2007).
Claudia Winograd University of Illinois at Urbana-Champaign
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