DEFINITION OF CLOUDS

Visible masses of water droplets or ice crystals suspended

in the lower atmosphere, generally confined to the troposphere.

The water droplets and ice crystals condense from

water vapor around small dust, pollen, salt, ice, or pollution

particles that aggregate into cloud formations, classified

according to their shape and height in the atmosphere. Luke

Howard, an English naturalist, suggested the classification

system still widely used today in 1803. He suggested Latin

names based on 10 genera, then broken into species. In 1887

Abercromby and Hildebrandsson further divided the clouds

into high, middle, and low-level types, as well as clouds that

form over significant vertical distances. The basic types of

clouds include the heaped cumulus, layered stratus, and wispy

cirrus. If rain is falling from a cloud the term nimbus is added,

as in cumulonimbus, the common thunderhead cloud.

High clouds form above 19,685 feet (6,000 m) and are

generally found at mid to low latitudes. The air at this elevation

is cold and dry, so the clouds consist of ice crystals and

appear white to the observer at the ground except at sunrise

and sunset. The most common high clouds are the cirrus,

thin, wispy clouds typically blown into thin horsehair-like

streamers by high winds. Most cirrus clouds are blown from

west to east by prevailing high-level winds and are a sign of

good weather. Cirrocumulus clouds are small, white, puffy

clouds that sometimes line up in ripple-like rows and at other

times form individually. Their appearance over large parts of

the sky is often described as a mackerel sky, because of the

resemblance to fish scales. Cirrostratus are thin, sheet-like

clouds that typically cover the entire sky. They are so thin

that the Sun, Moon, and some stars can be seen through

them. They are composed of ice crystals, and light that

refracts through these clouds often forms a halo or sun dogs.

These high clouds often form in front of an advancing storm

and typically foretell of rain or snow in 12–24 hours.

Middle clouds form between 6,560 feet and 22,965 feet

(2,000 m and 7,000 m), generally in middle latitudes. They

are composed mostly of water droplets, with some ice crystals

in some cases. Altocumulus clouds are gray, puffy masses that

often roll out in waves, with some parts appearing darker

than others. Altocumulus are usually less than 0.62 mile (1

km) thick. They form with rising air currents at cloud level,

and morning appearance often predicts thunderstorms by the

late afternoon. Altostratus are thin blue-gray clouds that often

cover the entire sky, and the sun may shine dimly through,

appearing as a faint irregular disk. Altostratus clouds often

form in front of storms that bring regional steady rain.

Low clouds have bases that may form below 6,650 feet

(2,000 m) and are usually composed entirely of water

droplets. In cold weather they may contain ice and snow.

Nimbostratus are the dark gray rather uniform-looking

clouds associated with steady light to moderate rainfall. Rain

from the nimbostratus clouds often causes the air to become

saturated with water, and a group of thin ragged clouds that

move rapidly with the wind may form. These are known as

stratus fractus, or scud clouds. Stratocumulus clouds are low,

lumpy-looking clouds that form rows or other patterns, with

clear sky visible between the cloud rows. The sun may form

brilliant streaming rays known as crepuscular rays through

these clouds. Stratus clouds have a uniform gray appearance

and may cover the sky, resembling fog but not touching the

ground. They are common around the seashore especially in

summer months.

Some clouds form over a significant range of atmospheric

levels. Cumulus are flat-bottomed puffy clouds with

irregular, domal, or towering tops. Their bases may be lower

than 3,280 feet (1,000 m). On warm summer days, small

cumulus clouds may form in the morning and develop significant

vertical growth by the afternoon, forming a towering

cumulus or cumulus congestus cloud. These may continue to

develop further into the giant cumulonimbus, giant thunderheads

with bases that may be as low as a few hundred

meters, and tops extending to more than 39,370 feet (12,000

m) in the tropopause. Cumulonimbus clouds release tremendous

amounts of energy in the atmosphere and may be associated

with high winds, vertical updrafts and downdrafts,

lightning, and tornadoes. The lower parts of these giant

clouds are made of water droplets, the middle parts may be

both water and ice, whereas the tops may be made entirely

of ice crystals.

There are many types of unusual clouds that form in different

situations. Pileus clouds may form over rising cumulus

tops, looking like a halo or fog around the cloud peak. Banner

clouds form over and downwind of high mountaintops,

sometimes resembling steam coming out of a volcano. Lenticular

clouds form wave-like figures from high winds moving

over mountains and may form elongate pancake-like shapes.

Unusual, and even scary-looking, mammatus clouds form

bulging bag-like sacks underneath some cumulonimbus

clouds, forming when the sinking air is cooler than the surrounding

air. Mammatus-like clouds may also form underneath

clouds of volcanic ash. Finally, jet airplanes produce

condensation trails, produced when water vapor from the

jet’s exhaust mixes with the cold air, which becomes suddenly

saturated with water and forms ice crystals. Pollution particles

from the exhaust may provide the nuclei for the ice. In

dry conditions condensation, or contrails, will evaporate

quickly. However, in more humid conditions the contrails

may persist as cirrus-like clouds. With the growing numbers

of jet flights in the past few decades, contrails have rapidly

become a significant source of cloudiness, contributing to the

global weather and perhaps climate.

Clouds have a large influence on the Earth’s climate.

They are highly reflective and reflect short wavelength radiation

from the Sun back to space, cooling the planet. However,

since they are composed of water they also stop the longer

wavelength radiation escaping in a greenhouse effect. Together

these two apparently opposing effects of clouds strongly

influence the climate of the Earth. In general, the low and

middle level clouds cool the Earth whereas abundant high

clouds tend to warm the Earth with the greenhouse effect.

See also ATMOSPHERE; CLIMATE.

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