Saturday, 26 July 2014 06:35



A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body.

In general, clouds form when rising air is cooled to its dew point (the temperature at which the air becomes saturated). Water vapour normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds. If sufficient condensation particles are not present, the air will become supersaturated and the formation of cloud or fog will be inhibited.

The cloud formation is generally due to adiabatic cooling.

Adiabatic Change of Temperature

Generally the temperature decreases with increasing height at the rate of 6.5o C per 1000m. A definite ascending air with given volume and temperature expands due to decrease in pressure and gets cooled with increasing height. On the other hand, a descending air contracts and thus its volume decreases and temperature increases. Hence there is change in temperature of air due to ascent or descent but without addition or subtraction of heat. Such change of temperature is known as adiabatic change of temperature.

Thus in cloud formation the heated air moves up and gets cooled due to expansion of air and leads to formation of clouds.

Sources of water vapour

The main sources from which water vapour is added to the air are: wind convergence over water or moist ground into areas of upward motion, precipitation, daytime heating leading to evaporation of water from the surface of oceans, water bodies or wet land and transpiration from plants.

Significance of clouds

Clouds are very significant because

• They cause all forms of precipitation.

• They play a major role in the heat budget of the earth.

• They reflect, absorb some part of incoming solar radiation as well as some part of long-wave terrestrial radiation re-radiated by the earth.

Types of clouds

1. Cirrus clouds: These clouds form above 20,000 feet (6,000 meters) and since the temperatures are extremely low at such high elevations, these clouds are primarily composed of ice crystals. High-level clouds are typically thin and white in appearance, but can appear in a magnificent array of colors when the sun is low on the horizon.

2. Cirro-cumulus: These clouds appear as small, rounded white puffs. The small ripples in the cirrocumulus sometimes resemble the scales of a fish. A sky with cirrocumulus clouds is sometimes referred to as a "mackerel sky."

3. Cirro-stratus: These clouds are thin, sheet-like high clouds that often cover the entire sky. They are so thin that the sun and the moon can be seen through them.

Cirrus clouds           Cirro-cumulus                Cirro-stratus

4. Altocumulus clouds: These are middle level clouds that are made up of water droplets and appear as gray, puffy masses, sometimes rolled out in parallel waves or bands.

5. Altostratus clouds: These are gray or blue-gray middle level clouds composed of ice crystals and water droplets. These clouds usually cover the entire sky. In the thinner areas of the cloud, the sun may be dimly visible as a round disk. Altostratus clouds often forms ahead of storms that produces continuous precipitation.

Altocumulus clouds                    Altostratus clouds

6. Stratus clouds: These are uniform grayish clouds that often cover the entire sky. They resemble fog that does not reach the ground. Usually no precipitation falls from stratus clouds, but sometimes they may drizzle. When a thick fog "lifts," the resulting clouds are low stratus.

7. Nimbostratus clouds: These form a dark gray, "wet" looking cloudy layer associated with continuously falling rain or snow. They often produce precipitation that is usually light to moderate.

Stratus clouds                                                Nimbo-Stratus clouds

8. Cumulonimbus clouds: These are thunderstorm clouds that form if cumulus congestus clouds continue to grow vertically. Their dark bases may be no more than 300 m (1000 ft) above the Earth's surface. Their tops may extend upward to over 12,000 m (39,000 ft). Tremendous amount of energy is released by the condensation of water vapour within a cumulonimbus. Lightning, thunder, and even violent tornadoes are associated with the cumulonimbus.

9. Cumulus clouds: These are puffy clouds that sometimes look like pieces of floating cotton. The base of each cloud is often flat and may be only 1000 m (330 ft) above the ground. The top of the cloud has rounded towers. When the top of the cumulus resembles the head of a cauliflower, it is called cumulus congestus or towering cumulus. These clouds grow upward, and they can develop into a giant cumulonimbus, which is a thunderstorm cloud.

Cumulonimbus clouds                             Cumulus clouds