The amount of water vapor present in the air is called humidity. It is greater in summer than in winter, during day it is more than night and also it is more at lower altitudes than at higher altitudes. Humidity varies from place to place and from time to time. There is, however, a limit to how much water vapor air can contain. When the air reaches that limit, at this stage and for the given temperature, air is said to be saturated. If the amount of water vapor present in the air is less than the maximum amount it can hold at a particular temperature, the air is said to be unsaturated. If the temperature of saturated air is further raised, it becomes unsaturated. Similarly unsaturated air can be made saturated by lowering the temperature of air. The temperature at which air gets saturated is called dew point. At this point, water vapor begins to change into minute droplets of water into ice crystals. The change of water vapor into water droplet (liquid) on cooling is known as condensation. For condensation to take place it is necessary that there be a cold surface on which the gaseous water vapor can condense. In the air, tiny particles of matter such as dust, sea salt, pollen grains, and smoke particles are present and are called condensation nuclei. On cooling the ground, blades of grass, buildings, poles etc. also act as surfaces.
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There are different forms of condensation such as dew, frost, fog, mist and clouds.
Dew:
It is formed when warm moist air comes in contact with any cold surface such as blades of grass, leaves, cemented surfaces, and ground, to form tiny drops of water.
Frost:
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When the temperature of the cold surface is lower than 0˚ C the water vapor condenses to form white feathery crystals called frost. It is frozen dew.
Fog:
When a complete layer of moist air cools and condenses. This happens in particular on cool winter mornings.
Mist:
Mist is a thin form of fog. A combination of fog and smoke is called Smog. It is a common phenomenon to be seen is polluted cities in winters. Fog, mist and smog remain suspended in the air till they get evaporated back into the air by sunshine.
Clouds
While all the above mentioned forms of condensation occur close to the ground, clouds are formed high in the air. When moist air is lifted upwards, at a certain height it begins to cool. As soon as dew point is reached the water vapor condenses to form tiny droplets of water or ice crystals (depending on the temperature) suspended in the air. Millions of minute droplets of water or ice crystals float with the air currents. These droplets are visible and they form clods.
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Clouds are classified on the basis of their shape and height into different categories. Cirrus (curled or leathery clouds), Stratus (sheet like spread clouds), Cumulus (heap clouds like cauliflowers), Nimbus (dark rain clouds).