Key Points

Water exists in the atmosphere in three forms: gaseous as water vapor, liquid as water droplets, and solid as ice crystals. It enters the atmosphere through evaporation and transpiration.

Humidity is the amount of water vapor in the air. Absolute humidity is the actual mass of water vapor in a unit volume of air, while relative humidity is the percentage of moisture present compared to the air's full capacity at a given temperature.

Air is considered saturated when it contains the maximum amount of water vapor it can hold at a specific temperature. The temperature at which saturation occurs in a sample of air is called the dew point.

Evaporation is the process of liquid water turning into water vapor, driven by heat. Condensation is the opposite process, where water vapor transforms into liquid water due to cooling.

In free air, condensation occurs around tiny particles like dust, smoke, and salt, which are known as hygroscopic condensation nuclei. These particles attract water molecules.

After condensation, moisture in the atmosphere takes forms like dew, frost, fog, and clouds. The form depends on factors like temperature and location.

Dew is moisture deposited as water droplets on cool surfaces when the dew point is above freezing. Frost is moisture deposited as ice crystals when the dew point is at or below freezing point.

Fog is a cloud with its base near the ground, reducing visibility. Mist contains more moisture than fog, and smog is a mixture of fog and smoke common in industrial areas.

A cloud is a mass of water droplets or ice crystals suspended in the atmosphere. The four main types of clouds are cirrus, cumulus, stratus, and nimbus.

Cirrus clouds are high-altitude clouds (8,000-12,000 m) that are thin and feathery in appearance. They are always white in color.

Cumulus clouds are puffy, cotton-like clouds with a flat base. Stratus clouds are horizontal, layered clouds that cover large areas of the sky.

Nimbus clouds are dark gray or black clouds that are dense and form at middle or low altitudes. They are associated with precipitation.

Precipitation is the release of moisture from the atmosphere that falls to the Earth. It can be in liquid form like rain, or solid forms like snow, sleet, and hail.

Sleet consists of frozen raindrops or refrozen melted snow. Hailstones are solid pieces of ice formed when raindrops pass through very cold layers of air.

This type of rain occurs when air, heated by the surface, rises, expands, and cools, leading to condensation and heavy, short-lived showers. It is common in equatorial regions.

Orographic rain is caused when a moist air mass is forced to rise over a mountain. The windward side receives heavy rain, while the leeward side, or rain-shadow area, remains dry.

Cyclonic rainfall occurs when large masses of air with different temperatures meet, and the warmer air is forced to rise over the colder air, causing condensation and precipitation.

Generally, rainfall decreases from the equator to the poles. Coastal areas and windward mountain slopes receive more rainfall than continental interiors and leeward slopes.