WATER IN THE ATMOSPHERE 

• Water vapour present in the air is called humidity.
• The actual amount of water vapour present in the atmosphere is said to be absolute humidity.
• The weight of the water vapour per unit volume is expressed in terms of grams per cubic meter.
• The absolute humidity differs from place to place on the surface of the earth.
• The percentage of moisture present in the atmosphere at a given temperature is called relative humidity.
• The air containing moisture to its full capacity at a given temperature is said to be saturated.
• The temperature at which saturation occurs in a given sample of air is known as the dew point.

1. Evaporation and condensation

Evaporation is the process by which water is transformed from liquid to gaseous. The amount of water vapour in the atmosphere is added or withdrawn due to evaporation and condensation respectively. Heat is the main cause of evaporation. Water starts evaporating is referred to as the latent heat of vaporization. The transformation of water vapour into water is called condensation; it is caused by the loss of heat.

Sublimation means the excess water vapour condenses from liquid form to solid form.

Condensation is influenced by the volume of air, temperature, pressure and humidity. Condensation takes place

1. When the temperature of the air is reduced to dew point with its volume remaining constant;
2. When both the volume and the temperature are reduced;
3. When the moisture is added to the air through evaporation.

• The most favourable condition for condensation is a decrease in air temperature.
• Dew, fog, frost and clouds form after the condensation of the water vapour or moisture of the atmosphere.
• Condensation takes place when the dew point is lower or higher than the freezing point.

Dew: When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects such as stones, grass blades and plant leaves is known as dew. The ideal conditions for its formation are the clear sky, calm air, high relative humidity and cold and long nights. The dew point is above the freezing point.

Frost: It forms on cold surfaces when condensation takes place below freezing points. The dew point is at or below the freezing point. Moisture in the form of minute ice crystals instead of water droplets.

Fog and mist: Fog is a cloud with its base at or near the ground. Because of fog and mist, the visibility becomes poor to zero. In urban and industrial centres smoke provides plenty of nuclei for the formation of fog and mist. When the fog is mixed with smoke is described as smog. The mist contains more moisture than fog. Mists are frequent on mountains as the rising warm air up the slopes meets a cold surface. Fogs are drier than mist and they are prevalent where the warm currents of the air come in contact with cold currents. Fogs are mini clouds in which condensation takes place around nuclei provided by dust, smoke, and salt particulars.

Clouds: A cloud is a mass of minute water droplets or tiny crystals of ice formed by the condensation of the water vapour in free air at considerable elevations. Clouds are formed at some height over the surface of the earth. Based on the height, expanse, density and transparency or opaqueness clouds are grouped under four types. They are,

• Cirrus: Cirrus clouds formed at high altitudes of 8,000-12,000 meters, in thin and detached clouds having a feather appearance. They are always in white.
• Cumulus: These are like cotton wool, formed at a height of 4,000- 7,000 meters.
• Stratus: These are layered clouds covering large portions of the sky. These clouds are generally formed either due to loss of heat or the mixing of air masses with different temperatures.
• Nimbus: Nimbus clouds are black or dark in colour with shapeless masses of thick vapour. They formed at middle levels or very near to the surface of the earth. Sometimes they seem to touch the ground. These are extremely dense and opaque to the rays of the sun.

1. A combination of these four basic types of clouds; 
2. High clouds- cirrus, cirrostratus, cirrocumulus:
3. Middle clouds- altostratus and altocumulus;
4. Low clouds- stratocumulus and nimbostratus
5. Clouds with extensive vertical development – cumulus and cumulonimbus.

2. Precipitation

After the condensation of water vapour, the release of moisture is called precipitation. It is in the form of water called rainfall. When the temperature is lower than 0®C, precipitation takes place in the form of fine flakes of snow is called snowfall. Another form of precipitation includes sleet and hail. Sleet is frozen raindrops and frozen melted snow water. Solidified raindrops into small rounded solid pieces of ice are called hailstones. These are formed by the rainwater passing through the colder layers.

Types of rainfall

 Based on origin, rainfall may be classified into three main types.

Convectional rain: The air being heated becomes light and rises in convection currents. As it rises, it expands and loses heat and consequently, condensation takes place and cumulous clouds are formed. Heavy rainfall with thunder and lightning takes place but this does not last long. Such rain is common in the summer or the hotter part of the day. It is very common in the equatorial regions and interior parts of the continents, particularly in the northern hemisphere.

Orographic rain: When the saturated air mass comes across a mountain, it is forced to rise. The rising air expands, and eventually, the temperature falls, and the moisture gets condensed. The main characteristic of this type of rain is that the windward slopes get more rainfall. After giving rain on the windward side, when these winds reach the other slope, they descend, and their temperature increases. Then their ability to take in moisture increases and hence, these leeward slopes remain dry and rainless. The area situated on the leeward side is known as the rain-shadow area or relief rain.

Cyclonic rain: Cyclonic rain, also known as frontal rain, is a type of precipitation associated with low-pressure systems like cyclones or tropical storms. It's characterized by heavy and often persistent rainfall, often occurring in bands or bursts. 

3. World distribution of rainfall

Different places on the earth’s surface receive different amounts of rainfall annually based on seasons. From the equator towards the poles, rainfall decreases steadily. The coastal area of the world receives a higher amount of rainfall than the interior of the continents.   Rainfall is more in oceans compared to the landmasses. Between the latitudes 35® and 40®N and S of the equator, the rain is heavier on the eastern coasts and decreases towards the west. Due to the westerlies, [between 45® and 65® N and S] the rainfall is first received on the western margins of the continents and goes on decreasing towards the east. If mountains run parallel to the coast, the rain is greater on the coastal plain and on the windward side, it decreases towards the leeward side.

The equatorial belt

The windward slopes of the mountains along the western coasts in the cool temperature zone and the coastal areas of the monsoon land receive heavy rainfall of 200 cm per annum. Interior continental areas receive moderate rainfall varying from 100-200 per annum. The coastal areas of the continents receive a moderate amount of rainfall.  The central parts of the tropical land and the eastern and interior parts of the temperature lands receive rainfall varying between 50-100 cm per annum. The interior of the continents and high latitudes receive very low rainfall –less than 50 cm per annum