Water In The Atmosphere

The atmosphere is an important agent in the circulation of water across the land, water bodies and air. If water vapour is absent in the air, then there is no way for water to circulate, and polluted water would not be renewed, instead be lost forever.

Other options are incorrect.

The atmosphere has a fixed water-holding capacity, dependent on its temperature at any time. When the water content in the air is equal to this capacity, the air is saturated.

A is incorrect. Relative Humidity refers to the amount of water content in the air, as compared to its saturated counterpart.

B is incorrect. Specific Humidity refers to the amount of water vapour present in the air per gram of air mass.

C is incorrect. Absolute humidity refers to the amount of water vapour present in the air per metre cube, regardless of the temperature of the air.

(i) The three types of precipitation are:

• Rainfall: Precipitation in the form of water.

• Sleet: Frozen rainfall

• Hailstones: Solid pieces of ice which reach the surface.

(ii) The atmosphere, at any time, has the capacity to hold a certain amount of water depending on its temperature. If the air holds maximum possible amount of vapour, it is called saturated. However, the percentage of water vapour held by the air, measured against the capacity of saturated air, is called relative humidity.

(iii) The amount of water vapour decreases rapidly with altitude due to the absence of water bodies which are a source of vapour. Also, temperature decreases with increasing height, which reduces the water-holding capacity.

(iv) Clouds are formed by the condensation of water vapour at higher altitudes around dust particles.

They are of the following types:

• Cirrus

• Cumulus

• Stratus

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(i) The following constitute the salient features of world distribution of precipitation:

• Rainfall decreases as we move from equator to higher altitudes.

• The coastal areas and water bodies themselves receive greater amount of rainfall than the interior areas of landmasses.

• Between 35 degrees north and 40 degrees north, the rainfall is received due to easterlies, which leads to higher precipitation along the eastern coasts than the western counterparts. However, due to the flow of westerlies in 45 degrees to 65 degrees north, the trend is reversed.

• Mountains act as barriers to the flow of wind, and cause rainfall on the windward side, leaving the leeward side sans rain.

(ii) The various forms of condensation are:

• Frost

• Clouds

• Dew

• Fog

Formation of Dew: Dew is formed when water vapour condenses over surfaces like grass and leaves. Clear sky, warm air, high relative humidity and cold, long nights help in the formation of dew. Usually, the dew point must be higher than the freezing point of water for this purpose.

Formation of Frost: Frost forms on cold surfaces when condensation occurs below the freezing point. Consequently, the deposition is in the form of ice crystals instead of water droplets. The air needs to fall to a temperature below freezing point for this to occur.

Early June – Kerala- Break of Monsoon

Mid June – Mumbai

July-August : Assam Floods

October-November: Chennai

December-January: Andaman and Nicobar Islands

the figures show the following clouds:

Figure 11.1 shows Cumulus clouds, as they look like cotton wool. These clouds are found in patches across the sky.

Figure 11.2 shows Nimbus clouds, as they are dark grey and shapeless mass. These clouds also seem to extend the ground.