NCERT Solutions for Class 11 Geography - Water in the Atmosphere

Q1: Multiple choice questions.
(i) Which one of the following is the most important constituent of the atmosphere for human beings?
(a) Water vapour
(b) Nitrogen
(c) Dust particle
(d) Oxygen
Ans: (a) Water vapour

(ii) Which one of the following process is responsible for transforming liquid into vapour?
(a) Condensation
(b) Transpiration
(c) Evaporation
(d) Precipitation
Ans: (c) Evaporation

(iii) The air that contains moisture to its full capacity :
(a) Relative humidity
(b) Specific humidity
(c) Absolute humidity
(d) Saturated air
Ans: (d) Saturated air

(iv) Which one of the following is the highest cloud in the sky?
(a) Cirrus
(b) Stratus
(c) Nimbus
(d) Cumulus
Ans: (a)

Q2: Answer the following questions in about 30 words.
(i) Name the three types of precipitation.
Ans: The three types of precipitation are:

• Rainfall: The precipitation in the form of water.
• Snowfall: The precipitation in the form of fine flakes of snow.
• Hailstones: Sometimes, drops of rain after being released by the clouds become solidified into small rounded solid pieces of ice and which reach the surface of the earth.

(ii) Explain relative humidity.
Ans: The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as the relative humidity. 

(iii) Why does the amount of water vapour decreases rapidly with altitude?
Ans: The water vapour is greater over the oceans and least over the continents. There is absence of these water bodies on high altitudes. Temperature decreases with higher altitudes, therefore, rates of evaporation also decreases as a result water vapour also decreases rapidly with altitude.

(iv) How are clouds formed? Classify them.
Ans: 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.
According to their height, expanse, density and transparency or opaqueness clouds are grouped under four types:

• Cirrus
• Cumulus
• Stratus
• Nimbus.

Q3: Answer the following questions in about 150 words.
(i) Discuss the salient features of the world distribution of precipitation.
Ans: Different places on the earth’s surface receive different amounts of rainfall in a year and that too in different seasons.

• The rainfall decreases as we go from equator towards poles.
• The coastal areas of the world receive greater amounts of rainfall than the interior of the continents.
• The rainfall is more over the oceans than on the landmasses of the world because of being great sources of water.
• Between the latitudes 35° and 40° N and S of the equator, the rain is heavier on the eastern coasts and goes on decreasing towards the west.
• Between 45° and 65° N and S of equator, due to the westerlies, the rainfall is first received on the western margins of the continents and it goes on decreasing towards the east.
• Wherever mountains run parallel to the coast, the rain is greater on the coastal plain, on the windward side and it decreases towards the leeward side.

(ii) What are forms of condensation? Describe the process of dew and frost formation.
Ans: The transformation of water vapour into water is called condensation. Condensation is caused by the loss of heat.
The forms of condensation are dew, frost, fog and clouds.

• Formation of 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, it is known as dew. The ideal conditions for its formation are clear sky, calm air, high relative humidity, and cold and long nights. For the formation of dew, it is necessary that the dew point is above the freezing point.
• Formation of frost: Frost forms on cold surfaces when condensation takes place below freezing point i.e. the dew point is at or below the freezing point. The excess moisture is deposited in the form of minute ice crystals instead of water droplets. The ideal conditions for the formation of white frost are the same as those for the formation of dew, except that the air temperature must be at or below the freezing point.