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Water in the Atmosphere – Class 11 Geography notes, important questions and MCQs

NCERT Class 11 Geography, chapter 10. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Water vapour in the atmosphere varies from place to place and is measured as humidity. Water changes state through evaporation, condensation and precipitation, which are part of the hydrological cycle. Condensation produces dew, frost, fog, mist and clouds, and precipitation falls as rain, snow, sleet or hail. Rainfall is of three types: convectional, orographic and cyclonic.

Humidity, evaporation and condensation

Absolute humidity is the actual amount of water vapour in a unit volume of air; relative humidity is the percentage of moisture present compared to the full capacity at that temperature.
Air is saturated when it holds its full capacity; the temperature at which saturation occurs is the dew point.
Condensation occurs when air cools to its dew point, often around condensation nuclei such as dust and salt particles; its forms include dew, frost, fog, mist and clouds.

Clouds and precipitation

Clouds are classified as cirrus (high, feathery), cumulus (puffy, cotton-like), stratus (layered) and nimbus (dark, rain-bearing).
Precipitation forms include rain, snow, sleet and hail.
Convectional rainfall is common in equatorial regions in the afternoon; orographic rainfall occurs on the windward side of mountains, leaving a rain-shadow area on the leeward side; cyclonic rainfall occurs along fronts.

Key terms

Humidity: the amount of water vapour in the air
Relative humidity: moisture present as a percentage of the air's capacity at that temperature
Dew point: the temperature at which air becomes saturated
Orographic rainfall: rainfall caused when moist air rises over mountains
Rain-shadow area: the dry leeward side of a mountain

Important questions with answers

What is relative humidity?2 marks

Relative humidity is the ratio of the actual amount of water vapour in the air to the maximum amount it can hold at that temperature, expressed as a percentage. It increases when air cools and decreases when air warms, even if the moisture content stays the same.

What are the forms of condensation?3 marks

When air cools to its dew point, condensation forms dew (water droplets on cool surfaces), frost (ice crystals when the dew point is below freezing), fog and mist (tiny droplets suspended near the ground, reducing visibility) and clouds (masses of droplets or ice crystals at height).

Explain the types of rainfall.5 marks

Rainfall occurs when moist air rises, cools to its dew point and condenses into clouds whose droplets grow large enough to fall. There are three main types. Convectional rainfall occurs when the ground is strongly heated, the air above it warms and rises in convection currents, cools and forms cumulonimbus clouds, giving heavy showers with thunder and lightning, usually in the afternoon; it is common in equatorial regions and in summer in the interiors of continents. Orographic or relief rainfall occurs when moist winds are forced to rise over mountains; the air cools and gives heavy rain on the windward side, while the leeward side receives little rain because the descending air warms and becomes dry, creating a rain-shadow area. The Western Ghats receive heavy orographic rain from the south-west monsoon, while the Deccan plateau to their east is in a rain shadow. Cyclonic or frontal rainfall occurs when warm, moist air rises over cold air along a front or in a cyclone, giving prolonged rainfall, as in the middle latitudes, and tropical cyclones bring very heavy rain to coastal areas.

Quick revision

Absolute and relative humidity; dew point.
Condensation: dew, frost, fog, mist, clouds.
Clouds: cirrus, cumulus, stratus, nimbus.
Rainfall: convectional, orographic, cyclonic.

Exam tips

Draw orographic rainfall with windward and leeward sides.
Learn the four basic cloud types.

Practice MCQs

The amount of water vapour in the air is called
1. humidity
2. pressure
3. albedo
4. insolation
Show answer
Answer: (a) humidity. It varies with temperature.
Dark rain-bearing clouds are
1. cirrus
2. cumulus
3. stratus
4. nimbus
Show answer
Answer: (d) nimbus. They are thick and low.
The temperature at which air becomes saturated is the
1. boiling point
2. melting point
3. freezing point
4. dew point
Show answer
Answer: (d) dew point. Condensation begins there.
The dry leeward side of a mountain is called a
1. windward slope
2. delta
3. floodplain
4. rain-shadow area
Show answer
Answer: (d) rain-shadow area. Descending air warms and dries.
Rainfall common in equatorial regions in the afternoon is
1. orographic
2. cyclonic
3. frontal only
4. convectional
Show answer
Answer: (d) convectional. Heated air rises and cools.
High, thin feathery clouds are
1. cumulus
2. stratus
3. cirrus
4. nimbus
Show answer
Answer: (c) cirrus. They are made of ice crystals.
Relative humidity increases when
1. temperature rises with the same moisture
2. temperature falls with the same moisture
3. air becomes drier
4. wind stops always
Show answer
Answer: (b) temperature falls with the same moisture. Cooler air holds less vapour.
Why does the western side of the Western Ghats receive heavy rain
1. it is a rain-shadow area
2. it lies far from the sea
3. it has no winds
4. moist south-west monsoon winds rise over the mountains and cool
Show answer
Answer: (d) moist south-west monsoon winds rise over the mountains and cool. This is orographic rainfall.

All chapters of Class 11 Geography

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.