Class | Subject | Book | Chapter | Session | Questions covered |
|---|---|---|---|---|---|
XI | Geography | Fundamentals of Physical Geography (NCERT) | 10 – Water in the Atmosphere | 2026-27 | 10 of 10 |
Chapter at a glance
Topic | Key points |
|---|---|
Humidity | Absolute (g/m³), specific, relative (%); saturated air at dew point |
Evaporation and condensation | Evaporation turns liquid to vapour (latent heat absorbed); condensation on hygroscopic nuclei when air cools below dew point |
Forms of condensation | Dew, frost, fog, mist, clouds |
Clouds | Cirrus (high), cumulus, stratus, nimbus |
Precipitation | Rain, snow, sleet, hail; types: convectional, orographic, cyclonic (frontal) |
World distribution | Heavy at equator and on windward coasts; decreases poleward and inland; rain-shadow areas |
Exercise Questions
Question 1 (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
Answer
In the context of this chapter, water vapour is the most important: it is the source of all precipitation and regulates temperature.
- Water vapour
Question 1 (ii)
Which one of the following process is responsible for transforming liquid into vapour?
(a) Condensation (b) Transpiration (c) Evaporation (d) Precipitation
Answer
- Evaporation
Question 1 (iii)
The air that contains moisture to its full capacity:
(a) Relative humidity (b) Specific humidity (c) Absolute humidity (d) Saturated air
Answer
- Saturated air
Question 1 (iv)
Which one of the following is the highest cloud in the sky?
(a) Cirrus (b) Stratus (c) Nimbus (d) Cumulus
Answer
Cirrus clouds form at 8,000–12,000 m.
- Cirrus
Question 2 (i)
Name the three types of precipitation.
Answer
On the basis of origin: convectional rainfall, orographic (relief) rainfall and cyclonic (frontal) rainfall. (On the basis of form: rain, snow, sleet and hail.)
Convectional, orographic and cyclonic (frontal) rainfall.
Question 2 (ii)
Explain relative humidity.
Answer
Relative humidity is the percentage of moisture present in the air compared with the maximum it can hold at that temperature:
It changes with temperature: warm air can hold more moisture, so RH falls when temperature rises (if moisture is unchanged). Saturated air has RH of 100 %.
The percentage of moisture in the air compared with its full capacity at that temperature; saturated air is at 100 %.
Question 2 (iii)
Why does the amount of water vapour decreases rapidly with altitude?
Answer
Water vapour enters the air by evaporation from the surface (oceans, lakes, plants), so it is concentrated in the lower layers. With height, temperature falls, and cold air can hold less vapour; the rising air cools, condenses and loses moisture as clouds and precipitation. Hence vapour decreases rapidly with altitude.
Because water vapour comes from surface evaporation and cooler air at height holds less moisture, so it condenses out as air rises.
Question 2 (iv)
How are clouds formed? Classify them.
Answer
Formation: when moist air rises, it expands and cools adiabatically. When it cools below its dew point, the water vapour condenses around hygroscopic nuclei (dust, salt, smoke) into tiny water droplets or ice crystals, forming clouds at considerable heights.
Classification (by form and height):
- Cirrus — high (8,000–12,000 m), thin, feathery, white.
- Cumulus — like cotton wool, at 4,000–7,000 m, with flat bases.
- Stratus — layered clouds covering large parts of the sky.
- Nimbus — black or dark grey, low, thick, rain-bearing.
Combinations: cirrostratus, cirrocumulus, altostratus, altocumulus, stratocumulus, nimbostratus, cumulonimbus.
Rising air cools below the dew point and its vapour condenses on nuclei into droplets/ice; clouds are cirrus, cumulus, stratus and nimbus (and their combinations).
Question 3 (i)
Discuss the salient features of the world distribution of precipitation.
Answer
- Decreases from the equator to the poles: equatorial regions receive heavy rainfall throughout the year (over 200 cm), due to convection.
- Coastal areas receive more rain than interiors; rainfall decreases from coasts towards the interiors of continents.
- Oceans receive more than land.
- Between 35° and 40° N and S, east coasts receive more rain (trade winds), decreasing westward; between 45° and 65°, west coasts receive more rain (westerlies), decreasing eastward.
- Windward slopes of mountains receive heavy orographic rainfall; leeward sides are rain-shadow areas.
- Subtropical high-pressure belts (around 30°) and the interiors of continents have deserts with very little rain.
- Classification by amount: heavy (over 200 cm) — equatorial belt, windward coasts and monsoon lands; moderate (100–200 cm); low (50–100 cm) — interiors; very low (under 50 cm) — deserts and polar areas.
- Seasonality: some areas get rain all year (equatorial, west coasts of temperate lands); others mainly in summer (monsoon areas) or in winter (Mediterranean).
Precipitation is heaviest near the equator and on windward coasts and slopes, decreases poleward and inland, is scant in subtropical highs and continental interiors (deserts) and polar regions, and varies in seasonality (all-year, summer monsoon, winter Mediterranean).
Question 3 (ii)
What are forms of condensation? Describe the process of dew and frost formation.
Answer
Forms of condensation: dew, frost, fog, mist and clouds. They are classified by the temperature and location of condensation.
Dew: when moisture condenses as water droplets on cool surfaces (grass, leaves, stones) rather than on nuclei in the air. Ideal conditions: clear sky, calm air, high relative humidity, cold and long nights. The surface cools by radiation below the dew point, and the dew point must be above freezing.
Frost: forms on cold surfaces when condensation takes place below freezing point (0°C), i.e. the dew point is at or below freezing. Excess moisture is deposited as tiny ice crystals instead of water droplets. Conditions are the same as for dew, except that air temperature must be at or below freezing.
Forms: dew, frost, fog, mist and clouds. Dew forms as droplets on surfaces cooled below dew point (above 0°C) on clear, calm nights; frost forms the same way as ice crystals when the dew point is at or below 0°C.
Topic checklist
- Humidity: absolute, specific and relative
- Evaporation and condensation
- Forms of condensation; types of clouds
- Precipitation: forms and types
- World distribution of rainfall