Water in the Atmosphere — NCERT Solutions

Given: We need to explain the concept of relative humidity.

Concept: Relative humidity expresses the moisture content of air in relation to its maximum moisture-holding capacity at a given temperature.

Answer:

Relative humidity is the ratio of the actual amount of water vapour present in the air to the maximum amount of water vapour the air can hold at a given temperature, expressed as a percentage.

$$\text{Relative Humidity} = \frac{\text{Actual amount of water vapour in air}}{\text{Maximum capacity of air to hold water vapour at that temperature}} \times 100$$

• When relative humidity is 100%, the air is fully saturated and condensation begins.
• Relative humidity increases when temperature falls (as the capacity of air to hold moisture decreases).
• Relative humidity decreases when temperature rises.
• It is an important factor in determining the comfort level of weather and the likelihood of precipitation.

Given: We need to explain why water vapour decreases with increasing altitude.

Concept: The source of water vapour is the Earth's surface (oceans, water bodies, soil, vegetation). Temperature decreases with altitude in the troposphere.

Answer:

The amount of water vapour decreases rapidly with altitude due to the following reasons:

1. Source of water vapour is at the surface: Water vapour enters the atmosphere mainly through evaporation and transpiration from the Earth's surface (oceans, lakes, rivers, and vegetation). As altitude increases, the distance from this source increases, so less water vapour is available.

2. Decrease in temperature with altitude: Temperature decreases with increasing altitude in the troposphere. Since the capacity of air to hold water vapour depends on temperature — warmer air holds more moisture — the colder air at higher altitudes can hold very little water vapour.

3. Gravity: Water vapour, being lighter than dry air, still tends to remain concentrated in the lower layers of the atmosphere due to the overall density distribution of the atmosphere.

Thus, about 90% of the total water vapour in the atmosphere is found in the lower 5 km of the troposphere.

Given: We need to explain cloud formation and their classification.

Concept: Clouds form through the process of condensation when moist air rises, cools, and reaches the dew point.

Formation of Clouds:

When air containing water vapour rises (due to convection, orographic lifting, or frontal activity), it expands and cools at the dry adiabatic lapse rate. When the temperature of the rising air falls to the dew point, the water vapour condenses around tiny dust particles (condensation nuclei) to form tiny water droplets or ice crystals. A large collection of such droplets or ice crystals suspended in the atmosphere forms a cloud.

Classification of Clouds:

Clouds are classified on the basis of their height and form into four main groups:

Given: We need to discuss the major features of how precipitation is distributed across the world.

Answer:

The distribution of precipitation across the world is highly uneven and is influenced by latitude, distance from the sea, prevailing winds, ocean currents, and relief. The salient features are as follows:

1. Decrease from Equator to Poles:
In general, rainfall decreases as we move from the equator towards the poles. The equatorial regions receive heavy rainfall (over 200 cm per annum) due to high temperatures, high humidity, and strong convectional currents. Polar regions receive very little precipitation (less than 50 cm), mostly as snow.

2. Coastal vs. Interior Areas:
Coastal areas receive more rainfall than the interior parts of continents. This is because coastal areas are closer to the sea, which is the primary source of moisture. Interior continental areas are far from the sea and receive moderate to low rainfall.

3. Oceans vs. Landmasses:
Oceans receive more precipitation than landmasses because they are the major sources of water vapour through evaporation.

4. Effect of Latitudinal Belts:

• Between 35°–40° N and S of the equator, rainfall is heavier on the eastern coasts and decreases towards the west (due to trade winds bringing moisture from the east).
• Between 45°–65° N and S, due to the westerlies, rainfall is first received on the western margins of continents and decreases towards the east.

5. Effect of Mountains (Relief):
Wherever mountains run parallel to the coast, the windward side receives heavy rainfall while the leeward side (rain-shadow area) remains dry and receives very little rainfall.

6. Major Precipitation Regimes:

• Heavy rainfall (over 200 cm/year): Equatorial belt, windward slopes of coastal mountains in cool temperate zones, coastal areas of monsoon lands.
• Moderate rainfall (100–200 cm/year): Interior continental areas and coastal areas of continents.
• Low to moderate rainfall (50–100 cm/year): Central parts of tropical lands, eastern and interior parts of temperate lands.
• Very low rainfall (less than 50 cm/year): Rain-shadow zones, interior of continents, and high-latitude regions.

7. Seasonal Distribution:
In some regions, rainfall is distributed evenly throughout the year (e.g., equatorial belt, western parts of cool temperate regions). In other regions, rainfall is seasonal, such as in monsoon lands (summer rainfall) or Mediterranean regions (winter rainfall).

Conclusion: The world distribution of precipitation is thus governed by a complex interplay of atmospheric, oceanic, and terrestrial factors, resulting in great regional variations.

Given: We need to identify the forms of condensation and explain how dew and frost are formed.

Concept: Condensation is the process by which water vapour is converted into liquid water (or ice) when the temperature of the air falls below the dew point.

Forms of Condensation:

Condensation occurs in different forms depending on the temperature and the surface on which it takes place:

1. Dew — Water droplets deposited on cool surfaces at night when temperature is above freezing.
2. Frost — Ice crystals deposited on surfaces when temperature falls below freezing point (0°C).
3. Fog — Condensation near the ground surface forming a thick layer of tiny water droplets suspended in air, reducing visibility.
4. Mist — Similar to fog but less dense; visibility is not as severely reduced.
5. Clouds — Condensation at higher altitudes forming visible masses of water droplets or ice crystals.

Process of Dew Formation:

• Condition required: Clear sky, calm air, high relative humidity, and long nights (especially in summer and early autumn).
• Process: During the night, the Earth's surface loses heat rapidly through terrestrial radiation (especially on clear nights when there is no cloud cover to trap heat). The objects on the surface — such as grass, leaves, and stones — cool down quickly.
• When the temperature of these surfaces falls to the dew point (the temperature at which air becomes saturated), the water vapour in the air in contact with these cool surfaces condenses into tiny liquid water droplets.
• These droplets deposited on the cool surfaces are called dew.
• Dew is most commonly seen on grass and leaves in the early morning hours.

Condition: Temperature of the surface must be above 0°C.

Process of Frost Formation:

• Condition required: Clear sky, calm air, very low temperature (below 0°C), and low humidity.
• Process: Frost forms under conditions similar to dew, but when the temperature of the surface falls below the freezing point (0°C).
• In this case, the water vapour does not first condense into liquid water; instead, it directly solidifies (sublimates) into ice crystals on the surface.
• Alternatively, dew that has already formed may subsequently freeze if the temperature drops below 0°C.
• The white, feathery ice crystals seen on surfaces on cold winter mornings are called frost (also known as hoar frost).

Condition: Temperature of the surface must be at or below 0°C.

Key Difference between Dew and Frost: