Water in the Atmosphere Class 11 Geography Notes

This chapter explains atmospheric moisture, humidity, evaporation, condensation, cloud formation and precipitation. It highlights how water circulates through the atmosphere and influences weather and climate across the world.

• Atmospheric moisture
• Humidity
• Relative humidity
• Dew point
• Evaporation
• Condensation
• Forms of condensation
• Clouds
• Precipitation
• Types of rainfall
• World distribution of rainfall

Water Vapour in Atmosphere

• Water vapour exists in atmosphere
• 0–4% by volume of atmosphere
• Exists in gaseous, liquid and solid forms

Sources of Atmospheric Moisture

1. Evaporation from water bodies
2. Transpiration from plants

Water Cycle in Atmosphere

• Evaporation
• Transpiration
• Condensation
• Precipitation

Humidity

Humidity is the amount of water vapour present in air.

Absolute Humidity

• Weight of water vapour per unit volume of air
• Measured in grams per cubic metre

Relative Humidity

Relative humidity is the percentage of moisture present in atmosphere compared to its full capacity at a given temperature.

Characteristics of Relative Humidity

• Greater over oceans
• Least over continents

Saturated Air

Air holding moisture to full capacity is called saturated air.

Dew Point

Dew point is the temperature at which saturation occurs.

Evaporation

Evaporation is the transformation of water from liquid to gaseous state mainly due to heat.

Latent Heat of Vaporisation

Heat required to convert liquid into vapour without temperature change.

Factors Increasing Evaporation

1. Increase in temperature
2. Low moisture content
3. Greater air movement

Condensation

Condensation is the transformation of water vapour into water caused by loss of heat.

Hygroscopic Nuclei

• Dust particles
• Smoke particles
• Salt particles

Conditions for Condensation

1. Air reaches dew point
2. Air temperature decreases
3. Moisture is added

Major Forms of Condensation

1. Dew
2. Frost
3. Fog
4. Mist
5. Clouds

Dew

• Water droplets deposited on cool surfaces
• Found on grass, leaves and stones
• Forms during clear sky and calm air conditions

Frost

• Ice crystals deposited on cold surfaces
• Occurs below 0°C temperature

Fog

• Cloud near ground surface
• Visibility less than 1 km

Mist

• Similar to fog
• Visibility between 1–2 km

Smog

Fog mixed with smoke is called smog.

Meaning of Clouds

Clouds are masses of minute water droplets or ice crystals formed by condensation in free air.

Major Cloud Types

1. Cirrus
2. Cumulus
3. Stratus
4. Nimbus

Cirrus Clouds

• Highest altitude clouds
• Height between 8,000–12,000 m
• Thin and feather-like

Cumulus Clouds

• Cotton wool appearance
• Flat base
• Height between 4,000–7,000 m

Stratus Clouds

• Layered clouds
• Cover large sky area

Nimbus Clouds

• Black or dark grey clouds
• Dense and opaque
• Found near Earth’s surface

Combination Cloud Types

Cloud Type	Category
Cirrostratus	High cloud
Cirrocumulus	High cloud
Altostratus	Middle cloud
Altocumulus	Middle cloud
Stratocumulus	Low cloud
Nimbostratus	Low cloud
Cumulonimbus	Vertical cloud

Meaning of Precipitation

Precipitation is the release of moisture after condensation in liquid or solid form.

Forms of Precipitation

• Rainfall
• Snowfall
• Sleet
• Hailstones

Snowfall

• Occurs in form of snowflakes
• Occurs below 0°C temperature

Sleet

• Frozen raindrops
• Refrozen melted snow-water

Hailstones

• Rounded ice pieces
• Formed by rainwater passing through cold layers

Major Types of Rainfall

1. Convectional rainfall
2. Orographic rainfall
3. Cyclonic rainfall

Convectional Rainfall

• Air gets heated and rises
• Cooling and condensation occur
• Heavy rainfall takes place
• Common in equatorial regions

Characteristics

• Short duration
• Heavy rainfall
• Thunder and lightning

Orographic Rainfall

• Moist air rises over mountains
• Condensation causes rainfall on windward side

Rain Shadow Area

• Leeward side receives less rainfall
• Known as rain shadow region

Cyclonic Rainfall

Caused by cyclones and fronts.

General Distribution Pattern

• Rainfall decreases from equator towards poles
• Oceans receive more rainfall than land
• Coastal areas receive more rainfall

Rainfall between 35°–40° Latitude

• Eastern coasts receive more rainfall

Rainfall between 45°–65° Latitude

• Western margins receive more rainfall
• Due to westerlies

Mountain Rainfall Pattern

• Windward side receives heavy rainfall
• Leeward side receives less rainfall

Major Rainfall Regimes

Region	Rainfall
Equatorial belt	Over 200 cm
Interior continental areas	100–200 cm
Tropical interiors	50–100 cm
Rain shadow areas	Less than 50 cm

Very Short Answer Questions

1. What is humidity?
2. What is dew point?
3. What is smog?
4. What is precipitation?
5. Which is the highest cloud?

Short Answer Questions

1. Explain relative humidity.
2. Differentiate between dew and frost.
3. Explain cloud formation.
4. Discuss fog and mist.
5. Explain types of rainfall.

Long Answer Questions

1. Explain the forms of condensation.
2. Discuss the world distribution of rainfall.
3. Explain cloud types and their characteristics.
4. Describe the process of evaporation and condensation.
5. Explain orographic rainfall with diagrammatic understanding.

1. What is relative humidity?

Relative humidity is the percentage of moisture present in air compared to its maximum capacity.

2. What is dew point?

It is the temperature at which air becomes saturated.

3. What is smog?

Smog is fog mixed with smoke.

4. What is rain shadow area?

Dry area on the leeward side of mountains.

5. Which cloud is found at highest altitude?

Cirrus clouds are found at highest altitude.

• Water Vapour:
  • 0–4% of atmosphere
• Humidity:
  • Water vapour in air
• Relative Humidity:
  • Moisture percentage
• Dew Point:
  • Saturation temperature
• Evaporation:
  • Liquid to vapour
• Condensation:
  • Vapour to liquid
• Forms of Condensation:
  • Dew
  • Frost
  • Fog
  • Mist
  • Clouds
• Cloud Types:
  • Cirrus
  • Cumulus
  • Stratus
  • Nimbus
• Precipitation:
  • Rainfall
  • Snowfall
  • Sleet
  • Hailstones
• Rainfall Types:
  • Convectional
  • Orographic
  • Cyclonic
• Rainfall Pattern:
  • Decreases from equator to poles
• Windward Side:
  • Heavy rainfall
• Leeward Side:
  • Rain shadow region