Notes Class 8 Chapter 3 Geography Maharashtra Board

Notes For All Chapters – Geography Class 8

Humidity and Clouds

1. Introduction to Weather and Moisture in Air

• Weather conditions differ in various places due to changes in humidity.
• Examples of different weather conditions:
• Rajasthan: Hot and dry, very little moisture.
• Kashmir: Cold and dry, minimal moisture.
• Mumbai: Hot and humid, high moisture content.
• Moisture in the air is invisible but affects rainfall and climate.

2. Evaporation

Definition: The process where water converts into water vapour due to heat from the sun.

• Factors affecting evaporation:

1. Temperature: Higher temperature increases evaporation.
2. Humidity: Evaporation slows in humid air.
3. Wind Speed: Faster wind speeds up evaporation.
4. Dryness of Air: Evaporation is quicker in dry conditions.

Activity: Observe which season clothes dry faster and why.

3. Humidity in the Air

• Definition: The amount of water vapour in the air.
• Air’s moisture-holding capacity:
• Warm air holds more moisture.
• Cold air holds less moisture.
• Saturation: When air holds maximum moisture at a given temperature.

Activity: Perform a sponge experiment to understand air’s moisture-holding capacity.

4. Types of Humidity

1. Absolute Humidity:

• Definition: The actual water vapour present in 1 cubic meter of air.
• Example: Coastal regions have higher absolute humidity.

2. Relative Humidity:

• Definition: The ratio of absolute humidity to the air’s vapour-holding capacity, expressed as a percentage.
• Formula: Relative Humidity=(Absolute HumidityVapour Holding Capacity)×100\text{Relative Humidity} = \left( \frac{\text{Absolute Humidity}}{\text{Vapour Holding Capacity}} \right) \times 100Relative Humidity=(Vapour Holding CapacityAbsolute Humidity​)×100
• Example: If absolute humidity = 15 gm/m³ and vapour-holding capacity = 30 gm/m³, RH=(1530)×100=50%RH = \left( \frac{15}{30} \right) \times 100 = 50\%RH=(3015​)×100=50%

3. Saturation and Dew Point:

• Saturation: When air cannot hold more moisture.
• Dew Point: The temperature at which air reaches saturation and condensation begins.

Activity: Observe how humidity affects food spoilage and fungus formation.

5. Condensation and Densification

• Condensation: Water vapour turns into liquid when cooled.
• Sublimation: Direct conversion of vapour into solid (ice/snow).
• Forms of Condensation:
• Dew: Water droplets on cool surfaces.
• Frost: Ice crystals form when temperature is below freezing.
• Fog: Water vapour forms mist near the ground.
• Clouds: Water vapour condenses at high altitudes.

Activity: Observe steam from boiling water condensing on a cooler surface.

6. Clouds

• Definition: Mass of condensed water droplets or ice particles in the air.
• Formation of Clouds:
• Sun heats land and water → Evaporation occurs.
• Warm air rises, cools, and condenses → Cloud formation.
• Difference Between Fog and Smog:
• Fog: Made of tiny water droplets.
• Smog: Fog mixed with pollutants.

7. Types of Clouds

Clouds are classified based on altitude:

(A) High Clouds (Above 7000m)

1 Cirrus:

• Wispy, feather-like clouds.
• Made of ice crystals.
• Indicate good weather.

2. Cirrocumulus:

• Small, white wave-like clouds.

3. Cirrostratus:

• Thin, sheet-like clouds covering the sky.
• Sun appears as if seen through frosted glass.

(B) Medium Clouds (2000m – 7000m)

1. Altocumulus:

• White and grey wave-like layers.

2. Altostratus:

• Thin, uniform grey clouds.

(C) Low Clouds (Below 2000m)

1. Stratocumulus:

• Thick, round cloud clusters.

2. Stratus:

• Low, grey clouds covering the sky.

3. Nimbostratus:

• Thick, dark clouds bringing continuous rain or snow.

(D) Vertical Clouds

1. Cumulus:

• Puffy, white clouds.
• Indicate fair weather.

2. Cumulonimbus:

• Dark, dense, towering clouds.
• Cause thunderstorms and heavy rain.

8. Cloudburst and Thunderstorms

• Cloudburst:
• Definition: Sudden, intense rainfall in a small area.
• Cause: Strong vertical winds prevent raindrops from falling until they become too heavy.
• Common in: Himalayan regions.
• Thunderstorms:
• Cumulonimbus clouds store electrical charges.
• Lightning occurs due to charge imbalance.
• Thunder is caused by air expansion due to heat from lightning.

9. Importance of Humidity and Clouds

• Weather Prediction: Cloud formations help forecast rain and storms.
• Effect on Climate: High humidity leads to heavy rainfall, while low humidity causes dryness.
• Impact on Humans:
• High humidity causes discomfort and sweating.
• Low humidity results in dry skin and dehydration.