Humidity Chapter Notes | Geography Class 9 ICSE PDF Download

Water is the essence of life, and its journey through the skies, oceans, and land is nothing short of magical! The chapter on Humidity in ICSE Class 9 Geography takes us on an exciting exploration of the water cycle, where water transforms from liquid to vapor and back again, shaping weather patterns and climates across the globe. From the formation of clouds to the fall of rain, snow, or hail, this chapter unravels the fascinating processes of evaporation, condensation, and precipitation. It also dives into how clouds and rainfall influence our environment, with real-world examples like Mumbai’s heavy rains and Pune’s drier climate. Get ready to discover the dynamic world of water in the atmosphere!

Humidity

• Humidity is the amount of water vapor present in a given volume of air at any time.
• Weather feels “humid” when the atmosphere contains large amounts of water vapor.
  

Types of Humidity:

• Humidity is expressed in two ways: Relative Humidity and Absolute Humidity.
• Relative Humidity: Measures how much water vapor is in the air compared to the maximum it can hold at a specific temperature.
• Expressed as a percentage, showing how close the air is to being fully saturated with moisture.
• Higher temperatures allow air to hold more water vapor, increasing its capacity.
• Relative humidity only accounts for invisible water vapor, excluding mist, clouds, fog, or water aerosols.

Example: On a hot day, if the air has 50% relative humidity, it holds half the water vapor it could at that temperature, making it feel less humid than a 90% relative humidity day.

Phenomena Associated with the Water Cycle

• The water cycle involves evaporation, condensation, and precipitation.
• Sun’s heat causes water to evaporate, forming water vapor.
• Cooled water vapor condenses to form clouds.
• Water falls from clouds as rainfall, hail, or snow.
• The water cycle is the continuous movement of water between oceans, atmosphere, and land.

Evaporation

• Evaporation is the process where water turns into water vapor due to heat.
• It occurs in oceans, seas, lakes, and rivers.

Factors increasing evaporation rate:

• Humidity: Dry air with low humidity speeds up evaporation.
• Heat Supply: Higher temperatures increase evaporation speed.
• Winds: Strong, dry winds accelerate evaporation, e.g., clothes dry faster on windy days.

Latent Heat

• Latent heat is the hidden energy that changes water’s state (solid to liquid or liquid to vapor) without changing temperature.
• Example: When ice melts into water, it absorbs heat without getting warmer until fully melted.

Condensation

• Condensation is the process where water vapor turns into tiny water droplets or ice.
• It happens when water vapor is added to saturated air or when air cools below its dew point.

Conditions required for condensation:

• High amount of water vapor in the atmosphere.
• Presence of tiny particles like dust, salt, or smoke for water vapor to condense around.
• Air temperature below the dew point.
• Example: Water droplets forming on a cold glass of water on a warm day show condensation.

Clouds

• Clouds are masses of tiny water droplets or ice crystals floating in the atmosphere.
• Formed when warm air rises, cools, and condenses around dust particles, creating water droplets.
• Billions of droplets combine to form clouds.
• Types of Clouds by Shape: Clouds are classified into three types based on their shapes.
  

Types of Clouds by Height:

• Low Clouds: Rain-bearing clouds with bases below 2 km above sea level, e.g., Cumulus.
• Middle Clouds: Found between 2 km and 6 km above sea level, e.g., Altocumulus.
• High Clouds: Located above 6 km, made of ice crystals, e.g., Cirrostratus.

Precipitation

• Precipitation is the process where condensed water vapor falls to Earth as rain, snow, hail, etc.
  

Forms of Precipitation:

• Rain: The most common form, where water droplets fall from clouds.
• Frost: Ice crystals formed when water vapor freezes on surfaces like leaves or rocks below 0°C.
• Dew: Water droplets formed on cool surfaces at night or early morning due to condensation.
• Mist: Tiny water droplets suspended near the ground, less dense than fog.
• Fog: Thick layer of water droplets or mist near the ground.
• Smog: A mix of fog, smoke, and dust, common in industrial areas.
• Hail: Solid ice pieces falling from clouds.
• Sleet: A mix of rain and ice.
• Rainfall is measured using a rain gauge, a metal cylinder with a funnel and glass bottle to collect rain.
• Snowfall is measured by the thickness of snow.

Factors Affecting Rainfall

Land and Sea Contrast: Coastal areas get more rain than inland regions.

• Direction of Winds: Winds from oceans to land bring more rain than winds from inland.
• Presence of Mountains: Windward sides of mountains receive more rain, while leeward sides get less.

Example: Mumbai (windward side of Western Ghats) gets heavy rainfall, while Pune (leeward side) gets less.

Rainfall and Its Types

• Rainfall is classified into three types: convectional, orographic, and cyclonic.

• Common in equatorial regions due to high temperatures causing rapid evaporation.
• Hot air rises, expands, and cools, forming cumulonimbus clouds.
• These clouds cause heavy rainfall with thunder and lightning.
• Example: The Amazon river basin experiences daily rain due to convectional rainfall.

• Caused by mountains forcing moisture-bearing winds to rise.
• Rising air cools and condenses, causing rain on the windward side.
• Leeward side gets little rain due to dry winds.
• Example: Mumbai (windward side of Western Ghats) gets heavy rain, while Pune (leeward side) gets scanty rain.

• Occurs when warm and cold air masses meet.
• Lighter warm air rises over cold air, cools, and causes heavy rainfall.
• Lasts a few hours, often during tropical cyclones.
• Example: Heavy rain during cyclones in coastal areas is cyclonic rainfall.

Thunderstorms

• Thunderstorms involve heavy rain and lightning.
• Lightning occurs when raindrops move due to convectional air currents, forming ice particles.
• Ice particles rubbing together create electric charges: positive ions at the cloud’s top, negative ions at the bottom.
• Attraction between positive and negative ions causes lightning flashes.

Seasonal Distribution of Rainfall

• Rainfall varies by season across regions.
• Year-Round Rainfall: Western margins of mid-latitude continents, like Western Europe, get rain throughout the year.
• Summer Rainfall: Sub-tropical regions experience convectional rainfall in summer.
• Winter Rainfall: Mediterranean countries like France, Albania, and Italy get rain in winter.

World Patterns of Rainfall Distribution

• May to October: Northern Hemisphere’s equatorial belt, Eastern Asia, and eastern North/South America get heavy rain from onshore winds.
• Western sides of these regions and Polar Regions get no rain during this period.
• November to April: Southern Hemisphere’s Tropic of Capricorn gets direct sunlight, so eastern coasts of Argentina, Africa, Brazil, and Australia receive rain from trade winds.

World Distribution of Rainfall

• Regions of Heavy Rainfall(more than 200 cm):
  • Equatorial regions of Africa, South America, and Southeast Asia.
  • Coastal monsoon regions of Asia.
  • West coastal regions in the westerly wind belt of mid-latitudes in North America and Western Europe.
• Regions of Moderate Rainfall(100–200 cm):
  • Sub-tropical areas like East China, Eastern Brazil, and Southeast Asia.
• Regions of Low Rainfall(less than 25 cm):
  • Tropical deserts on western continental margins and inland areas far from oceans.
  • Cold deserts or Polar Regions.