Chapter – 10

Water in the Atmosphere

In this post we have given the detailed notes of class 11 Geography Book 1 Chapter 10 (Water in the Atmosphere) in English. These notes are useful for the students who are going to appear in class 11 board exams.

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Chapter 10: Water in the Atmosphere

Introduction:

Water vapor, present in the air at varying amounts (0-4% by volume), plays a crucial role in weather phenomena. It exists in three forms: gaseous, liquid, and solid. The atmosphere’s moisture comes from evaporation from water bodies and transpiration from plants, creating a continuous water exchange between the atmosphere, oceans, and continents.  

Humidity:

• Humidity is the amount of water vapor in the air.
  • Absolute humidity: The actual weight of water vapor per unit volume of air (grams per cubic meter). It varies geographically.  
  • Relative humidity: The percentage of moisture in the air compared to its maximum capacity at a given temperature. It changes with temperature, as warmer air can hold more moisture. Relative humidity is higher over oceans and lower over continents.  

Saturation:

• Saturated air is air containing the maximum possible water vapor at a given temperature.  
• Dew point is the temperature at which a given air sample becomes saturated.  

Evaporation and Condensation:

• Evaporation: The process of water changing from liquid to gas, driven by heat. The temperature at which evaporation starts is called the latent heat of vaporization. Higher temperatures and lower moisture content increase the air’s capacity to absorb and retain moisture. Air movement enhances evaporation by replacing saturated air with unsaturated air.  
• Condensation: The conversion of water vapor to liquid water, triggered by cooling. When moist air cools, it loses its capacity to hold water vapor, leading to condensation. In the atmosphere, condensation happens around tiny particles called hygroscopic condensation nuclei, like dust, smoke, and salt, which attract water. Condensation also occurs when moist air contacts a colder surface or when the temperature nears the dew point. The extent of cooling and the relative humidity influence condensation, along with factors like air volume, temperature, pressure, and humidity.  

Conditions Favouring Condensation:

• Air temperature dropping to the dew point at a constant volume.  
• Reduction in both air volume and temperature.  
• Increased moisture in the air through evaporation.  
• The most favourable condition is a decrease in air temperature.  

Forms of Condensation:

• Dew: Water droplets forming on cool surfaces (like stones, grass, leaves) when the dew point is above freezing. Ideal conditions include clear skies, calm air, high relative humidity, and cold, long nights.  
• Frost: Minute ice crystals deposited on cold surfaces when condensation occurs below freezing (0°C). Ideal conditions are similar to dew formation, but the temperature must be at or below freezing.  
• Fog: A cloud at or near ground level, formed by condensation within an air mass due to sudden cooling, reducing visibility. Smoke in urban areas contributes to fog formation, creating smog.  
• Mist: Similar to fog but with more moisture, creating thicker layers of moisture around each nuclei. Mists are common over mountains where warm, rising air meets cold surfaces. Fogs are drier and occur where warm and cold air currents meet.  
• Clouds: Masses of minute water droplets or tiny ice crystals formed by condensation at significant heights. Their shapes vary due to their height.  

Types of Clouds:

• Cirrus: High-altitude (8,000-12,000m), thin, feathery, white clouds.  
• Cumulus: Cotton-like clouds with a flat base, forming at 4,000-7,000m.  
• Stratus: Layered clouds covering large areas, often formed by heat loss or mixing of air masses with different temperatures.  
• Nimbus: Dark, shapeless clouds at middle levels or near the surface, associated with rain.  

Cloud Combinations:

• High clouds: Cirrus, cirrostratus, cirrocumulus  
• Middle clouds: Altostratus, altocumulus  
• Low clouds: Stratocumulus, nimbostratus  
• Extensive vertical development: Cumulus, cumulonimbus  

Precipitation:

• Precipitation: The release of moisture from the atmosphere after condensation, occurring in liquid or solid forms.
  • Rainfall: Liquid precipitation.  
  • Snowfall: Precipitation as fine flakes of snow when the temperature is below 0°C.  
  • Sleet: Frozen raindrops or refrozen melted snow. It forms when rain falls through a subfreezing layer of air near the ground.  
  • Hail: Small, rounded pieces of ice formed when raindrops solidify while passing through colder air layers. Hailstones have multiple ice layers.  

Types of Rainfall:

• Convectional rain: Heavy, short-lived rainfall with thunder and lightning, common in summer or hot parts of the day, especially in equatorial regions and continental interiors.  
• Orographic rain: Rainfall caused by moist air forced to rise and cool over mountains, resulting in more rain on the windward slopes and a rain-shadow area on the leeward side.  
• Cyclonic rain: Rainfall associated with cyclonic activity, where warm and cold air masses meet (refer to Chapter 9 for details).  

World Distribution of Rainfall:

• Rainfall amounts vary geographically and seasonally.  
• Generally decreases from the equator towards the poles.  
• Coastal areas receive more rainfall than inland areas.  
• Oceans have higher rainfall than landmasses.  
• Between 35° and 40° N and S, rainfall is heavier on eastern coasts.  
• Between 45° and 65° N and S, rainfall is heavier on western coasts due to the westerlies.  
• Mountains parallel to the coast cause higher rainfall on the windward side.  

Major Precipitation Regimes:

• Heavy rainfall (over 200 cm per annum): Equatorial belt, windward slopes of mountains in the cool temperate zone, coastal areas of monsoon lands.  
• Moderate rainfall (100-200 cm per annum): Interior continental areas, coastal areas of continents.  
• Moderate rainfall (50-100 cm per annum): Central parts of tropical land, eastern and interior parts of temperate lands.  
• Low rainfall (less than 50 cm per annum): Rain shadow zones, interiors of continents, high latitudes.  

Seasonal Distribution of Rainfall:

• Even Distribution: In some regions, rainfall is distributed relatively evenly throughout the year. This pattern is characteristic of the equatorial belt and the western parts of cool temperate regions.  
• Seasonal Concentration: In many other regions, rainfall is concentrated in specific seasons. This can lead to distinct wet and dry seasons. The timing and intensity of these seasons are influenced by factors such as the movement of the Intertropical Convergence Zone (ITCZ), monsoon winds, and the position of high and low-pressure systems.  

Impact of Seasonal Rainfall Distribution:

• Agriculture: The seasonal distribution of rainfall has a significant impact on agriculture. Farmers need to plan their cropping patterns and irrigation strategies based on the timing and amount of rainfall.  
• Water Resources: Seasonal rainfall distribution affects the availability of water resources. Regions with distinct wet and dry seasons may face challenges in managing water storage and distribution.  
• Ecosystems: The timing and amount of rainfall influence the types of plants and animals that can thrive in a particular ecosystem. Seasonal variations in rainfall can affect the life cycles of plants and animals.  

Extreme Rainfall Events:

• Floods: Extreme rainfall events can lead to floods, causing damage to property and infrastructure, and posing risks to human life.  
• Droughts: Conversely, prolonged periods of low rainfall can result in droughts, impacting agriculture, water resources, and ecosystems.  

Climate Change and Rainfall:

• Changes in Patterns: Climate change is expected to alter rainfall patterns, leading to changes in the intensity, frequency, and seasonal distribution of rainfall.  
• Impacts: These changes can have significant impacts on agriculture, water resources, ecosystems, and human societies.

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