Water in the Atmosphere Class 11 Geography Chapter 10 CBSE Notes 2025-26

Air contains water vapour, which makes up from zero to four percent of its volume and has a vital influence on weather patterns. Moisture in the atmosphere comes from evaporation over water bodies and transpiration from plants, creating a continuous exchange between the atmosphere, oceans, and continents. This cycling takes place through processes like evaporation, transpiration, condensation, and precipitation.

Humidity and Its Types Humidity refers to the amount of water vapour present in the air. It is measured in different ways, with absolute humidity being the actual weight of water vapour per unit volume of air, expressed in grams per cubic metre. Relative humidity represents the percentage of water vapour in the air relative to the maximum the air can hold at that temperature. Warmer air can hold more moisture, so both temperature and location affect humidity. Relative humidity is higher over oceans and lowest above continents.

Saturation and Dew Point If air holds moisture to its total capacity at a specific temperature, it is said to be saturated, meaning it can’t hold any more water vapour at that temperature. The dew point is the temperature at which this saturation happens and condensation begins.

Evaporation and Condensation Processes Evaporation adds water vapour to the atmosphere and happens when liquid water transforms into a gas, driven mainly by heat. The latent heat of vaporization is the energy needed for this process. Higher temperatures raise the air’s ability to absorb and retain more water vapour. Wind helps evaporation by moving saturated air away from the surface and replacing it with drier air.

Condensation is the opposite process, where water vapour turns back into liquid form when air loses heat. This often occurs when air cools to its dew point or when it comes in contact with a colder object. For condensation to occur, “hygroscopic condensation nuclei” like dust, smoke, or sea salt are needed for vapour to settle on. More cooling and higher relative humidity favour condensation.

Condensation happens when:

• Air temperature drops to dew point while volume remains constant
• Both temperature and air volume are reduced simultaneously
• Moisture is added to the air, raising its water vapour content

The most favourable condition for condensation is a drop in air temperature.

Forms of Condensation: Dew, Frost, Fog, and Clouds After condensation, atmospheric moisture takes several forms depending on temperature and location:

• Dew: Water droplets condensing on cool surfaces like grass or stones. Conditions: clear sky, calm air, humid nights, and temperatures above freezing.
• Frost: Forms when condensation occurs below 0°C on cold surfaces, creating ice crystals (not water droplets). Similar conditions as dew, but with freezing temperatures.
• Fog and Mist: When air cools suddenly, condensation happens near the ground, forming fog (dense) or mist (holds more moisture than fog). Common in mountains and cities, where dust or smoke particles act as nuclei.

Types and Classification of Clouds Clouds form from condensation high in the atmosphere and can consist either of water droplets or tiny ice crystals. Their classification is based on their height, appearance, and transparency. The four main types are:

• Cirrus: High-altitude clouds (8,000–12,000 m), thin, feathery, and always white.
• Cumulus: Fluffy, cotton-like clouds with flat bases, located at 4,000–7,000 m, seen in patches.
• Stratus: Large, layered clouds covering broad sky areas, usually formed by air mixing or cooling.
• Nimbus: Dense, dark grey, and thick clouds found near the earth’s surface or mid-levels, often bringing rain. Sometimes these clouds appear so low they nearly touch the ground.

Other combinations include cirrostratus (high), altostratus (middle), stratocumulus (low), and cumulonimbus (vertical development).

Precipitation and Its Forms Continuous condensation in the air causes water particles to grow until gravity causes them to fall to earth—a process called precipitation. This can occur as either liquid or solid:

• Rainfall: The most common form, occurs as water droplets when the temperature is above 0°C.
• Snowfall: When air temperature is below 0°C, water vapour forms hexagonal ice crystals that combine into flakes.
• Sleet: Raindrops or melted snow refreeze before hitting the ground, forming small ice pellets.
• Hail: Frozen raindrops or ice layers formed as rainwater passes through cold air, leading to rounded, layered hailstones.

Types of Rainfall Rainfall is divided into three main types by origin:

• Convectional Rain: Occurs when heated air rises, cools, and condenses into cumulus clouds. It brings heavy, but short-duration rainfall, mainly in the summer or in the hottest part of the day, especially in equatorial regions and continental interiors.
• Orographic (Relief) Rain: Happens when moist air is forced up by mountains, causing cooling and condensation on the windward slope; the other side (leeward) becomes rain-shadow and dry.
• Cyclonic (Frontal) Rain: Generated by cyclonic activity where different air masses meet, causing widespread and steady rainfall (details discussed in previous chapters).

World Distribution of Rainfall Rainfall varies greatly across the globe. In general, rainfall decreases from the equator towards the poles. Coastal areas typically receive more rainfall than the inner parts of continents, and oceans receive more rain than landmasses because of their vast water surfaces.

Between 35° and 40° latitudes (N and S), eastern coasts receive heavier rainfall, which decreases towards the west. In the 45° to 65° zones, westerly winds ensure western coastal regions get more rain, tapering off towards eastern areas. Mountains parallel to coasts cause more rain on the windward side and less on the leeward side.

The main rainfall and precipitation zones based on annual averages:

• Equatorial belt, windward slopes of coastal mountains, and monsoon lands: over 200 cm/year (heavy rainfall)
• Interior continental and continental coastal areas: 100–200 cm/year (moderate rainfall)
• Central tropical and temperate interiors: 50–100 cm/year (low to moderate rainfall)
• Rain-shadow areas and high latitudes: less than 50 cm/year (very low rainfall)

Some areas, like the equatorial belt and western cool temperate regions, receive rain more evenly throughout the year, while in other regions, seasonal distribution is more pronounced.

Practice and Application The chapter also includes exercises, with multiple-choice questions testing understanding of key concepts like the importance of water vapour, the process of evaporation, and cloud types. Short and long answer questions help consolidate understanding of types of precipitation, role of relative humidity, rapid decrease of water vapour with altitude, and classification of clouds. Students are encouraged to analyze rainfall news for further application and awareness of real-world weather patterns.

Class 11 Geography Chapter 10 Notes – Water in the Atmosphere Revision Key Points

These comprehensive Class 11 Geography Chapter 10 notes cover all main aspects of “Water in the Atmosphere” for fast and effective revision. Understanding concepts like humidity, condensation, clouds, precipitation types, and rainfall patterns will help you master both objective and descriptive questions in exams.

Key terms, classification tables, and concise definitions are highlighted to aid memory retention. With these notes, students can easily revise crucial subtopics like the forms of precipitation and the global rainfall distribution, making last-minute studying simpler and more effective.