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When preparing for the NCERT Class 11 Geography Fundamentals of Physical Geography Chapter 10: Water in the Atmosphere Notes, you can go through the important concepts of the chapter in this blog. This blog provides simple and clear notes to help you understand the key concepts of this chapter. Whether you’re preparing for exams or need a quick revision, these notes will help you grasp the essential ideas without going through the entire textbook. So, let’s start!
Table of Contents
- 1 Introduction
- 2 Water Vapour and Humidity
- 3 Evaporation and Condensation
- 4 Dew
- 5 Frost
- 6 Fog and Mist
- 7 Clouds
- 8 Precipitation
- 9 Types of Rainfall
- 10 World Distribution of Rainfall
- 11 Important Definitions in NCERT Notes Class 11 Geography Fundamentals of Physical Geography Chapter 10: Water in the Atmosphere Notes
- 12 FAQs
Explore Notes of Class 11 Fundamentals of Geography
Introduction
Water vapour, varying from 0 to 4% by volume in the atmosphere, plays a crucial role in weather phenomena. It exists in gaseous, liquid, and solid forms, derived from the evaporation of water bodies and transpiration from plants. There is a continuous exchange of water between the atmosphere, oceans, and continents through evaporation, transpiration, condensation, and precipitation.
Water Vapour and Humidity
Water vapour in the air is known as humidity, expressed as:
- Absolute Humidity: The actual weight of water vapour per unit volume of air, measured in grams per cubic meter. It varies by location due to temperature differences.
- Relative Humidity: The percentage of moisture in the air compared to its full capacity at a given temperature. It increases or decreases with temperature changes, being higher over oceans and lower over continents.
- Saturated Air: Air holding maximum moisture at a given temperature, unable to hold more. The temperature at which saturation occurs is the dew point.
Evaporation and Condensation
Understand the concepts of evaporation and condensation by looking at the definition given below:
- Evaporation: The process of water transforming from liquid to gas, driven by heat (latent heat of vaporisation). Higher temperatures and air movement increase evaporation, while low moisture content enhances the air’s capacity to absorb water.
- Condensation: The transformation of water vapour into liquid due to cooling. It occurs when air reaches its dew point, often around hygroscopic condensation nuclei (e.g., dust, smoke, salt particles). Condensation depends on cooling, relative humidity, air volume, pressure, and moisture addition. The most favourable condition is a decrease in air temperature.
- Condensation forms include dew, frost, fog, and clouds, classified by temperature and location, occurring when the dew point is above or below the freezing point.
Also Read:
- NCERT Class 6 Geography: Chapter 3 Motions of the Earth
- NCERT Class 6 Geography: Chapter 1 The Earth in the Solar System
Dew
Dew forms when moisture condenses as water droplets on cooler surfaces like stones, grass, or leaves. Ideal conditions include clear skies, calm air, high relative humidity, cold and long nights, and a dew point above freezing.
Frost
Frost occurs when condensation happens below the freezing point (0°C), forming minute ice crystals on cold surfaces. Conditions are similar to dew, but the air temperature and dew point must be at or below freezing.
Fog and Mist
Look below to understand the difference between Fog and Mist:
- Fog: A cloud near or at the ground, formed when moist air cools suddenly and condenses on fine dust particles, reducing visibility. Common areas where warm and cold air currents meet, or in urban areas with smoke (forming smog).
- Mist: Similar to fog but with more moisture per nuclei, frequent over mountains where warm air meets cold surfaces. Fogs are drier than mists.
Clouds
Clouds are masses of water droplets or ice crystals formed by condensation at high altitudes. They are classified by height, expanse, density, and transparency into four types:
- Cirrus: Thin, feathery, white clouds at high altitudes (8,000–12,000 m).
- Cumulus: Puffy, cotton-like clouds at 4,000–7,000 m with a flat base, often scattered.
- Stratus: Layered clouds covering large sky areas, formed by heat loss or mixing of air masses with different temperatures.
- Nimbus: Dense, dark gray clouds at middle or low levels, often near the ground, appearing shapeless and opaque.
- Combinations include high clouds (cirrus, cirrostratus, cirrocumulus), middle clouds (altostratus, altocumulus), low clouds (stratocumulus, nimbostratus), and vertically extensive clouds (cumulus, cumulonimbus).
Precipitation
Precipitation occurs when condensed particles grow too heavy for air to hold, falling to the Earth’s surface due to gravity. Forms include:
- Rainfall: Liquid precipitation when temperatures are above 0°C.
- Snowfall: Precipitation as snowflakes (hexagonal ice crystals) when temperatures are below 0°C.
- Sleet: Frozen raindrops or refrozen melted snow, formed when rain passes through a subfreezing air layer near the ground.
- Hail: Solid, rounded ice pieces (hailstones) formed when raindrops solidify in colder layers, with concentric ice layers.
Types of Rainfall
Rainfall is classified by origin into three types:
- Convectional Rain: Occurs when heated air rises in convection currents, cools, and condenses into cumulus clouds, causing heavy but short-lived rain with thunder and lightning. Common in equatorial regions and summer afternoons.
- Orographic Rain: Occurs when moist air is forced up by mountains, cooling and condensing on windward slopes, causing heavy rainfall. Leeward slopes (rain-shadow areas) remain dry due to descending, warming air.
- Cyclonic Rain: Associated with extratropical cyclones, as described in Chapter 9.
World Distribution of Rainfall
Rainfall varies globally by location and season:
- Rainfall decreases from the equator to the poles.
- Coastal areas receive more rainfall than continental interiors due to proximity to water sources.
- Between 35°–40° N and S, eastern coasts receive heavier rainfall, decreasing westward.
- Between 45°–65° N and S, westerlies cause heavier rainfall on western coasts, decreasing eastward.
- Mountainous coastal areas receive more rain on windward sides.
- Precipitation Regimes:
- Heavy rainfall (>200 cm/year): Equatorial belt, windward slopes of western coasts in cool temperate zones, and monsoon coastal areas.
- Moderate rainfall (100–200 cm/year): Interior continental areas.
- Moderate rainfall (50–100 cm/year): Central tropical lands and eastern/interior temperate lands.
- Low rainfall (<50 cm/year): Rain-shadow zones and high latitudes.
- Seasonal distribution varies, with even rainfall in equatorial belts and western cool temperate regions.
Also Read:
- NCERT Class 6 Geography: Chapter 5 Major Domains of the Earth
- NCERT Class 7 Geography Chapter 3 ‘Our Changing Earth’: Notes and Solutions (Free PDF)
Important Definitions in NCERT Notes Class 11 Geography Fundamentals of Physical Geography Chapter 10: Water in the Atmosphere Notes
Here we have explained the key concepts and terms of this chapter to make it easy for you to understand.
- Humidity: Water vapour in the air, influencing weather phenomena.
- Absolute Humidity: Weight of water vapour per unit volume of air, in grams per cubic meter.
- Relative Humidity: Percentage of moisture in the air compared to its full capacity at a given temperature.
- Dew Point: Temperature at which air becomes saturated and cannot hold more moisture.
- Evaporation: Transformation of liquid water into vapour, driven by heat.
- Condensation: Transformation of water vapour into liquid due to cooling, often around hygroscopic nuclei.
- Dew: Water droplets condensed on cooler surfaces when the dew point is above freezing.
- Frost: Ice crystals form on cold surfaces when condensation occurs below 0°C.
- Fog: A cloud near the ground, formed by the sudden cooling of moist air on dust particles.
- Mist: Similar to fog but with more moisture per nuclei, reducing visibility less.
- Clouds: Masses of water droplets or ice crystals at high altitudes, classified as cirrus, cumulus, stratus, and nimbus.
- Precipitation: Release of moisture as rain, snow, sleet, or hail when condensed particles fall due to gravity.
- Convectional Rain: Rainfall from rising, cooling air forming cumulus clouds, common in equatorial regions.
- Orographic Rain: Rainfall on windward mountain slopes, with dry leeward rain-shadow areas.
- Cyclonic Rain: Rainfall associated with cyclonic systems, as described in Chapter 9.
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FAQs
Water vapour, varying from 0 to 4% by volume, drives weather phenomena through processes like evaporation, condensation, and precipitation, forming dew, frost, fog, and clouds.
Condensation occurs when moist air cools to its dew point, losing the capacity to hold water vapour, which then condenses into liquid or solid forms around nuclei like dust or smoke.
Rainfall types include convectional (from rising, cooling air), orographic (on windward mountain slopes), and cyclonic (associated with cyclones, as in Chapter 9).
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