NCERT Class 11 Geography Chapter 11: Water In The Atmosphere YouTube Lecture Handouts

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• Water Vapour is

• Water as solid, liquid or gas

• Moisture by evaporation or transpiration

• Continuous exchange b/w evaporation, transpiration, condensation and precipitation

• Absolute Humidity - actual amount of the water vapour present in the atmosphere – ability to hold water depends on temperature

• Relative Humidity - moisture as compared to full capacity; greater over the oceans and least over the continents

• Saturated Air – air containing moisture to its full capacity at given temperature

• Dew point – temperature at which saturation occurs in the sample of air

• Evaporation – water from liquid to gas

• Latent Heat of Vaporization – Temperature at which water starts evaporating

• Increase in temperature increases water absorption and retention capacity of the given parcel of air

• Condensation – transformation of water vapor to water (by loss of heat). In free air, condensation results from cooling around very small particles termed as hygroscopic condensation nuclei or when the moist air comes in contact with some colder object. It is influenced by temperature, pressure, volume of air and humidity.

Condensation Occurs When

• temperature of the air is reduced to dew point with its volume remaining constant

• when both the volume and the temperature are reduced

• when moisture is added to the air through evaporation

Sublimation – Solid to Gas

• Forms of condensation can be classified based on temperature and location. Condensation takes place when the dew point is lower than or higher than the freezing point.

• Dew: When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects - clear sky, calm air, high relative humidity, and cold and long nights. For dew formation, dew point should be above freezing point

• Frost: Forms on cold surfaces when condensation takes place below freezing point – minute ice crystals - clear sky, calm air, high relative humidity, and cold and long nights

• Fog & Mist: fog is a cloud with its base at or very near to the ground; condensation takes place within itself on fine dust particles (smoke as nuclei to form fog)

• Smoke + Fog = Smog

• Mist contains more moisture than the fog. In mist each nuclei contains a thicker layer of moisture. Mists are frequent over mountains as the rising warm air up the slopes meets a cold surface.

• Fog prevalent where warm currents meet cold currents. Fogs are mini clouds in which condensation takes place around nuclei provided by the dust, smoke

Clouds

Water droplets or ice crystals formed by condensation at free air at reasonable heights - expanse, density and transparency or opaqueness clouds are grouped under four types:

• Cirrus – high altitude, thin, detached, feathery and white

• Cumulus – cotton wool, patches, scattered with flat base

• Stratus – layered due to loss of heat or mixing of air mass with different temperature

• Nimbus – black and dark grey, dense, low, shapeless with thick vapor

Combination

• High clouds – cirrus, cirrostratus, cirrocumulus

• Middle clouds – altostratus and altocumulus

• Low clouds – stratocumulus and nimbostratus

• Clouds with extensive vertical development – cumulus and cumulonimbus

Precipitation

When resistance of the air fails to hold condensed particles against the force of gravity, they fall on to the earth’s surface & release moisture

• Rainfall – as rain

• Snowfall – as snow when temperature is less than - hexagonal crystals

• Sleet - frozen raindrops and refrozen melted snow-water. When a layer of air with the temperature above freezing point overlies a subfreezing layer near the ground

• Hailstones – rain solidified as round pieces as concentric layers of ice one over another

Types of Rainfall

• Conventional – heating of air and rising air mass – in summer and hot part of day – equatorial regions & interior of continents

• Orographic (Relief) - saturated air mass comes across a mountain, it is forced to ascend and as it rises, it expands; the temperature falls, and the moisture is condensed. Windward side receives more rainfall - winds reach the other slope, they descend, and their temperature rises. Then their capacity to take in moisture increases and leeward (Rainshadow) side is dry

• Cyclonic – due to cyclone formation

Distribution of Rainfall

• Rain decreases towards poles

• Coastal areas have more rain

• Rain is more over ocean than land

• Rain is more on windward side than leeward side

• B/w and N and S of the equator, rain is heavier on the eastern coasts and goes on decreasing towards the west.

• B/w and N and S of equator, due to westerlies, rainfall is first received on the western margins of the continents and it goes on decreasing towards the east

-Manishika