Since Atmosphere contains water in form of vapour, it causes various changes in the environment. The presence of water in the atmosphere causes various weather phenomena. This water is present in all three forms in the atmosphere – solid, liquid and gas.
Absolute Humidity – actual amount of water vapour present in the atmosphere. It is measured in grams per cubic metre.
Moisture in atmosphere comes from –
- water bodies, through evaporation.
- Plants, through transpiration.
Ability of air to hold water vapour depends entirely on its temperature.
Relative humidity – percentage of moisture present in atmosphere as compared to its full capacity at a given temperature. It is greater over the oceans and least over the continents.
Saturated air – That air which contains moisture to its full capacity at a given temperature.
Dew point – Temperature at which saturation occurs in a given sample of air is known as its dew point.
Latent heat of Vaporisation – Temperature at which water starts evaporating.
Condensation – It is transformation of water vapour into water. It is caused by loss of heat.
Sublimation – If water vapour directly condenses into solid form.
Most favourable conditions for Condensation are decrease in air temperature. Condensation occurs when dew point is lower than freezing point as well as higher than the freezing point.
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Dew in the atmosphere:
- When moisture is deposited in form of water droplets on cooler surfaces of solid objects, it is called dew.
- Ideal conditions: Clear sky, calm air, high relative humidity, cold and long nights.
- Dew point > above freezing point.
- Saturation of air with moisture should happen above Freezing point.
Frost in Atmosphere:
- When condensation takes place below freezing point, frost is formed.
- Dew point is at or below freezing point.
- Air temperature must be at or below the freezing point.
Fog and Mist:
- Fog is a cloud with its base at or very near to the ground.
- Mist contains more moisture than fog.
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Clouds:
Clouds are a mass of water droplets or crystals of ice of minute size located at considerable elevations. These clouds take up various shapes and sizes. This allows for the study of different types of clouds to understand the behaviour of water in atmosphere.
- Cirrus – at high altitudes
- Cumulus – cottoning
- Stratus – layered clouds
- Nimbus – thick vapour.
Precipitation : release of moisture after condensation.
Sleet: Frozen raindrops and refrozen melted snow water.
Hailstorms: Released as raindrops, pass through colder layers so freeze before reaching the surface.
Types of Rainfall:
According to its basis of origin, rainfall is primarily classified into 3 major types:
Convectional rainfall: cumulous clouds in equatorial regions and continent interiors especially in Northern hemisphere.
Orographic or relief rain: Windward slopes receives greater rainfall because air is forced to ascend mountain and expands, temperature decreases hence results in condensation.
Cyclonic rain: The rains occurring due to extra tropical cyclones.
Precipitation regimes:
The water in Atmosphere is eventually falls on Earth in form of precipitation, The pattern of precipitation regimes is helpful in understanding the distribution of rainfall.
- Equatorial belt, windward slopes along western coast in cool temperate zone and coastal areas of monsoon land receives heavy rainfall of over 200 cm/annum.
- Moderate 100-200 cm/ anuum rainfall in interior of continental areas.
- 50-100 cm/annum rainfall in central part pf tropical land and eastern and interior parts of temperate lands receive rainfall 50-100cm/annum.
- 50 cm/annum rainfall in rain shadow areas of interior of continents and high latitudes.
There is even distribution of rainfall ocurrs in equatorial belt and western parts of cool temperate regions.
Rainfall is more over ocean s than land.
Heavier rain on eastern coast and decreases towards west – between lattitudes 35 degrees and 40 degrees north and south of equator.
Due to westerlies, first rainfall on western margins of continents and it goes on decreases towards east of 45 degrees and 65 degrees N and S of equator.
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