Water In The Atmosphere Class 11 Notes Geography Chapter 10 - CBSE

Chapter : 10

What Are Water In The Atmosphere ?

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Water is present in the atmosphere in three forms namely: gaseous, liquid, and solid.
The moisture in the atmosphere is derived from water bodies through evaporation and from plants through
transpiration. Thus, there is a continuous exchange of water between the atmosphere, the oceans, and the
continents through the processes of evaporation, transpiration, condensation, and precipitation.
The ability of the air to hold water vapour depends entirely on its temperature. The absolute humidity differs from place to place on the surface of the earth. The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as relative humidity.
The movement of air replaces the saturated layer with the unsaturated layer. Hence, the greater the movement
of air, the greater the evaporation.
The transformation of water vapour into water is called condensation. Condensation is caused by the loss of
heat. When moist air is cooled, it may reach a level when its capacity to hold water vapour ceases.
Condensation also occurs when the moist air comes in contact with some colder object and may also occur when the temperature is close to the dew point.
After condensation, the atmosphere’s moisture or humidity gets converted into dew, fog, mist, frost, and clouds.
The ideal conditions for the formation of dew are the clear sky, calm air, high relative humidity, and cold and long nights.
Frost forms on cold surfaces when condensation takes place below the freezing point (0°C), i.e., the dew point
is at or below the freezing point.
The only difference between mist and fog is that mist contains more moisture than fog. In the mist, each nuclei has a thicker layer of moisture.
Fogs are drier than mist and are prevalent where warm currents of air come in contact with cold currents. Fogs
are mini clouds where condensation occurs around nuclei provided by dust, smoke, and salt particles.
Cloud is a mass of minute water droplets or tiny ice crystals formed by the condensation of the water vapour
in free air at considerable elevations. According to their height, expanse, density, and transparency or opaqueness, clouds are grouped under four types: cirrus, cumulus, stratus, nimbus.
Precipitation: After the condensation of water vapour, the release of moisture is known as precipitation. This may take place in liquid or solid form.
Rainfall: The precipitation in the form of water is called rainfall.
Snowfall: When the temperature is lower than 0°C, precipitation occurs in the form of fine snow flakes and is
called snowfall.
On the basis of origin, rainfall may be classified into three main types: convectional, orographic or relief, and
cyclonic or frontal.
Orographic rain: When the 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. It is also known as the relief rain.
Rain shadow area: The area situated on the leeward side, which gets less rainfall, is known as the rain-shadow
area.
Cyclonic rain: Rain caused by a cyclone is called cyclonic rain.
Hailstones: Sometimes, raindrops, after being released by the clouds become solidified into small, rounded,
solid pieces of ice that reach the surface of the earth and are called hailstones.