• Water vapour varies from 0% to 4% by volume of the atmosphere and plays an important role in the weather phenomena.
• 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.
Water vapour and Humidity
Water vapour present in the air is known as humidity. It is expressed quantitatively in different ways.
Absolute Humidity
• The actual amount of the water vapour present in the atmosphere is known as the absolute humidity. It is the weight of water vapour per unit volume of air and is expressed in terms of grams per cubic metre.
• 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.
Relative Humidity
• The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as the relative humidity.
• With the change of air temperature, the capacity to retain moisture increases or decreases and the relative humidity is also affected. It is greater over the oceans and least over the continents
Saturation and Dew point
• The air containing moisture to its full capacity at a given temperature is said to be saturated.
• At this temperature air is incapable of holding any additional amount of moisture at that stage.
• The temperature at which saturation occurs in a given sample of air is known as dew point
EVAPORATION AND CONDENSATION
• The amount of water vapour in the atmosphere is added or withdrawn due to evaporation and condensation respectively.
• Evaporation is a process by which water is transformed from liquid to gaseous state. Heat is the main cause for evaporation.
• The temperature at which the water starts evaporating is referred to as the latent heat of vapourisation.
• Increase in temperature increases water absorption and retention capacity of the given parcel of air.
• If the moisture content is low, air has a potentiality of absorbing and retaining moisture.
• Movement of air replaces the saturated layer with the unsaturated layer.
• Hence, the greater the movement of air, the greater is the evaporation
• The transformation of water vapour into water is called condensation.
Condensation and Sublimation
• 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.
• Then, the excess water vapour condenses into liquid form. If it directly condenses into solid form, it is known as sublimation
• After condensation the water vapour or the moisture in the atmosphere takes one of the following forms
Dew
When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects is called Dew
Frost
Frost forms on cold surfaces when condensation takes place below freezing point (0 degree C), i.e. the dew point is at or below the freezing point
Fog and Mist
When the temperature of an air mass containing a large quantity of water vapour falls all of a sudden, condensation takes place within itself on fine dust particles.
The only difference between the mist and fog is that mist contains more
moisture than the fog
Clouds
Cloud is a mass of minute water droplets or tiny crystals of ice formed by the condensation of the water vapour in free air at considerable
elevations.
As the clouds are formed at some height over the surface of the earth, they take various shapes.
According to their height, expanse, density and transparency or opaqueness clouds are grouped under four types
Cirrus
Cirrus clouds are formed at high altitudes (8,000 – 12,000m). They are thin and detached clouds having a feathery appearance.
They are always white in colour.
Cumulus
• Cumulus clouds look like cotton wool. They are generally formed at a height of 4,000 – 7,000 m.
• They exist in patches and can be seen scattered here and there. They have a flat base.
Stratus
• As their name implies, these are layered clouds covering large portions of the sky.
• These clouds are generally formed either due to loss of heat or the mixing of air masses with different temperatures.
Nimbus
• Nimbus clouds are black or dark grey. They form at middle levels or very near to the surface of the earth.
• These are extremely dense and opaque to the rays of the sun.
• Sometimes, the clouds are so low that they seem to touch the ground.
• Nimbus clouds are shapeless masses of thick vapour
Sleet
• Sleet is 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, precipitation takes place in the form of sleet.
• Raindrops, which leave the warmer air, encounter the colder air below.
• As a result, they solidify and reach the ground as small pellets of ice not bigger than the raindrops from which they are formed.
Types of Rainfall
On the basis of origin, rainfall may be classified
As,
Convectional Rain
• The, Heated air becomes light and rises up in convection currents.
• As it rises, it expands and loses heat and consequently, condensation takes place and cumulous clouds are formed.
• With thunder and lightening, heavy rainfall takes place but this does not last long.
• Such rain is common in the summer or in the hotter part of the day.
• It is very common in the equatorial regions and interior parts of the continents
Orographic Rain
• When the saturated air mass comes across a mountain, it is forced to ascend and as it rises, it expands and the temperature falls, and the moisture is condensed.
• The chief characteristic of this sort of rain is that the windward slopes receive greater rainfall.
• After giving rain on the windward side, when these winds reach the other slope, they descend, and their temperature rises.
• Then their capacity to take in moisture increases and hence, these leeward slopes remain rainless and dry.
• The area situated on the leeward side, which gets less rainfall is known as the rain-shadow area. It is also known as the relief rain.
