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The amount of water vapour in the air may change mainly due to two phenomena: 1. Evaporation 2. Condensation.
1. Evaporation—through Gain of Heat:
These two activities always change the form of water. The transformation of liquid water into gaseous form is called evaporation. To transform one gram of liquid water into water vapour as much as 600 calories of energy is needed.
The amount of heat energy required to raise temperature of one gram of water by one degree celsius is known as a calorie. During evaporation water vapour absorbs heat energy and releases it at the time of condensation. This hidden heat energy in the water vapour is called Latent Heat.
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The rate of evaporation depends upon:
i. Degree of dryness of air,
ii. Temperature, and
iii. Wind.
2. Condensation—through Loss of Heat:
Dry air is capable of retaining more water vapour and so is the case with the increase of temperature and amount of air movement. However, air cannot retain infinite amount of water vapour and at certain stage it becomes saturated.
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The temperature at which the air becomes completely saturated and transforms into water (or condensation takes place) is called dew point. And if the water vapour directly transforms into snow (solid water) it is called freezing point.
The process by which water vapour changes into liquid or solid water is known as condensation. When a mass of air is completely saturated, any addition of extra water vapour will transform into liquid or solid water depending on temperature. Similarly, condensation also will take place without addition of extra water vapour, when the temperature falls below dew or freezing point.
Condensation, therefore, depends on:
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(a) Relative humidity, and
(b) Amount of cooling.
Condensation generally takes place around certain very small particles of dust, smoke and salt. These small particles are called hygroscopic nuclei or condensation nuclei.
The air near the earth surface gets warm through the process of radiation from the earth. Being warm, air moves upward and gradually expands in volume. As a result, the temperature of the unit volume of air falls.
Such a decrease of temperature that takes place without addition or subtraction of additional heat from outside, only through the process of upward movement and expansion of air, is known as Adiabatic Temperature change. The adiabatic rate of saturated air is called Wet Adiabatic rate and that of unsaturated air is called Dry Adiabatic rate.
Forms of Condensation:
After condensation, the water vapour or the moisture in the atmosphere takes various forms.
The forms of condensation can be classified on the basis at which dew point is reached. Dew, fog and clouds are formed when temperature is higher than the freezing point and frost, snow, etc., are formed when temperature is below the freezing point.
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Forms of condensation may also be classified according to the location of occurrence:
(i) Near the earth’s surface-Dew, fog, frost, mist, snow, etc.
(ii) Above the earth’s surface-clouds.
Dew:
The calm air mass of which temperature drops down to dew point, condensation takes place. The condensed droplets collect over cooler surface like stone, grass and plant leaves. This form of condensation is known as dew.
The ideal conditions for the formation of dew are:
(i) Clear sky and calm air
(ii) High relative humidity
(iii) Cold and long nights.
All these help in greater radiation of heat from the earth’s surface. It is necessary to have the temperature to be above freezing point.
Frost:
When condensation takes place at a temperature below freezing point (0°C), the excess water vapours take the shape of ice crystals instead of water droplets. It is called frost. The same set of conditions necessary for dew formation is also required for frost.
Fog:
Fog is a special type of cloud seen at the lowest layer of troposphere very near to the earth’s surface. It looks like smoke, visibility becomes very poor and it covers the surface completely. Over the big cities, fog mixes up with smoke, dust and other gaseous materials. This type of fog is generally known as Haze. Haze is injurious to health.
On the basis of the process of condensation, fog may be classified as:
(i) Radiation fog, and
(ii) Advection fog.
Radiation fog:
Radiation fog is formed by the process of radiation, cooling of the earth’s surface and adjacent air. Hence, condensation takes place very near to the surface and as such it is also known as Ground fog. It is not very thick (10-30 metres).
Advection fog:
Advection fog is formed when cold air moves to a relatively warm region. This type of fog is common near sea coasts. Advection fogs are thick (300-600 metres) and persistent.
According to the places of origin, fogs may also be classified as—Sea fog, Stream fog, Frontal fog and Airmass fog.
Mist:
When the visibility of the fog becomes less than two metres but more than one metre the type of fog is called mist.
Clouds:
The water vapour present in the air up to a height of nearly 12000 metres from the surface transforms into clouds by the process of condensation. Clouds form when the temperature of the air falls below dew point by the process of adiabatic cooling.
Clouds are generally classified according to their appearances (shape, structure and vertical extent) and their heights or altitudes. Altogether, 10 types of clouds have been identified and they are placed under four classes.
They are:
A-Class:
Highest in the sky. It is seen between (000-12000 metres above the (High) ground. Cirrus, Cirrostratus and Cirrocumulus clouds fall in this class.
B-Class:
Medium clouds occur between 2000-6000 metres. Altocumulus and Alto- stratus (Medium) clouds fall in this group.
C-Class:
Low clouds occur up to a height of 2000 metres from the ground. (Low) Stratocumulus, Stratus and Nimbostratus clouds fall under this class.
D-Class:
Cumulus and Cumulonimbus clouds fall under D-Class. These two types of clouds are vertical clouds and have very high vertical extension.
Cirrus clouds are very thin and white in colour. They are formed of ice crystals and give a feathery appearance. They are the highest clouds. Sometimes, they carry indication of a cyclone or a stormy weather. However, widely scattered cirrus clouds indicate clear weather. Cirrostratus clouds look like a white sheet of cloth.
Halo is formed around the sun and the moon which indicates incoming of a stormy weather. The cirrocumulus clouds look like soap foams. Sometimes, they appear in lumps otherwise they scattered all over. Precipitation does not occur directly by these three types of clouds and they are called high clouds.
The altocumulus clouds belong to medium clouds. It looks like piled up cotton wool and may form in rows like waves. The colour of the opposite side of the sun rays is black. Alto- stratus clouds are similar in appearance to cirrostratus but they are much thicker. The formation of alto-stratus clouds reduces the brightness of the sunshine or the moon light and they indicate the possibility of wide-spread rainfall.
Stratus clouds resemble to fog and are made up of almost equal layers. Occasionally, they are broken into patches separated by blue sky and by the incoming wind. Stratus clouds are grey in colour. Many a time these cloud-forms get mixed up and take joint shapes as stratocumulus and nimbostratus, stratocumulus is smoky in colour. Nimbostratus is another dark smoky cloud from which widespread rain occurs. All these clouds are low clouds.
There is another type of cauliflower shaped, thick, dense and black cloud forms at lower level called cumulus. The widely scattered cumulus clouds indicate clear weather but the composite Cumulo-Nimbus brings very heavy rain or storm.
Cumulonimbus looks like a mountain with a uniform base. It is very black and may extend up to a height of 6000 metres. It is the source of hail, storm and heavy rainfall and it brings many destruction to the property and lives.
