The atmosphere is heated or cooled by the following processes which have been briefly discussed here:
(1) Partial absorption of solar radiation by the atmosphere
(3) Terrestrial radiation
(4) Convection and advection
(5) Latent heat of condensation
(6) Expansion and compression of the air.
(1) Partial absorption of solar radiation:
As we know, the atmosphere is not heated directly by the sun’s rays passing through it. Even then, the dust particles and water vapour present in the lower layers of the atmosphere directly absorb about 10 per cent of the incoming solar radiation.
About 50 per cent of this absorption occurs in the lower 2 km of air where most of the water vapour is found. However, the process of absorption is not very effective in raising the surface-air temperature.
That is why even on a clear sunny day, the temperature near the surface remains low. In winter, people enjoy the warmth of sunshine by sitting close to the side of a wall of their houses exposed to the sun, because the short-wave incoming solar radiation is being converted into long-wave terrestrial energy.
Conduction is a slow process of heat transfer as regards warming of the atmosphere. Since air is a very poor conductor of heat, the conduction process affects only the lowermost layers of air closest to the earth’s surface.
As a means of heat transfer for the atmosphere as a whole, conduction is the least important and can be neglected when considering a majority of meteorological phenomena.
(3) Terrestrial radiation:
It has already been pointed out in an earlier chapter that about two-thirds of the radiant solar energy reaches the earth’s surface directly or indirectly in the form of short-wave electro-magnetic waves, where it is converted into terrestrial heat by the surface.
According to Kirchoffs Law, the earth radiates heat in the form of long waves or infrared radiation. Remember that the terrestrial radiation is a process which continues for all the 24 hours.
During the sunlight hours the receipt of heat through insolation exceeds its loss by terrestrial radiation. On the contrary, during night the heat is lost through long-wave earth radiation.
The earth radiates heat as a black body, while the radiation from the atmosphere is selective.
Thus, most of the atmospheric gases, especially carbon dioxide and water vapour, that are almost transparent to the short-wave solar radiation and are able to absorb only about 19 percent of it, absorb about 85 per cent of the terrestrial long-wave or infrared radiation.
Thus, it is clear that the atmosphere receives a larger part of its energy supply from the earth and not directly from the sun.
Since the atmosphere is almost transparent to most of the solar radiation and absorbs a large part of the terrestrial radiation, it acts to conserve the heat energy of the earth. This conservation is called the greenhouse effect.
Water vapour, carbon dioxide, dust particles and ozone are directly involved in the process of absorption of terrestrial radiation. But water vapour is the most important.
It is on account of this that in arid regions, characterized by the minimum amount of water vapour present in the atmosphere, nights are cooler because of the earth’s radiation of heat.
On the contrary, even in the long winter nights, when there is a thick cover of clouds, the temperatures remain relatively higher because of the absorption of long-wave terrestrial radiation by the clouds.
The earth radiates heat to the layers of air which re-radiate it. The radiation from the atmospheric layers is upward and downward both. That is why there is a gradual decrease in temperature with increasing altitude.
However, a certain amount of heat is radiated to space from the topmost layer of the atmosphere.
Thus, whereas the earth’s surface receives heat from the incoming solar radiation, it also radiates back the same amount of heat to the atmosphere as well as to the outer space.
In this way, a balance is maintained between the receipt of the radiant energy from the sun and that lost to the space and atmosphere by terrestrial radiation.
(4) Convection and advection:
Since the atmosphere is a gaseous medium, convection is the most significant mechanism of heat transfer. Heat gained by the layers of air at or near the earth’s surface from radiation or conduction is usually transferred to the upper atmospheric layers by the process of convection.
Transfer of heat by convection takes place in two forms: (a) sensible heat content of the air which is transferred directly by the rising and mixing of heated air, and (b) latent heat which is the indirect form of energy transfer by convection.
When condensation in the free atmosphere takes place, the latent heat of evaporation is released and made available to the air. This is known as the latent heat of condensation.
Whereas the term convection is used to describe the vertical motions in the atmosphere, the term advection is reserved for horizontal convection transport of heat. It is worthwhile to remember that the horizontal convection is on a much larger scale.
In fact, advection is responsible for slow heat transfer from the equatorial to the Polar Regions. Vertical convection, on the other hand, is more localized in character. In other words, the process of convection redistributes heat from equatorial regions to the poles and from the surface upward.
When pockets of air are heated by contact with the warm earth’s surface, they expand in volume and become less dense than the surrounding air and, therefore, rise.
The rising air currents are replaced by cooler and denser air from above. Thus a convectional circulation is set up with horizontal as well as vertical motions.
(5) Latent heat of condensation:
There is no doubt that the atmosphere receives a large percentage of its total heat energy from the long-wave terrestrial radiation, conduction and convection. But in the heating and cooling of the atmosphere, the latent heat of evaporation as well as the latent heat of condensation also plays a significant role.
The latent heat of condensation is made available to different layers of the atmosphere by the process of evaporation taking place at the surfaces of oceans, humid ground, and natural vegetation.
Oceans with their extensive water surfaces exposed to the sun are the most important source for the latent heat of evaporation. It is estimated that half of the insolation received at the ocean surfaces is consumed in the evaporation of surface water, because the process of evaporation needs a certain amount of energy.
This transformed solar energy is thus contained in the air in potential form. When water vapour is condensed, the latent energy is again released into atmosphere and is used in heating it.
The released energy is known as the latent heat of condensation. It may be pointed out that the latent energy of evaporation does not raise the temperature of the water vapour.
When condensation or sublimation takes place, the water vapour is converted into liquid or solid form. Then the latent heat is released and it raises the temperature of the air.
The scientists have proved by experiments that at constant temperature the amount of energy required to convert a given quantity of liquid into vapour is again released when the process is reversed.
At 20°C temperature each gram of water vapour, when condensed into liquid water, releases 585 calories of heat. When water is frozen into ice, each gram of it provides additional 80 calories.
When we recall that about two-thirds of the earth’s surface is covered with water, the importance of the latent heat of evaporation as the principal source of atmospheric heat can easily be appreciated.
(6) Expansion and compression of the air:
Whenever air moves upward it passes through regions of successively lower pressure. Consequently the rising air expands and cools adiabatically. In the same way, as the air descends, it comes under increasingly higher pressures so that it compresses and is heated.
These temperature changes caused by a change of pressure alone that the rising or falling air is subjected to are called adiabatic temperature changes.
As a process leading to adiabatic cooling it is common whenever the earth’s surface is warmer than the air above. Thus, the temperature changes brought about in the air aloft simply due to changes in the air pressure are very important in the heating or cooling of the atmosphere.