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Table of contents
Introduction
The solar energy received by the earth’s surface including both ground surface and water surface is converted into heat energy in the form of sensible heat and is temporarily stored. This stored energy is radiated from the ground and water surface in the form of long waves into the atmosphere. The process of radiation of heat energy from the earth’s surface is called ground radiation (including radiation from both, ground surface and water surface).
The part of ground radiation after being absorbed by the atmosphere is again radiated back to the earth’s surface. This process of radiation of terrestrial heat energy from the atmosphere back to the earth’s surface is called counter-radiation. The counter-radiation is affected mainly by water vapor and atmospheric carbon dioxide.
Heating and Cooling of the Atmosphere
The heating and cooling of the atmosphere is accomplished through the processes of direct absorption of solar radiation, conduction, terrestrial radiation, convection, condensation, adiabatic mechanism etc.
1. Heating of the Atmosphere by Direct Solar Insolation
The heat energy is radiated from the outer surface of the sun (photosphere) in the form of short waves . The atmosphere absorbs 14 percent of incoming shortwave solar radiation through ozone, oxygen, water vapor etc. present therein. Seven percent of this energy is spread in the lower atmosphere up to the height of 2 km. It is apparent that this amount is too low to heat the atmosphere significantly.
2. Conduction
The transfer of heat through the molecules of matter in any body is called conduction. The transfer of heat under the process of conduction may be accomplished in two ways viz.
(i) from one part of a body to the other part of the same body, and
(ii) from one body to the other touching body.
Conduction may be effective only when there is difference in temperatures in different parts of a single body or in two bodies and the process continues till the temperatures of all parts of a body or of two touching bodies become the same. It is obvious that heat moves from a warmer body to the cooler body through molecular movement.
The rate of transfer of heat through molecular movement depends on the heat conductivity of the substance. The substance or a body which allows transfer of heat through conduction at a very fast rate is called a good conductor of heat while the substance or a body which retards conduction of heat is called a bad or poor conductor of heat.
Metal is a good conductor of heat while air is a very poor conductor of heat. The earth’s surface is heated during day time after receiving solar radiation. The air coming in contact with the warmer ground surface is also heated because of transfer of heat (conduction of heat) from the ground surface through the molecules to the air.
Since air is very poor conductor of heat and hence the transfer of heat from the ground surface through conduction is effective only up to a few meters in the lower atmosphere and thus the lower atmosphere is heated. The ground surface becomes colder than the air above during winter nights and thus heat is transferred from the lower portion of the atmosphere to the ground surface and thus the atmosphere is cooled.
3. Terrestrial Radiation
The process of transfer of heat from one body to the other body without the aid of a material medium (e.g. solid, liquid or gas) is called radiation. There are two basic laws which govern the nature of flow of heat energy through radiation.
(a) Wien’s displacement law states that ‘the wavelength of the radiation is inversely proportional to the absolute temperature of the emitting body’.
(b) Stefan-Boltzmann law states that ‘flow, or flux of radiation is proportional to the fourth power of the absolute temperature of the radiating body’.
The earth’s surface after receiving insolation from the sun through shortwave electromagnetic radiation gets heated and radiates heat to the atmosphere in the form of longwave or infrared radiation throughout 24 hours. It may be remembered that the atmosphere is more or less transparent for incoming shortwave solar radiation but it absorbs more than 90 percent of outgoing longwave terrestrial radiation through water vapor, carbon dioxide, ozone etc.
Thus, the terrestrial radiation is the most important source of heating of the atmosphere. The process of radiation of heat from the earth’s surface is called ground radiation. The part of this ground radiation after being absorbed by the atmosphere is radiated back to the earth’s surface.
This process of radiation of terrestrial heat energy from the atmosphere back to The earth’s surface is called counter-radiation which is affected mainly by water vapor and atmospheric carbon dioxide. This mechanism known as greenhouse effect keeps the lower atmosphere and the ground surface relatively warmer.
Thus, the atmosphere acts as a window pane which allows the shortwave solar radiation to come in and prevent the longwave terrestrial radiation from escaping into space.
It is obvious that the increase in the concentration of carbon dioxide in the atmosphere will increase the greenhouse effect and thus the temperature of the earth’s surface would increase. It may be pointed out that carbon dioxide also absorbs longwave terrestrial radiation and helps in keeping the lower atmosphere and the ground surface warmer.
Water vapor absorbs both the incoming shortwave solar radiation and outgoing! longwave terrestrial radiation. Since most of water vapor is concentrated in the lower atmosphere (90 per cent of the total atmospheric water vapor found up to the height of 5 km in the lower atmosphere) and hence both the incoming solar radiation and outgoing terrestrial radiation increase with increasing height. This is the reason that high mountains are called radiation windows.
4. Convection
The transfer of heat energy through the movement of a mass of substance from one place to another place is called convection. The process of convection becomes effective only in fluids or gasses because their internal mass motion activates convection of heat energy.
The earth’s surface gets heated after receiving heat energy (insolation) from the sun. Consequently, the air coming in contact with the warmer earth’s surface also gets heated and expands in volume. Thus, warmer air becomes lighter and rises upward and a vertical circulation of air is set in.
Conversely, the relatively colder air aloft becomes heavier because of contraction in volume and thus descends to reach the earth’s surface. The descending air is warmed because of dry adiabatic rate and warm ground surface. This warm air again ascends because of increase in volume and decrease in density. The whole mechanism of ascent of warmer air and descent of colder air generates convection currents in the lower atmosphere.
This convective mechanism transports heat from the ground surface to the atmosphere and thus helps in the heating of the lower atmosphere. Similarly, horizontal convection currents are also generated on the ground surface.
5. Adiabatic Heating and Cooling
The adiabatic heating and cooling of the atmosphere takes place through the ascent and descent of a parcel of air respectively. The general trend is that temperature decreases with increasing height at the rate of 6.5°C per 1000 m or 3.6 P 1000 feet. This rate of decrease of temperature with increasing height is called normal lapse rate.
A definite ascending air with given volume and temperature expands due to decrease in pressure and thus cools. For example, an air with the volume of one cubic foot and air pressure of 1016 mb at sea level rises to the height of 17,500 feet, its volume is doubled because of expansion.
On the other hand, a descending air contracts and thus its volume decreases but its temperature increases. It is apparent that there is change in temperature of air due to ascent or descent but without addition or subtraction of heat. Such type of change of temperature of air due to contraction or expansion of the air is called adiabatic change of temperature.
Adiabatic change of temperature is of two types viz.
(i) Dry adiabatic change and
(ii) Moist adiabatic change.
The temperature of unsaturated ascending air decreases with increasing height at the rate of 5.5°F per 1000 feet or 10°C per 1000 m. This type of change of temperature of unsaturated ascending or descending air is called dry adiabatic rate. It may be pointed out that if an air descends its temperature increases at the above mentioned rate.
The rate of decrease of temperature of an ascending air beyond condensation level is lowered due to addition of latent heat of condensation to the air. This is called moist adiabatic rate wherein temperature of a parcel of ascending air beyond condensation level decreases (and hence air cools) at the rate of 3°F per 1000 feet or 6°C per 1000 meters. This is also called retarded adiabatic rate and cooling. Conversely, the descending parcel of air contracts in volume due to increase of pressure and hence is warmed at the rate of 10°C per 1000 meters.
References:
- Climatology, Savindra Singh
- Physical Geography: Atmosphere, K. Bhardwaj
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