SOLAR RADIATION, HEAT BALANCE AND TEMPERATURE
The earth receives almost all of its energy from the sun.
The earth in turn radiates back to space the energy received from the sun.
As a result, the earth neither warms up nor does it get cooled over a period of time.
Thus, the amount of heat received by different parts of the earth is not the same.
This variation causes pressure differences in the atmosphere.
This leads to transfer of heat from one region to the other by winds.
This chapter explains the process of heating and cooling of the atmosphere and the resultant temperature distribution over the earth’s surface
The earth’s surface receives most of its energyin short wavelengths. The energy received by the earth is known as incoming solar radiation which in short is termed as insolation.
As the earth is a geoid resembling a sphere, the sun’s rays fall obliquely at the top of the atmosphere and the earth intercepts a very small portion of the sun’s energy.
The solar output received at the top of the atmosphere varies slightly in a year due to the variations in the distance between the earth and the sun.
During its revolution around the sun, the earth is farthest from the sun (152 million km on 4th July). This position of the earth is called aphelion.
On 3rd January, the earth is the nearest to the sun (147 million km). This position is called perihelion.
Therefore, the annual insolation received by the earth on 3rd January is slightly more than the amount received on 4th July. However, the effect of this variation in the solar output is masked by other factors like the distribution of land and sea and the atmospheric circulation.
Variability of Insolation at the Surface of the Earth
The amount and the intensity of insolation vary during a day, in a season and in a year. The factors that cause these variations in insolation are :
- the rotation of earth on its axis;
- the angle of inclination of the sun’s rays;
- the length of the day; (iv) the transparency of the atmosphere;
- The configuration of land in terms of its aspect. The last two however, have less influence.
The fact that the earth’s axis makes an angle of 66½ with the plane of its orbit round the sun has a greater influence on the amount of insolation received at different latitudes
Spatial Distribution of Insolation at the Earth’s Surface
The insolation received at the surface varies from about 320 Watt/m2 in the tropics to about 70 Watt/m2 in the poles.
Maximum insolation is received over the subtropical deserts, where the cloudiness is the least.
Equator receives comparatively less insolation than the tropics.
HEATING AND COOLING OF ATMOSPHERE
The atmosphere is largely transparent to short wave solar radiation
The earth after being heated by insolation transmits the heat to the atmospheric layers near to the earth in long wave form.
The heat energy from the earth’s surface is transferred to the lower atmosphere also get heated which is directly in contact with the surface by the process of conduction.
Conduction takes place when two bodies of unequal temperature are in contact with one another, there is a flow of energy from the warmer to cooler body. The transfer of heat continues until both the bodies attain the same temperature or the contact is broken. Conduction is important in heating the lower layers of the atmosphere.
The air in contact with the earth rises vertically on heating in the form of currents and further transmits the heat of the atmsphere. This process of vertical heating of the atmosphere is known as convection. The convective transfer of energy is confined only to the troposphere.
The transfer of heat through horizontal movement of air is called advection. Horizontal movement of the air is relatively more important than the vertical movement.
In middle latitudes, most of dirunal (day and night) variation in daily weather are caused by advection alone.
In tropical regions particularly in northern India during summer season local winds called ‘loo’ is the outcome of advection process.