The earth receives almost all of its energy from the sun. This energy is radiated back to space, ensuring the earth neither warms up nor cools down over time. However, the distribution of this heat varies across the globe, causing pressure differences and atmospheric circulation.
The incoming solar radiation received by the earth is known as insolation. The earth receives an average of 1.94 calories per square centimetre per minute at the top of its atmosphere. The solar output received varies slightly throughout the year due to the earth's elliptical orbit:
Consequently, the insolation received on 3rd January is slightly higher than on 4th July, though its effects are masked by atmospheric circulation and land-sea distribution.
The variability of insolation at the earth's surface depends on several factors:
The earth's axis makes an angle of 66½° with the plane of its orbit, which heavily influences the insolation received at different latitudes. Slanted rays cover a larger area, reducing the net energy received per unit area, and must pass through a greater depth of the atmosphere, leading to more absorption and scattering.
The atmosphere is largely transparent to short-wave solar radiation. Within the troposphere, gases like water vapour and ozone absorb much of the near-infrared radiation. Small suspended particles scatter visible light, which gives the sky its blue colour and causes the red hues of sunrise and sunset.
Insolation varies from about 320 Watt/m² in the tropics to about 70 Watt/m² at the poles. The maximum insolation is received over subtropical deserts where cloudiness is least. The equator receives less insolation than the tropics due to cloud cover. At any given latitude, continents receive more insolation than oceans.
The atmosphere is heated and cooled through three primary processes:
The earth's surface, heated by short-wave insolation, becomes a radiating body and emits energy in the form of long waves. This terrestrial radiation heats the atmosphere from below. Greenhouse gases, particularly carbon dioxide, absorb this long-wave radiation, indirectly heating the atmosphere.
The earth maintains a constant temperature because the amount of heat received as insolation equals the amount lost through terrestrial radiation.
Assuming 100 units of insolation reach the top of the atmosphere:
| Heat Budget Component | Units |
|---|---|
| Total Incoming Insolation | 100 |
| Reflected back to space (Albedo of the Earth) | 35 |
| - Reflected by clouds | 27 |
| - Reflected by snow and ice | 2 |
| - Scattered by atmosphere | 6 |
| Net Insolation Absorbed | 65 |
| - Absorbed directly by the atmosphere | 14 |
| - Absorbed by the Earth's surface | 51 |
The earth's surface radiates back its 51 units as follows:
| Radiation from Earth's Surface | Units |
|---|---|
| Radiated directly to space | 17 |
| Absorbed by the atmosphere (direct, convection, latent heat) | 34 |
| Total absorbed by atmosphere (14 from insolation + 34 from terrestrial) | 48 |
| Radiated from atmosphere back to space | 48 |
| Total returned to space (17 direct + 48 from atmosphere) | 65 |
There are latitudinal variations in the net radiation balance. A surplus of net radiation exists between 40° N and 40° S, while the polar regions experience a deficit. Heat is redistributed from the surplus tropical regions to the deficit polar regions by winds and ocean currents, preventing the tropics from overheating and the poles from freezing completely.
Temperature is the measurement of the hotness or coldness of a place, resulting from the interaction of insolation with the atmosphere and the earth's surface.
Temperature distribution is mapped using isotherms (lines joining places of equal temperature). In January, isotherms bend northwards over oceans and southwards over continents in the Northern Hemisphere due to land-sea contrasts. In the Southern Hemisphere, isotherms run more parallel to latitudes due to the dominance of oceans.
Under certain conditions, the normal lapse rate is reversed, and temperature increases with altitude. This is known as inversion of temperature.
A long winter night with clear skies and still air is ideal for temperature inversion. The ground radiates heat rapidly, becoming cooler than the air above. This promotes stability in the lower atmosphere, trapping smoke and dust particles to form dense morning fogs.
In hills and mountains, cold air produced at night flows downslope under gravity. This heavy, dense cold air piles up in valley bottoms, leaving warmer air above. This process, called air drainage, protects valley plants from frost damage.