Heating and Cooling of Atmosphere

How the Atmosphere Gets Heated and Cooled

The atmosphere is not heated directly by the Sun. Instead, the Sun's energy first warms the Earth's surface, and the Earth then passes that heat upward to the air. This happens through three distinct processes, each operating at a different scale.

Conduction is the transfer of heat between two bodies of unequal temperature that are in direct contact with one another. Energy flows from the warmer body to the cooler one, and this transfer continues until both bodies reach the same temperature or the contact is broken. In the atmosphere, conduction works only at the very bottom — the air in contact with the land gets heated slowly, and that heat is then passed upward to the layers above, one by one. Because air is a poor conductor, this process is important only for heating the lowest layers of the atmosphere.

Convection is the vertical transfer of heat. When the air in contact with the Earth's surface gets heated, it rises vertically in the form of currents, carrying the heat upward with it. This is how the atmosphere gets heated from below. Convective transfer is confined entirely to the troposphere — it does not operate in the layers above.

Advection is the transfer of heat through the horizontal movement of air. This is relatively more important than vertical movement. In middle latitudes, most of the day-to-day variation in weather is caused by advection alone. In tropical regions, particularly in northern India during summer, the hot, dry local winds called 'loo' are a direct outcome of the advection process.

Terrestrial Radiation

The insolation received by the Earth comes in short-wave form and heats up the surface. Once heated, the Earth itself becomes a radiating body and emits energy back toward the atmosphere — but this time in long-wave form. This process is known as terrestrial radiation.

This long-wave radiation is absorbed by atmospheric gases, particularly carbon dioxide and the other greenhouse gases. So the atmosphere is heated indirectly, from below, by the Earth's radiation rather than directly by the Sun. The atmosphere in turn radiates and transmits this heat out to space. In this way, the amount of heat received from the Sun is eventually returned to space, and the Earth's surface and atmosphere maintain a constant temperature.

Heat Budget of the Planet Earth

The Earth as a whole neither accumulates nor loses heat — it maintains its temperature. This can happen only if the amount of heat received as insolation exactly equals the amount lost through terrestrial radiation.

Consider the insolation received at the top of the atmosphere as 100 units. While passing through the atmosphere, some energy is reflected, scattered, and absorbed. Only the remaining part reaches the Earth's surface.

The reflected amount — 35 units — is called the albedo of the Earth.

Now, the Earth radiates back those 51 units it absorbed, in the form of terrestrial radiation. Of these:

• 17 units are radiated to space directly.
• The remaining 34 units are absorbed by the atmosphere — 6 units absorbed directly, 9 units through convection and turbulence, and 19 units through the latent heat of condensation.

So the atmosphere absorbs a total of 48 units: 14 units directly from insolation plus 34 units from terrestrial radiation. These 48 units are also radiated back into space.

The total radiation returning from the Earth and the atmosphere is therefore 17 + 48 = 65 units, which exactly balances the 65 units received from the Sun. This balance is termed the heat budget or heat balance of the Earth. It explains why the Earth neither warms up nor cools down despite the enormous transfer of heat that constantly takes place.

Variation in the Net Heat Budget at the Earth's Surface

There are variations in the amount of radiation received at different parts of the Earth's surface. Some regions have a surplus radiation balance while others have a deficit.

Looking at the latitudinal variation in the net radiation balance of the Earth–atmosphere system:

• There is a surplus of net radiation between 40 degrees north and 40 degrees south.
• The regions near the poles have a deficit.

This surplus heat energy from the tropics is redistributed poleward. As a result, the tropics do not get progressively heated up from the accumulation of excess heat, and the high latitudes do not get permanently frozen from excess deficit. The redistribution of heat keeps the whole system in balance.

Factors Affecting the Temperature of a Place

Several factors determine the temperature of any particular place.

Latitude. The temperature of a place depends on the insolation it receives, and insolation varies with latitude. Since insolation is greater near the equator and decreases toward the poles, temperature varies accordingly.

Altitude. The atmosphere is indirectly heated by terrestrial radiation from below. Therefore, places near sea level record higher temperatures than places at higher elevations. In other words, temperature generally decreases with increasing height. The rate of this decrease is called the normal lapse rate, and it is 6.5°C per 1,000 metres. …