Role of Atmosphere in Earth Heat Balance

Sun is the major source of energy for the entire earth system. The earth does receive very small proportions of energy from other stars and from the interior of the earth itself (volcanoes and geysers provide certain amount of heat energy). However, when compared with the amount received from the sun, these other sources seem insignificant.

The energy emitted by the sun which reaches the surface of the earth is called Insolation. The sun, a mass of intensely hot gases, with a temperature at the surface be 6000°C emits  radiant  energy in the form of waves, which consists  of very short wave-length x-rays, gamma rays, and ultraviolet  rays; the visible light rays and the longer infrared rays. The earth receives only about one two-thousand-millionth of the total insolation poured out by the sun, but this is vital to it; the amount received at the outer limit of the atmosphere is called Solar Constant. Thus Solar Constant is the rate per unit area at which solar radiation is received at the outer limit of the atmosphere.

Effects of the Atmosphere on Solar Radiation

When the sun’s energy passes through the atmosphere several things happen to it. Around one fourth of this energy is directly reflected back to clouds and the ground. Around 8 percent is scattered by minute atmospheric particles and returned to space as diffuse radiation. Some 20 percent reaches the earth’s surface as diffuse radiation after being scattered. Approximately 27 percent reaches the earth’s surface as direct radiation and 19 percent is absorbed by the ozone layer and by water vapour in the clouds of the atmosphere.

On an average, 47 percent of the solar energy arriving at the outer limits of the atmosphere eventually reaches the surface, and 19 percent is retained in the atmosphere. This 19 percent of direct solar radiation that is retained by the atmosphere is locked up in the clouds and the ozone layer and is thus not available to heat the troposphere. The warmth of the atmosphere is due to the 47 percent of incoming solar energy reaching the earth’s surface (that is, both land and bodies of water) and in the transfer of heat energy from the earth back to the atmosphere through such physical processes such as Long-Wave Radiation, Conduction and Convection. Some related phenomena such as advection and Latent Heat of Condensation also contribute to the warmth of the atmosphere.

Radiation as method of Heat Energy Transfer

Radiation is the process by which most energy is transferred through space from the sun to the earth. Radiation is given off by all bodies including earth and human being. The hotter is the body, shorter are the waves.

We can simply say that the radiation from Sun comes to earth in the form of smaller waves and earth being cooler body, gives off energy in the form of long-wave. These are then radiated back to the atmosphere. This Long-Wave Radiation from the earth’s surfaces heats the lower layers of the atmosphere. It is evident that the atmosphere is primarily heated from below by radiation from the heated Earth surface.

As we discussed above, the most important cause of atmospheric temperature is the energy received from the sun. The atmosphere of the earth does not heat up directly as solar radiation is in the form of short waves and air cannot absorb the short waves. The earth absorbs the short wave energy and then radiates in the form of long wave terrestrial radiation that can be absorbed by the air. So, air heats up when comes in contact with the surface of the earth.

Conduction as Method of Heat Transfer

Conduction is the means by which heat is transferred from one part of a body to another or between two touching objects. Heat flows from the warmer to the cooler (part of a) body in order to equalize temperature. Conduction actually occurs through molecular movement, with one molecule bumping into another. The Atmospheric conduction occurs at the interface of (zone of contact between) the atmosphere and the earth’s surface. However, it is actually a insignificant method of heat transfer in terms of warming the atmosphere since it affects only the layers of air closest to the earth’s surface. This is because air is a very poor conductor of heat.

Convection as Source of Heat Transfer

When the pockets of air near the surface are heated, they expand in volume, become less dense than the surrounding air, and therefore rise. This vertical transfer of heat through the atmosphere is called convection, and is the same type of process by which heated water circulates in a pan while heating. The currents set into motion by the heating of a fluid (liquid or gas) make up a convectional system. Most vertical transfer of heat within the atmosphere & Oceans occurs via Convection and is a major cause of clouds and precipitation.

Advection as Source of Heat Transfer

Advection is the horizontal heat transfer within the atmosphere. Obviously the wind is the transfer agent of advection. Wind brings about the horizontal movement of large portions of lower atmosphere. This advection transports warmer or accounts for a major proportion of the lateral heat transfer that takes place within the atmospheric system.

Latent Heat of Condensation

A proportion of the solar energy is used to change liquid water from rivers, lakes, and oceans to water-vapour. The solar energy used to do this is then stored in the water-vapour as latent or potential energy. Later the water-vapour in the atmosphere may change to form liquid water again through a process called CONDENSATION. The energy released through this process is known as the Latent Heat of Condensation. Like other means of heat transfer in the earth system, latent heat of condensation plays a major role in warming of the atmosphere and in addition, is a source of energy for STORMS.

Earth’s Albedo

The ratio between the total solar radiation falling (incident) upon a surface and the amount reflected without heating  the earth, is called ALBEDO (expressed as a decimal or as a percentage).  The earth’s average albedo is about 0.4 (40 percent) ; that  is , 4/10 of the solar radiation  is reflected back into space. It varies from 0.03 for dark soil to 0.85 for a snow-failed. Water has a low albedo (0.02) with near-vertical rays, but a high albedo for low-angle slanting rays. The figure for grass is about 0.25. Over-pastured land and bare soil are more reflective of solar radiation than are crops and vegetation. A desert is much more reflective than a savanna or forest. If economic pressure on soil and vegetation increases, and drought then occurs, the effect overall is to increase the albedo of the surface.