Heat and 'Insolation'

Heat is the energy input that initiates all atmospheric processes. Its unequal distribution over the earth causes density differences in the atmospheric motion. Heat is the energy that evaporates water from the earth's surface and causes it to mix upward into the atmosphere. Heat is intimately exchanged with other forms of energy, such as molecular motion, kinetic energy of atmospheric flow, the latent energy of the change of state of water, and the potential energy of position above the earth's surface.

For practical purposes all the heat energy that the earth's atmosphere receives originates from the sun. What happens to the sunlight as it penetrates the earth-atmosphere system depends on the characteristics of the atmosphere and the underlying surface. For the earth as a whole the entirety of climate can be explained by the amount of sunlight received and the character of the surface receiving it. For any portion of the earth-atmosphere system, however, the climate is profoundly influenced by atmospheric motion, which itself is a product of heat receipt and nature of the earth's surface.

...The heat received from incoming sunlight is known as insolation. This has been measured approximately at many different points on the earth over a great number of years and, taking into account various states of the atmosphere, it has been estimated that at the top of the atmosphere slightly less than two calories of heat are received per minute on a square centimeter of surface oriented perpendicular to the sunlight. Within the range of error of calculations, this appears to be a constant, known as the solar constant (There appears to be a little variation in solar emission, perhaps of a cyclic nature. Recent measurements from satellites have yielded an average value for the "solar constant" of 1370 W/m2 - watts per square meter).

The sunlight is the result of radiation of heat outward from the sun which on the average is about 150 million kilometers (93 million miles) away from the earth. Since the sun is essentially a sphere, and heat is radiated perpendicular to its surface in all directions, sun rays diverge away from the sun's surface so that the heat flux passing through any unit area decreases rapidly with distance from the sun. The rule is that the heat flux through a unit area diminishes at the rate of distance squared that it has traveled. Thus, knowing the amount of heat received at the earth, and knowing (though astronomical measurement) the distance between the earth and the sun, one can compute the emission power of the sun. This turns out to be approximately 1023 kilowatts, an enormous magnitude that exceeds by many millions of times al the electrical generating capacity on earth. If this amount of energy were being produced by the combustion of coal, the sun would have burned up in 5000 years, even though it is a large body with a volume approximately one million times that of the earth. The sun's energy is not being produced by the combustion of coal, however, but by the fusion of hydrogen to helium...

By comparison to the sun and the distance between the sun and the earth, the earth is a small speck in space that intercepts only a minute portion of the total heat emission from the sun. The entire earth intercepts only about 1/2 billionth of the sun's energy output, but this still amounts to 1.8 X 1014 kilowatts, which is more than 300,000 times all the electrical generating capacity in the United States. Obviously, then, any efforts by man to add to or subtract from the heat budget of the earth by burning fossil fuels, or even by producing nuclear energy, will be puny compared to the amount of heat the earth receives from the sun every instant. Thus, man cannot hope to alter the earth's climate over broad areas by the artificial addition of heat. But man can hope to find ways to divert and convert the sun's energy into more-useful forms and processes.

--The Climate of the Earth, Paul E. Lydolph (1985)

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