To the inorganic or physical energy cycle, there must be added an organic phase, the biochemical energy cycle, in which a part of the incoming solar energy is used and given up by plants and animals. Briefly, this cycle consists of the absorption of solar energy by plants in the manufacture of carbohydrate compounds, which provide the food for animals. Eventually, after much recycling, most of these compounds are oxidized in a process known as respiration and the energy is returned to the atmosphere.
Solar radiation. A systematic approach to the earth's heat budget begins with an examination of the input, or source, of energy from solar radiation. This radiation is traced as it penetrates the earth's atmosphere and is absorbed or transformed. We then turn to the mechanism of output of heat energy by the earth as a secondary radiator.
Our sun, has a surface temperature of about 11,000°F (6000°C). The highly heated, incandescent gas that comprises the sun's surface emits a form of energy known as electromagnetic radiation. This form of energy transfer can be thought of as a collection, or spectrum, of waves of a wide range of lengths traveling at the uniform velocity of 186,000 mi (300,000 km) per second. The energy travels in straight lines radially outward from the sun, and requires about 9 1/3 minutes to travel the 93 million miles (150 million kilometers) from sun to earth. Although the solar radiation travels through space without energy loss, the intensity of radiation within a beam of given cross section (such as one square inch) decreases inversely as the square of the distance from the sun. The earth thus intercepts only about one two-billionth of the sun's total energy output.
The solar radiation spectrum consists of
(a) X-rays, gamma rays, and ultraviolet rays, carrying about 9 percent of the total energy,
(b) visible light rays, 41 percent, and
(c) invisible infrared and heat rays, 50 percent.
A micron is equivalent to one ten-thousandth of a centimeter. Hereafter, the term shortwave radiation is applied to the visible and ultraviolet portion of the spectrum (wavelengths less than 0.7 microns) as distinct from the infrared portion (wave-lengths longer than 0.7 microns). We see that the total energy of the radiation spectrum is about equally divided between shortwave and infrared portions.
In this fusion process, a vast quantity of heat is generated and finds its way by convection and conduction to the sun's surface. Because the rate of production of energy is constant, the output of solar radiation varies only slightly. Hence, at the average distance from the sun, the solar energy which is received upon a unit area of surface held at right angles to the sun's rays is also nearly constant. Known as the solar constant, this radiation rate has a value of about 2 gram calories per square centimeter per minute.
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