radiation traveling IN waves shorter than one micrometer (|im) is characterized as short wave, and includes gamma rays, x-rays, ultraviolet light, and visible light. Climatologically, short wave radiation commonly refers to the incoming radiation from the sun. There is an inverse relationship between the temperature of an object and the wavelengths at which it primarily emits. Because the sun is a hot object (approximately 5800 K), it emits radiation at short wavelengths. Since shorter wavelengths carry more energy than longer ones, they are more intense. Most of the short wave emitted by the sun is in the visible region of the electromagnetic spectrum, which spans from 0.4 |im (violet) to 0.7 |im (red). The sun's wavelength of maximum emission is found at 0.5 |im.
The amount of emitted solar radiation that reaches the Earth decreases inversely with the square of the distance between the Earth and sun, by the inverse square law. The total of short wave radiation that reaches the top of the Earth-atmosphere system is called the solar constant. Although the amount varies slightly throughout the year, due to the elliptical nature of the Earth's orbit around the sun, the solar constant averages around 1370 W m-2.
Because the eccentricity of the Earth's orbit varies between nearly zero to five percent (with a periodicity of 110,000 years), the amount of short wave received increases or decreases over time from present values. The current orbit is nearly circular, leading to little seasonal variation of incoming short wave radiation. However, a more highly elliptical orbit would render a difference of up to 30 percent between aphelion (maximum Earth-sun distance) and perihelion (minimum Earth-sun distance). Incoming solar radiation can take several avenues once it enters the atmosphere. Over a year, 30 percent of total short wave radiation is reflected back to space, either by gas molecules or other particles in the atmosphere, by clouds, or by the
Earth's surface; this is the Earth's albedo (the proportion of radiation that is reflected from a surface). The atmosphere and clouds absorb an additional 20 percent of short wave radiation. Approximately 50 percent strikes the surface, where it is absorbed. These values may vary locally and at shorter time scales.
Some of the short wave radiation reaching the Earth's surface arrives directly, unimpeded by clouds or atmospheric constituents; this is direct radiation. Some is scattered about the atmosphere and arrives at the surface indirectly. This is diffuse radiation. Diffuse radiation is a product of scattering, which occurs when short wave radiation strikes small particles in the atmosphere, including gas molecules. Upon impact, the radiation is scattered omnidirectionally. Some short wave radiation will be scattered toward the surface. Energy received at the Earth's surface is the total amount of direct and diffuse radiation.
The presence and type of clouds in the atmosphere reduces the amount of short wave radiation reaching the Earth's surface. Thin clouds, such as cirrus, have lower albedos than thick ones, such as cumulus. Once short wave solar radiation reaches the surface and is absorbed, it is converted to long wave forms of radiation.
Short wave radiation impacting the Earth also includes ultraviolet light. These wavelengths are shorter than visible radiation, so ultraviolet light carries more energy than visible light. Ultraviolet radiation is classed into three categories: UV-A (0.32-0.40 |im), UV-B (0.29-0.32 |im), and UV-C (0.20-0.29 |im). Excessive exposure to UV-A and UV-B radiation has been linked to skin cancers and skin damage. UV-C radiation is largely absorbed by stratospheric ozone. Absorption of ultraviolet radiation breaks ozone down into atomic and molecular oxygen. The presence of stratospheric ozone helps protect the Earth's surface from the damaging effects of this form of short wave radiation.
sEE ALso: Radiation, Ultraviolet; Sunlight.
BIBLioGRAPHY. C.D. Ahrens, Meteorology Today (Thompson Brooks/Cole, 2007); D.L. Hartmann, Global Physical Climatology (Academic Press, 1994); R.B. Stull, Meteorology for Scientists and Engineers (Brooks/Cole, 1999).
Petra A. Zimmermann
Ball State University
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