Overview of Radiation
	Principle	Radiation is an essential element of the sun-Earth-atmosphere system. Radiation obeys certain laws of physics. These laws provide us with the knowledge that (a) warmer bodies radiate more energy than cooler bodies, (b) warmer bodies radiate at shorter wavelengths than cooler bodies, (c) good emitters are good absorbers at a particular wavelength and (d) poor emitters are poor absorbers at the same wavelength.
	Figure 1 - Electromagnetic Spectrum
	Definition	Electromagnetic radiation, or simply radiation, is an advancing disturbance in electric and magnetic field existing in space or in the media.   The entire range of radiative wavelengths and their associated frequencies is called the electromagnetic (EM) spectrum. Various types of radiation exist, including cosmic rays (extremely high energy from the sun, mostly absorbed by the atmosphere), gamma rays (very high energy from radioactive decay), x-rays (very high energy which can penetrate through the body for a photograph), ultraviolet radiation (high energy which allows for formation of ozone and sunburns, can be very dangerous), visible radiation (medium energy to which the human eye is sensitive), infrared radiation (lower energy which heats the lower portion of the atmosphere), microwave radiation (lower energy used in microwave ovens to heat food), and radio waves (lowest energy used to transmit information and, in meteorology, used in radar technology).
	Figure 2- An Electromagnetic Wave
	Electromagnetic Waves	Electromagnetic waves travel at the speed of light and are described by their wavelength and frequency. The speed of light is 300 million meters per second. The wavelength of an electromagnetic wave is measured as the distance from one crest of the wave to the next. The frequency is defined as the rate at which wave crests pass a fixed point. The frequency equals the speed of light divided by the wavelength.
	Figure 3- Physics of Radiation
	Properties of Radiation	Warmer bodies radiate at shorter wavelengths than cooler bodies. The wavelength which is radiated by an object depends on the object's temperature. The warmer the object, the shorter the wavelength. Hence, the sun radiates at shorter wavelengths (e.g., visible) than the earth (e.g., infrared). Warmer bodies radiate more energy than cooler bodies. The energy emitted from an object is proportional to the fourth root of the object's temperature. As a result, the sun (temperature ~6000 degrees Celsius) radiates 160,000 times more energy than the earth (temperature ~25 degrees Celsius)! Good emitters are good absorbers at a particular wavelength. Poor emitters are poor absorbers at the same wavelength.
	Figure 4- Reflection, Absorption, Scattering, Refraction, and Transmission
	Radiative Transfer	Radiation can be absorbed, reflected, scattered, refracted, or transmitted. Absorbed: energy is retained by a substance. Reflected: the surface returns a portion of the energy. Scattered: the electromagnetic waves are changed from propagating in one direction to all directions. Refracted: the electromagnetic waves are changed from propagating in one direction to another direction. Transmitted: energy passes through space or the media.
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