Radiation Long Wave

long WAvE (or longwave) radiation is the part of the electromagnetic spectrum emitted at spectral wavelengths generally greater than one micrometer

(|im). Types of long wave radiation include infrared, microwave, and radio waves. Emittance of radiation is a function of temperature, and objects giving off long wave radiation are colder than those radiating at short wavelengths. For example, the sun (approximately 5800 K) radiates primarily in the short wave part of the spectrum (especially visible light from 0.4 to 0.7 micrometers), whereas the Earth (approximately 290 K) emits radiation at much larger wavelengths. Climatologically, long wave radiation generally refers to radiation emitted by the Earth-atmosphere system (also called terrestrial radiation), largely at wavelengths of 5-15 |im. Long wave radiation emitted by the Earth's surface and atmosphere falls primarily within the thermal infrared ("below the red") region of the electromagnetic spectrum. It can be sensed through the sensation of heat.

In the Earth-atmosphere system, short wave radiation from the sun is absorbed and converted to long wave radiation. Various components of the Earth-atmosphere system absorb the incoming short wave radiation; among those are the Earth's surface, gas and dust molecules in the atmosphere, and clouds. Long wave radiation is then reradiated from those components, after which it is referred to as outgoing long wave radiation or counterradiation. Counterradiation may be reabsorbed (and reradiated) by those very same components that initially absorbed short wave radiation. This process is behind the greenhouse effect.

Globally, the Earth's atmosphere is relatively transparent to radiation between 8 and 15 |im. This atmospheric window allows much long wave radiation to be lost to space. However, the window may be closed locally by the presence of large amounts of water vapor or clouds. Additionally, increasing amounts of greenhouse gases can also potentially close this window. Thus, the role of long wave radiation in the greenhouse effect is fundamental. In the absence of an atmosphere containing long wave-absorbing greenhouse gases (for example, water vapor, or carbon dioxide), the Earth's average temperature would be approximately 0 degrees F, (minus 18 degrees C or 255 K). However, due to the efficiency with which greenhouse gases reabsorb counterradiation, the Earth's average temperature is 59 degrees F (15 degrees C, or 288 K).

Water vapor and carbon dioxide are the most abundant greenhouse gases by volume. They are

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