Radiation, Long Wave

LONG WAVE (OR longwave) radiation is the part of the electromagnetic spectrum emitted at spectral wavelengths generally greater than one micrometer (μm). 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 μm. 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 Earths 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 Earths atmosphere is relatively transparent to radiation between 8 and 15 μm. 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 [Page 840]particularly effective at absorbing counterradiation. The amount of water vapor in the atmosphere is a direct response to temperature. Other long wave-absorbing gases can be produced by human activity. Carbon dioxide and certain other trace gases (such as methane, nitrous oxide, and chlorofluorocarbons) can potentially upset the long wave exchanges involved in the Earth's energy balance. Such deviations can lead to changes in the average global temperature. Potential consequences of an escalating amount of these atmospheric gases include a growing proportion of outgoing long wave radiation being “trapped” in the atmosphere, leading to an increase in the Earth's temperature.

Clouds serve as very effective absorbers of outgoing long wave radiation. This, in turn, affects surface and near-surface temperatures. The presence of clouds in a nighttime sky results in warmer temperatures than a cloudless one. In the absence of incoming short wave radiation, only long wave exchanges occur at night. With clouds trapping much of the outgoing long wave, substantial amounts are redirected back toward the surface. This increases the near-surface temperatures. Similarly, the lack of clouds leads to greater amounts of long wave radiation escaping the Earth-atmosphere system to space.

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