atmospheric window

An atmospheric window is a band of frequencies (and wavelengths) over which the Earth's atmosphere allows electromagnetic radiation (EMR) to pass, given that the Earth's atmospheric transparency is limited. To other wavelengths, the atmosphere is opaque, which is a major motivation for space telescopes. Major atmospheric windows:

• visible (optical) - includes visible light and near infrared (without this window, we would have no daylight).
• radio - a large part of the radio frequency range.
• infrared - varies with time and place.

Within infrared and microwave, there are additional narrower windows, such as the water hole, which allows the 18-cm and 21-cm line to be viewed from the ground. The windows vary as to how transparent they are. Higher altitudes avoid some of the opaqueness, opening some windows to practical use and generally enhancing all the windows, the motivation for locating observatories at high altitudes. Water vapor in air also is a source of opaqueness for some microwave and infrared wavelengths, and dry air (such as found in deserts, and in very cold climates) also improves windows for astronomical use.

Atmospheric windows are a major factor in the behavior of the atmosphere, i.e., weather and climate, as they influence the Sun's warming effect and the Earth's cooling that balances it, i.e., the degree of the greenhouse effect. The windows' characteristics result from the atmosphere's constituents, its density, and how the constituents and density are distributed.