An atmospheric window is a range of

wavelengths In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...

of the

electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...

that can pass through the

atmosphere of Earth The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing ...

. The

optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from aroun ...

and

radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transm ...

windows comprise the three main atmospheric windows. The windows provide direct channels for Earth's surface to receive electromagnetic energy from the Sun, and for

thermal radiation Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) is ...

from the surface to leave to space. Atmospheric windows are useful for

astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...

remote sensing Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Ear ...

telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than tha ...

and other science & technology applications.

Role in Earth's energy budget

Atmospheric windows, especially the optical and infrared, affect the distribution of energy flows and temperatures within Earth's energy balance. The windows are themselves dependent upon clouds, water vapor, trace greenhouse gases, and other components of the atmosphere. Out of an average 340

watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wa ...

s per square meter (W/m²) of

solar irradiance Solar irradiance is the power per unit area ( surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metr ...

at the top of the atmosphere, about 200 W/m² reaches the surface via windows, mostly the optical and infrared. Also, out of about 340 W/m² of reflected

shortwave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 m ...

(105 W/m²) plus

outgoing longwave radiation Outgoing Long-wave Radiation (OLR) is electromagnetic radiation of wavelengths from 3–100 μm emitted from Earth and its atmosphere out to space in the form of thermal radiation. It is also referred to as up-welling long-wave radiation an ...

(235 W/m²), 80-100 W/m² exits to space through the infrared window depending on

cloudiness Cloud cover (also known as cloudiness, cloudage, or cloud amount) refers to the fraction of the sky obscured by clouds on average when observed from a particular location. Okta is the usual unit for measurement of the cloud cover. The cloud c ...

. About 40 W/m² of this transmitted amount is emitted by the surface, while most of the remainder comes from lower regions of the atmosphere. In a complimentary manner, the infrared window also transmits to the surface a portion of down-welling thermal radiation that is emitted within colder upper regions of the atmosphere. Some sources, mainly the ones related to the study of the

greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...

, suggest a synonymity of the terms ''atmospheric window'' and ''

infrared window The infrared atmospheric window refers to a region of the Infrared spectrum where there is relatively little absorption of terrestrial thermal radiation by atmospheric gases. The window plays an important role in the atmospheric greenhouse effec ...

'', because the infrared window is examined as an escape route for a fraction of the thermal radiation emitted near the surface. Sources within other fields of science and technology have exhibited less ambiguity, namely

radio astronomy Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comi ...

and

. The "window" concept is useful to provide qualitative insight into some important features of atmospheric radiation transport. Full characterization of the absorption, emission, and

scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...

coefficients of the atmospheric medium is needed in order to perform a rigorous quantitative analysis (typically done with

atmospheric radiative transfer codes An atmospheric radiative transfer model, code, or simulator calculates radiative transfer of electromagnetic radiation through a planetary atmosphere. Methods At the core of a radiative transfer model lies the radiative transfer equation that ...

). Application of the Beer-Lambert Law may yield sufficient quantitative estimates for wavelengths where the atmosphere is optically thin. Window properties are mostly encoded within the absorption profile.

Other applications

In astronomy

Up until the 1940s, astronomers used

optical telescopes An optical telescope is a telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through electro ...

to observe distant

astronomical objects An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often use ...

whose radiation reached the earth through the optical window. After that time, the development of

radio telescopes A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency ...

gave rise to the more successful field of

that is based on the analysis of observations made through the

radio window The radio window is a range of frequencies of electromagnetic radiation that penetrate the earth's atmosphere. It plays an important role in astronomy; up until World War II, astronomers could only use the visible and near infrared portions of the ...

In telecommunications

Communications satellites A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. C ...

greatly depend on the atmospheric windows for the transmission and reception of signals: the satellite-ground links are established at frequencies that fall within the spectral bandwidth of atmospheric windows.

Shortwave radio Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 m ...

does the opposite, using frequencies that produce skywaves rather than those that escape through the radio windows.

In remote sensing

Both active (signal emitted by satellite or aircraft, reflection detected by sensor) and passive (reflection of sunlight detected by the sensor)

techniques work with wavelength ranges contained in the atmospheric windows.{{Cite book, last=Dwivedi, first=Ravi Shankar, url=https://www.worldcat.org/title/remote-sensing-of-soils/oclc/959595730, title=Remote sensing of soils, publisher= Srpinger-Verlag GmbH, year=2017, isbn=978-3-662-53738-1, pages=13, language=English, oclc=959595730

References