Atmospheric window

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The absorption bands of Earth's atmosphere (grey colour) delimit its atmospheric windows (middle panel) and the effect they have on both downgoing solar radiation and upgoing thermal radiation emitted near the surface is shown in the top panel. The individual absorption spectra of major greenhouse gases plus Rayleigh scattering are shown in the lower panel. Atmospheric Transmission.svg
Τhe absorption bands of Earth's atmosphere (grey colour) delimit its atmospheric windows (middle panel) and the effect they have on both downgoing solar radiation and upgoing thermal radiation emitted near the surface is shown in the top panel. The individual absorption spectra of major greenhouse gases plus Rayleigh scattering are shown in the lower panel.

An atmospheric window is a region of the electromagnetic spectrum that can pass through the atmosphere of Earth. The optical, infrared and radio windows comprise the three main atmospheric windows. [2] The windows provide direct channels for Earth's surface to receive electromagnetic energy from the Sun, and for thermal radiation from the surface to leave to space. [3] Atmospheric windows are useful for astronomy, remote sensing, telecommunications and other science and technology applications.

Contents

In the study of the greenhouse effect, the term atmospheric window may be limited to mean the infrared window , which is the primary escape route for a fraction of the thermal radiation emitted near the surface. [4] [5] In other fields of science and technology, such as radio astronomy [6] and remote sensing, [7] the term is used as a hypernym, covering the whole electromagnetic spectrum as in the present article.

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. [8]

Out of an average 340  watts per square meter (W/m2) of solar irradiance 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 (105 W/m2) plus outgoing longwave radiation (235 W/m2), 80-100 W/m2 exits to space through the infrared window depending on cloudiness. 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. [3]

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 coefficients of the atmospheric medium is needed in order to perform a rigorous quantitative analysis (typically done with atmospheric radiative transfer codes). 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. [9]

Other applications

In astronomy

Up until the 1940s, astronomers used optical telescopes to observe distant astronomical objects whose radiation reached the earth through the optical window. After that time, the development of radio telescopes gave rise to the more successful field of radio astronomy that is based on the analysis of observations made through the radio window. [10]

In telecommunications

Communications satellites 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. [11] [12] Shortwave radio does the opposite, using frequencies that produce skywaves rather than those that escape through the radio windows. [13]

In remote sensing

Both active (signal emitted by satellite or aircraft, reflection detected by sensor) and passive (reflection of sunlight detected by the sensor) remote sensing techniques work with wavelength ranges contained in the atmospheric windows. [14]

See also

Related Research Articles

<span class="mw-page-title-main">Electromagnetic spectrum</span> Range of frequencies or wavelengths of electromagnetic radiation

The electromagnetic spectrum is the full range of electromagnetic radiation, organized by frequency or wavelength. The spectrum is divided into separate bands, with different names for the electromagnetic waves within each band. From low to high frequency these are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. The electromagnetic waves in each of these bands have different characteristics, such as how they are produced, how they interact with matter, and their practical applications.

<span class="mw-page-title-main">Greenhouse effect</span> Atmospheric phenomenon causing planetary warming

The greenhouse effect occurs when greenhouse gases in a planet's atmosphere trap some of the heat radiated from the planet's surface, raising its temperature. This process happens because stars emit shortwave radiation that passes through greenhouse gases, but planets emit longwave radiation that is partly absorbed by greenhouse gases. That difference reduces the rate at which a planet can cool off in response to being warmed by its host star. Adding to greenhouse gases further reduces the rate a planet emits radiation to space, raising its average surface temperature.

<span class="mw-page-title-main">Infrared</span> Form of electromagnetic radiation

Infrared is electromagnetic radiation (EMR) in the spectral band between microwaves and visible light. It is invisible to the human eye. IR is generally understood to encompass wavelengths from around 750 nm to 1000 μm.

<span class="mw-page-title-main">Sunlight</span> Light emitted by the Sun

Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when the Sun is above the horizon. When direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat (Atmospheric). When blocked by clouds or reflected off other objects, sunlight is diffused. Sources estimate a global average of between 164 watts to 340 watts per square meter over a 24-hour day; this figure is estimated by NASA to be about a quarter of Earth's average total solar irradiance.

<span class="mw-page-title-main">Radio wave</span> Type of electromagnetic radiation

Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz (GHz) and below. At 300 GHz, the corresponding wavelength is 1mm, which is shorter than the diameter of a grain of rice. At 30 Hz the corresponding wavelength is ~10,000 kilometers, which is longer than the radius of the Earth. Wavelength of a radio wave is inversely proportional to its frequency, because its velocity is constant. Like all electromagnetic waves, radio waves in a vacuum travel at the speed of light, and in the Earth's atmosphere at a slightly slower speed. Radio waves are generated by charged particles undergoing acceleration, such as time-varying electric currents. Naturally occurring radio waves are emitted by lightning and astronomical objects, and are part of the blackbody radiation emitted by all warm objects.

<span class="mw-page-title-main">Thermal radiation</span> Electromagnetic radiation generated by the thermal motion of particles

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 is converted to electromagnetic radiation. All matter with a temperature greater than absolute zero emits thermal radiation. At room temperature, most of the emission is in the infrared (IR) spectrum. Particle motion results in charge-acceleration or dipole oscillation which produces electromagnetic radiation.

<span class="mw-page-title-main">Optical window</span>

The optical window is the portion of the optical spectrum that is not blocked by the Earth's atmosphere. The window runs from around 300 nanometers (ultraviolet-B) up into the range the human eye can detect, roughly 400–700 nm and continues up to approximately 2 μm. Sunlight mostly reaches the ground through the optical atmospheric window; the Sun is particularly active in most of this range.

<span class="mw-page-title-main">Microwave radiometer</span> Tool measuring EM radiation at 0.3–300-GHz frequency

A microwave radiometer (MWR) is a radiometer that measures energy emitted at one millimeter-to-metre wavelengths (frequencies of 0.3–300 GHz) known as microwaves. Microwave radiometers are very sensitive receivers designed to measure thermally-emitted electromagnetic radiation. They are usually equipped with multiple receiving channels to derive the characteristic emission spectrum of planetary atmospheres, surfaces or extraterrestrial objects. Microwave radiometers are utilized in a variety of environmental and engineering applications, including remote sensing, weather forecasting, climate monitoring, radio astronomy and radio propagation studies.

<span class="mw-page-title-main">Absorption spectroscopy</span> Spectroscopic techniques that measure the absorption of radiation

Absorption spectroscopy is spectroscopy that involves techniques that measure the absorption of electromagnetic radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a function of frequency, and this variation is the absorption spectrum. Absorption spectroscopy is performed across the electromagnetic spectrum.

Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It lies between the super high frequency band and the far infrared band, the lower part of which is the terahertz band. Radio waves in this band have wavelengths from ten to one millimetre, so it is also called the millimetre band and radiation in this band is called millimetre waves, sometimes abbreviated MMW or mmWave. Millimetre-length electromagnetic waves were first investigated by Jagadish Chandra Bose, who generated waves of frequency up to 60 GHz during experiments in 1894–1896.

<span class="mw-page-title-main">Black-body radiation</span> Thermal electromagnetic radiation

Black-body radiation is the thermal electromagnetic radiation within, or surrounding, a body in thermodynamic equilibrium with its environment, emitted by a black body. It has a specific, continuous spectrum of wavelengths, inversely related to intensity, that depend only on the body's temperature, which is assumed, for the sake of calculations and theory, to be uniform and constant.

<span class="mw-page-title-main">Atmospheric physics</span> The application of physics to the study of the atmosphere

Within the atmospheric sciences, atmospheric physics is the application of physics to the study of the atmosphere. Atmospheric physicists attempt to model Earth's atmosphere and the atmospheres of the other planets using fluid flow equations, radiation budget, and energy transfer processes in the atmosphere. In order to model weather systems, atmospheric physicists employ elements of scattering theory, wave propagation models, cloud physics, statistical mechanics and spatial statistics which are highly mathematical and related to physics. It has close links to meteorology and climatology and also covers the design and construction of instruments for studying the atmosphere and the interpretation of the data they provide, including remote sensing instruments. At the dawn of the space age and the introduction of sounding rockets, aeronomy became a subdiscipline concerning the upper layers of the atmosphere, where dissociation and ionization are important.

<span class="mw-page-title-main">Emissivity</span> Capacity of an object to radiate electromagnetic energy

The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is not visible to human eyes. A portion of the thermal radiation from very hot objects is easily visible to the eye.

<span class="mw-page-title-main">Radio window</span>

The radio window is the region of the radio spectrum that penetrate the Earth's atmosphere. Typically, the lower limit of the radio window's range has a value of about 10 MHz ; the best upper limit achievable from optimal terrestrial observation sites is equal to approximately 1 THz.

<span class="mw-page-title-main">Absorption (electromagnetic radiation)</span> Physical process by which matter takes up a photons energy and stores it

In physics, absorption of electromagnetic radiation is how matter takes up a photon's energy — and so transforms electromagnetic energy into internal energy of the absorber.

<span class="mw-page-title-main">Outgoing longwave radiation</span> Energy transfer mechanism which enables planetary cooling

In climate science, longwave radiation (LWR) is electromagnetic thermal radiation emitted by Earth's surface, atmosphere, and clouds. It may also be referred to as terrestrial radiation. This radiation is in the infrared portion of the spectrum, but is distinct from the shortwave (SW) near-infrared radiation found in sunlight.

<span class="mw-page-title-main">Electromagnetic absorption by water</span>

The absorption of electromagnetic radiation by water depends on the state of the water.

<span class="mw-page-title-main">Water vapor windows</span>

Water vapor windows are wavelengths of infrared light that have little absorption by water vapor in Earth's atmosphere. Because of this weak absorption, these wavelengths are allowed to reach the Earth's surface barring effects from other atmospheric components. This process is highly impacted by greenhouse gases because of the effective emission temperature. The water vapor continuum and greenhouse gases are significantly linked due to water vapor's benefits on climate change.

In the study of heat transfer, Schwarzschild's equation is used to calculate radiative transfer through a medium in local thermodynamic equilibrium that both absorbs and emits radiation.

<span class="mw-page-title-main">Thermal remote sensing</span>

Thermal remote sensing is a branch of remote sensing in the thermal infrared region of the electromagnetic spectrum. Thermal radiation from ground objects is measured using a thermal band in satellite sensors.

References

  1. ↑ "The Atmospheric Window". National Oceanographic and Atmospheric Administration . Retrieved 28 October 2022.
  2. ↑ "Introduction to the Electromagnetic Spectrum | Science Mission Directorate". science.nasa.gov. Retrieved 2021-12-28.
  3. 1 2 Kiehl, J. T.; Trenberth, Kevin E. (1 February 1997). "Earth's Annual Global Mean Energy Budget". Bulletin of the American Meteorological Society. 78 (2): 197–208. Bibcode:1997BAMS...78..197K. doi: 10.1175/1520-0477(1997)078<0197:eagmeb>2.0.co;2 .
  4. ↑ Cotton, William R.; Pielke, Roger A. (2007). Human impacts on weather and climate. Cambridge: Cambridge University Press. p. 180. ISBN   978-0-521-84086-6. OCLC   466742997.
  5. ↑ Rohli, Robert V; Vega, Anthony J (2012). Climatology. Sudbury, MA: Jones & Bartlett Learning. p. 287. ISBN   978-0-7637-9101-8. OCLC   569552317.
  6. ↑ Burke, Bernard F. (2019). An introduction to radio astronomy. Cambridge: Cambridge University Press. p. 5. ISBN   978-1-107-18941-6. OCLC   1199628889.
  7. ↑ Joseph, George (2005). Fundamentals of remote sensing. Hyderabad: Universities Press, India. p. 43. ISBN   978-81-7371-535-8. OCLC   474734434.
  8. ↑ US Department of Commerce, NOAA. "The Earth-Atmosphere Energy Balance". www.weather.gov. Retrieved 2021-12-29.
  9. ↑ "Remote Sensing: Absorption Bands and Atmospheric Windows". NASA Earth Observatory. 17 September 1999. Retrieved 28 October 2022.
  10. ↑ Wilson, Thomas (2016). Tools of Radio Astronomy. Springer-Verlag GmbH. pp. 1–2. ISBN   978-3-662-51732-1. OCLC   954868912.
  11. ↑ Banerjee, P. (2017). Satellite communication. New Delhi: Prentice-Hall of India. p. 181. ISBN   978-81-203-5299-5. OCLC   1223331096.
  12. ↑ Ngan, King N. (2001). Video Coding for Wireless Communication Systems. CRC Press. p. 183. ISBN   978-1-4822-9009-7. OCLC   1027783404.
  13. ↑ Nyre, Lars (2009-06-02). Sound Media: From Live Journalism to Music Recording. Routledge. p. 147. ISBN   978-1-135-25377-6.
  14. ↑ Dwivedi, Ravi Shankar (2017). Remote sensing of soils. Srpinger-Verlag GmbH. p. 13. ISBN   978-3-662-53738-1. OCLC   959595730.
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