View source for Atmospheric physics

{{Short description|Sub-field of physics dealing with the atmosphere's structure, composition, and motion}}
{{Atmospheric sciences| atmophys=true}}
Within the [[atmospheric sciences]], '''atmospheric physics''' is the application of [[physics]] to the study of the [[Earth's atmosphere|atmosphere]]. Atmospheric physicists attempt to model [[Earth's atmosphere]] and the atmospheres of the other [[planet]]s using [[fluid dynamics|fluid flow]] equations, [[radiation budget]], and energy transfer processes in the atmosphere (as well as how these tie into boundary systems such as the oceans). 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.

== Remote sensing ==
{{Weather}}
[[Image:weather radar.jpg|thumb|Brightness can indicate reflectivity as in this 1960 [[weather radar]] image (of [[1960 Atlantic hurricane season#Hurricane Abby|Hurricane Abby]]). The radar's frequency, pulse form, and antenna largely determine what it can observe.]]
{{Main|Remote sensing}}
Remote sensing is the small or large-scale acquisition of information of an object or phenomenon, by the use of either recording or real-time sensing device(s) that is not in physical or intimate contact with the object (such as by way of [[aircraft]], [[spacecraft]], [[satellite]], [[buoy]], or [[ship]]). In practice, remote sensing is the stand-off collection through the use of a variety of devices for gathering information on a given object or area which gives more information than sensors at individual sites might convey.<ref>COMET program (1999). [http://www.comet.ucar.edu/nsflab/web/remote/121.htm Remote Sensing.] {{Webarchive|url=https://web.archive.org/web/20130507000326/http://www.comet.ucar.edu/nsflab/web/remote/121.htm |date=2013-05-07 }} [[University Corporation for Atmospheric Research]]. Retrieved on 2009-04-23.</ref> Thus, [[Earth observation]] or [[weather satellite]] collection platforms, ocean and atmospheric observing [[weather buoy]] platforms, monitoring of a pregnancy via [[ultrasound]], [[magnetic resonance imaging]] (MRI), [[positron-emission tomography]] (PET), and [[space probes]] are all examples of remote sensing. In modern usage, the term generally refers to the use of imaging sensor technologies including but not limited to the use of instruments aboard aircraft and spacecraft, and is distinct from other imaging-related fields such as [[medical imaging]].

There are two kinds of remote sensing. Passive sensors detect natural radiation that is emitted or reflected by the object or surrounding area being observed. Reflected sunlight is the most common source of radiation measured by passive sensors. Examples of passive remote sensors include film [[photography]], infrared, [[charge-coupled devices]], and [[radiometer]]s. Active collection, on the other hand, emits energy in order to scan objects and areas whereupon a sensor then detects and measures the radiation that is reflected or backscattered from the target. [[radar]], [[lidar]], and [[SODAR]] are examples of active remote sensing techniques used in atmospheric physics where the time delay between emission and return is measured, establishing the location, height, speed and direction of an object.<ref>Glossary of Meteorology (2009). [http://amsglossary.allenpress.com/glossary/search?id=radar1 Radar.] [[American Meteorological Society]]. Retrieved on 2009-24-23.</ref>

Remote sensing makes it possible to collect data on dangerous or inaccessible areas. Remote sensing applications include monitoring [[deforestation]] in areas such as the [[Amazon Basin]], the [[effects of climate change]] on [[glacier]]s and Arctic and Antarctic regions, and [[depth sounding]] of coastal and ocean depths. Military collection during the [[Cold War]] made use of stand-off collection of data about dangerous border areas. Remote sensing also replaces costly and slow data collection on the ground, ensuring in the process that areas or objects are not disturbed.

Orbital platforms collect and transmit data from different parts of the [[electromagnetic spectrum]], which in conjunction with larger scale aerial or ground-based sensing and analysis, provides researchers with enough information to monitor trends such as [[El Niño]] and other natural long and short term phenomena. Other uses include different areas of the [[earth science]]s such as [[natural resource management]], agricultural fields such as land usage and conservation, and national security and overhead, ground-based and stand-off collection on border areas.<ref>[[NASA]] (2009). [http://hurricanes.nasa.gov/earth-sun/technology/remote_sensing.html Earth.] {{webarchive|url=https://web.archive.org/web/20060929081013/http://hurricanes.nasa.gov/earth-sun/technology/remote_sensing.html |date=2006-09-29 }} Retrieved on 2009-02-18.</ref>

== Radiation ==
[[Image:seasons.svg|frame|right|This is a diagram of the seasons.  In addition to the density of incident light, the [[dissipation]] of light in the [[Earth's atmosphere|atmosphere]] is greater when it falls at a shallow angle.]]
{{See also|Radiation|Effect of sun angle on climate}}
Atmospheric physicists typically divide radiation into solar radiation (emitted by the sun) and terrestrial radiation (emitted by Earth's surface and atmosphere).

Solar radiation contains variety of wavelengths. Visible light has wavelengths between 0.4 and 0.7&nbsp;micrometers.<ref>Atmospheric Science Data Center. [http://science-edu.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html What Wavelength Goes With a Color?] {{webarchive|url=https://web.archive.org/web/20110720105431/http://science-edu.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html |date=2011-07-20 }} Retrieved on 2008-04-15.</ref> Shorter wavelengths are known as the [[ultraviolet]] (UV) part of the spectrum, while longer wavelengths are grouped into the [[infrared]] portion of the spectrum.<ref>Windows to the Universe. [http://www.windows.ucar.edu/tour/link=/earth/Atmosphere/earth_atmosph_radiation_budget.html Solar Energy in Earth's Atmosphere.] {{Webarchive|url=https://web.archive.org/web/20100131001838/http://www.windows.ucar.edu/tour/link=/earth/Atmosphere/earth_atmosph_radiation_budget.html |date=2010-01-31 }} Retrieved on 2008-04-15.</ref> Ozone is most effective in absorbing radiation around 0.25&nbsp;micrometers,<ref name="UDE">[[University of Delaware]]. [http://www.udel.edu/Geography/DeLiberty/Geog474/geog474_energy_interact.html Geog 474: Energy Interactions with the Atmosphere and at the Surface.] Retrieved on 2008-04-15.</ref> where UV-c rays lie in the spectrum. This increases the temperature of the nearby [[stratosphere]]. Snow reflects 88% of UV rays,<ref name="UDE"/> while sand reflects 12%, and water reflects only 4% of incoming UV radiation.<ref name="UDE"/> The more glancing the angle is between the atmosphere and the [[sun]]'s rays, the more likely that energy will be reflected or absorbed by the [[atmosphere]].<ref>Wheeling Jesuit University. [http://www.cet.edu/ete/modules/ozone/ozatmo.html Exploring the Environment:  UV Menace.] {{webarchive |url=https://web.archive.org/web/20070830094306/http://www.cet.edu/ete/modules/ozone/ozatmo.html |date=August 30, 2007 }} Retrieved on 2007-06-01.</ref>

Terrestrial radiation is emitted at much longer wavelengths than solar radiation. This is because Earth is much colder than the sun.   Radiation is emitted by Earth across a range of wavelengths, as formalized in [[Planck's law]]. The wavelength of maximum energy is around 10 micrometers.

== Cloud physics ==
{{Main|Cloud physics}}
Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of [[cloud]]s. Clouds are composed of microscopic droplets of water (warm clouds), tiny crystals of ice, or both (mixed phase clouds). Under suitable conditions, the droplets combine to form [[precipitation (meteorology)|precipitation]], where they may fall to the earth.<ref>Oklahoma Weather Modification Demonstration Program. [http://www.evac.ou.edu/okwmdp/physics.html CLOUD PHYSICS.] {{webarchive|url=https://web.archive.org/web/20080723154301/http://www.evac.ou.edu/okwmdp/physics.html |date=2008-07-23 }} Retrieved on 2008-04-15.</ref> The precise mechanics of how a cloud forms and grows is not completely understood, but scientists have developed theories explaining the structure of clouds by studying the microphysics of individual droplets. Advances in radar and satellite technology have also allowed the precise study of clouds on a large scale.

== Atmospheric electricity ==
[[Image:Lightning over Oradea Romania 3.jpg|thumb|Cloud-to-ground [[lightning]] in the global atmospheric electrical circuit]]
{{Main|Atmospheric electricity}}
Atmospheric electricity is the term given to the electrostatics and electrodynamics of the atmosphere (or, more broadly, the atmosphere of any [[planet]]). The [[Continent|Earth's surface]], the [[ionosphere]], and the atmosphere is known as the [[global atmospheric electrical circuit]].<ref>Dr. Hugh J. Christian and Melanie A. McCook. [http://thunder.msfc.nasa.gov/primer/primer3.html Lightning Detection From Space: A Lightning Primer.] {{webarchive |url=https://web.archive.org/web/20080430142456/http://thunder.msfc.nasa.gov/primer/primer3.html |date=April 30, 2008 }} Retrieved on 2008-04-17.</ref> Lightning discharges 30,000 [[ampere]]s, at up to 100 million [[volt]]s, and emits light, radio waves, [[X-ray]]s and even [[gamma ray]]s.<ref>NASA.  [http://www.nasa.gov/vision/universe/solarsystem/rhessi_tgf.html Flashes in the Sky: Earth's Gamma-Ray Bursts Triggered by Lightning.] Retrieved on 2007-06-01.</ref> Plasma temperatures in lightning can approach 28,000 [[kelvin]]s and [[electron]] densities may exceed 10<sup>24</sup>/m<sup>3</sup>.<ref>Fusion Energy Education.[http://fusedweb.llnl.gov/CPEP/Chart_Pages/5.Plasmas/Lightning.html Lightning! Sound and Fury.] {{Webarchive|url=https://web.archive.org/web/20161123043211/http://fusedweb.llnl.gov/CPEP/Chart_Pages/5.Plasmas/Lightning.html |date=2016-11-23 }} Retrieved on 2008-04-17.</ref>

== Atmospheric tide ==
{{Main|Atmospheric tide}}
The largest-amplitude atmospheric tides are mostly generated in the [[troposphere]] and [[stratosphere]] when the atmosphere is periodically heated as water vapour and ozone absorb solar radiation during the day. The tides generated are then able to propagate away from these source regions and ascend into the [[mesosphere]] and [[thermosphere]]. Atmospheric tides can be measured as regular fluctuations in wind, temperature, density and pressure. Although atmospheric tides share much in common with ocean tides they have two key distinguishing features:

i) Atmospheric tides are primarily excited by the Sun's heating of the atmosphere whereas ocean tides are primarily excited by the Moon's gravitational field. This means that most atmospheric tides have periods of oscillation related to the 24-hour length of the solar day whereas ocean tides have longer periods of oscillation related to the lunar day (time between successive lunar transits) of about 24 hours 51 minutes.<ref>Glossary of Meteorology. [http://amsglossary.allenpress.com/glossary/search?id=atmospheric-tide1 Atmospheric Tide.] Retrieved on 2008-04-15.</ref>

ii) Atmospheric tides propagate in an atmosphere where density varies significantly with height. A consequence of this is that their amplitudes naturally increase exponentially as the tide ascends into progressively more rarefied regions of the atmosphere (for an explanation of this phenomenon, see below). In contrast, the density of the oceans varies only slightly with depth and so there the tides do not necessarily vary in amplitude with depth.

Note that although solar heating is responsible for the largest-amplitude atmospheric tides, the gravitational fields of the Sun and Moon also raise tides in the atmosphere, with the lunar gravitational atmospheric tidal effect being significantly greater than its solar counterpart.<ref>Scientific American. [http://www.sciam.com/article.cfm?id=does-the-moon-have-a-tida Does the Moon have a tidal effect on the atmosphere as well as the oceans?.] Retrieved on 2008-07-08.</ref>

At ground level, atmospheric tides can be detected as regular but small oscillations in surface pressure with periods of 24 and 12 hours.  Daily pressure maxima occur at 10 a.m. and 10 p.m. local time, while minima occur at 4 a.m. and 4 p.m. local time.  The absolute maximum occurs at 10 a.m. while the absolute minimum occurs at 4 p.m.<ref>Dr James B. Calvert. [http://mysite.du.edu/~jcalvert/geol/tides.htm Tidal Observations.] Retrieved on 2008-04-15.</ref> However, at greater heights the amplitudes of the tides can become very large. In the [[mesosphere]] (heights of ~ 50 – 100&nbsp;km) atmospheric tides can reach amplitudes of more than 50&nbsp;m/s and are often the most significant part of the motion of the atmosphere.

== Aeronomy ==
{{Main|Aeronomy}}
[[Image:Upperatmoslight1.jpg|250px|thumb|right|Representation of upper-atmospheric lightning and electrical-discharge phenomena]]
Aeronomy is the science of the upper region of the atmosphere, where dissociation and ionization are important. The term aeronomy was introduced by Sydney Chapman in 1960.<ref>Andrew F. Nagy, p. 1-2 in ''Comparative Aeronomy'', ed. by Andrew F. Nagy ''et al.'' (Springer 2008, {{ISBN|978-0-387-87824-9}})</ref> Today, the term also includes the science of the corresponding regions of the atmospheres of other planets.  Research in aeronomy requires access to balloons, satellites, and [[sounding rockets]] which provide valuable data about this region of the atmosphere.  [[Atmospheric tide]]s play an important role in interacting with both the lower and upper atmosphere.  Amongst the phenomena studied are [[upper-atmospheric lightning]] discharges, such as luminous events called red [[sprites (lightning)|sprites]], sprite halos, blue jets, and elves.

== Centers of research ==
In the UK, atmospheric studies are underpinned by the [[Met Office]], the [[Natural Environment Research Council]] and the [[Science and Technology Facilities Council]]. Divisions of the U.S. [[NOAA|National Oceanic and Atmospheric Administration (NOAA)]] oversee research projects and [[weather]] modeling involving atmospheric physics. The US [[Arecibo Observatory|National Astronomy and Ionosphere Center]] also carries out studies of the high atmosphere. In [[Belgium]], the [[Belgian Institute for Space Aeronomy]] studies the atmosphere and [[outer space]]. In France, there are several public or private entities researching the atmosphere, as an example météo-France ([[Météo-France]]), several laboratories in the national scientific research center (such as the laboratories in the [[Institut Pierre Simon Laplace|IPSL]] group). 

== See also ==
<!-- kinematic and dynamical meteorology (distinct) require articles -->
{{col-start}}
{{col-break}}
*[[Adiabatic lapse rate]]
*[[Atmospheric thermodynamics]]
*[[Baroclinic instability]]
*[[Barotropic vorticity equation]]
*[[Convective instability]]
{{col-break}}
*[[Coriolis effect]]
*[[Euler equations]]
*[[Exometeorology]]
*[[FluxNet]]
*[[Geostrophic wind]]
*[[Gravity wave]]
*[[Hydrostatic balance]]
{{col-break}}
*[[Kelvin–Helmholtz instability]]
*[[Madden–Julian oscillation]]
*[[Navier–Stokes equations]]
*[[Potential vorticity]]
*[[Pressure-gradient force]]
*[[Primitive equations]]
{{col-break}}
*[[Rossby number]]
*[[Rossby radius of deformation]]
*[[Space weather]]
*[[Space physics]]
*[[Thermal wind]]
*[[Vorticity equation]]
{{col-end}}

== References ==
{{Reflist}}

==Further reading==
* J. V. Iribarne, H. R. Cho, ''Atmospheric Physics'', D. Reidel Publishing Company, 1980.

==External links==
*{{Commons category-inline}}

{{Physics-footer}}
{{Atmospheric electricity}}
{{Authority control}}

[[Category:Atmospheric physics| ]]
[[Category:Branches of meteorology]]
[[Category:Fluid dynamics]]
[[Category:Applied and interdisciplinary physics]]