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Adiabatic process
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==Adiabatic compression and expansion== The adiabatic compression of a gas causes a rise in temperature of the gas. Adiabatic expansion against pressure, or a spring, causes a drop in temperature. In contrast, [[free expansion]] is an [[isothermal]] process for an ideal gas. '''Adiabatic compression''' occurs when the pressure of a gas is increased by work done on it by its surroundings, e.g., a [[piston]] compressing a gas contained within a cylinder and raising the temperature where in many practical situations heat conduction through walls can be slow compared with the compression time. This finds practical application in [[diesel engines]] which rely on the lack of heat dissipation during the compression stroke to elevate the fuel vapor temperature sufficiently to ignite it. Adiabatic compression occurs in the [[Earth's atmosphere]] when an [[air mass]] descends, for example, in a [[Katabatic wind]], [[Foehn wind]], or [[Chinook wind]] flowing downhill over a mountain range. When a parcel of air descends, the pressure on the parcel increases. Because of this increase in pressure, the parcel's volume decreases and its temperature increases as work is done on the parcel of air, thus increasing its internal energy, which manifests itself by a rise in the temperature of that mass of air. The parcel of air can only slowly dissipate the energy by conduction or radiation (heat), and to a first approximation it can be considered adiabatically isolated and the process an adiabatic process. '''Adiabatic expansion''' occurs when the pressure on an adiabatically isolated system is decreased, allowing it to expand in size, thus causing it to do work on its surroundings. When the pressure applied on a parcel of gas is reduced, the gas in the parcel is allowed to expand; as the volume increases, the temperature falls as its internal energy decreases. Adiabatic expansion occurs in the Earth's atmosphere with [[orographic lifting]] and [[lee waves]], and this can form [[Pileus (meteorology)|pilei]] or [[lenticular cloud]]s. Due in part to adiabatic expansion in mountainous areas, snowfall infrequently occurs in some parts of the [[Sahara desert]].<ref>{{cite web |last1=Knight |first1=Jasper |title=Snowfall in the Sahara desert: an unusual weather phenomenon |url=https://theconversation.com/snowfall-in-the-sahara-desert-an-unusual-weather-phenomenon-176037 |website=The Conversation |access-date=3 March 2022 |date=31 January 2022}}</ref> Adiabatic expansion does not have to involve a fluid. One technique used to reach very low temperatures (thousandths and even millionths of a degree above absolute zero) is via [[adiabatic demagnetization|adiabatic demagnetisation]], where the change in [[magnetic field]] on a magnetic material is used to provide adiabatic expansion. Also, the contents of an [[expanding universe]] can be described (to first order) as an adiabatically expanding fluid. (See [[heat death of the universe]].) Rising magma also undergoes adiabatic expansion before eruption, particularly significant in the case of magmas that rise quickly from great depths such as [[kimberlite]]s.<ref name="Kavanagh">{{cite journal|last1=Kavanagh|first1=J. L.|last2=Sparks |first2=R. S. J.|year=2009|title=Temperature changes in ascending kimberlite magmas|journal=Earth and Planetary Science Letters|publisher=[[Elsevier]]|volume=286|issue=3β4|pages=404β413|doi=10.1016/j.epsl.2009.07.011|url=https://monash.academia.edu/JanineKavanagh/Papers/114092/Temperature_changes_in_ascending_kimberlite_magma|access-date=18 February 2012|bibcode = 2009E&PSL.286..404K }}</ref> In the Earth's convecting mantle (the asthenosphere) beneath the [[lithosphere]], the mantle temperature is approximately an adiabat. The slight decrease in temperature with shallowing depth is due to the decrease in pressure the shallower the material is in the Earth.<ref>{{Cite book|title=Geodynamics|url=https://archive.org/details/geodynamics00dltu|url-access=limited|last=Turcotte and Schubert|publisher=Cambridge University Press|year=2002|isbn=0-521-66624-4|location=Cambridge|pages=[https://archive.org/details/geodynamics00dltu/page/n199 185]}}</ref> Such temperature changes can be quantified using the [[ideal gas law]], or the [[hydrostatic equation]] for atmospheric processes. In practice, no process is truly adiabatic. Many processes rely on a large difference in time scales of the process of interest and the rate of heat dissipation across a system boundary, and thus are approximated by using an adiabatic assumption. There is always some heat loss, as no perfect insulators exist.
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