Editing Lapse rate (section)

== Effect on weather ==
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[[File:Anvil shaped cumulus panorama edit crop.jpg|thumb|right|x150px|The latent heat of vaporization adds energy to clouds and storms.]]
The varying environmental lapse rates throughout the Earth's atmosphere are of critical importance in [[meteorology]], particularly within the [[troposphere]]. They are used to determine if the [[air parcel|parcel]] of rising air will rise high enough for its water to condense to form [[cloud]]s, and, having formed clouds, whether the air will continue to rise and form bigger shower clouds, and whether these clouds will get even bigger and form [[cumulonimbus cloud]]s (thunder clouds).

As unsaturated air rises, its temperature drops at the dry adiabatic rate.  The [[dew point]] also drops (as a result of decreasing air pressure) but much more slowly, typically about {{nowrap|2&nbsp;°C}} per 1,000&nbsp;m. If unsaturated air rises far enough, eventually its temperature will reach its [[dew point]], and condensation will begin to form. This altitude is known as the [[lifting condensation level]] (LCL) when mechanical lift is present and the [[convective condensation level]] (CCL) when mechanical lift is absent, in which case, the parcel must be heated from below to its [[convective temperature]]. The [[cloud base]] will be somewhere within the layer bounded by these parameters.

The difference between the dry adiabatic lapse rate and the rate at which the [[dew point]] drops is around {{nowrap|4.5&nbsp;°C}} per 1,000&nbsp;m. Given a difference in temperature and [[dew point]] readings on the ground, one can easily find the LCL by multiplying the difference by 125 m/°C.

If the environmental lapse rate is less than the moist adiabatic lapse rate, the air is absolutely stable — rising air will cool faster than the surrounding air and lose [[buoyancy]]. This often happens in the early morning, when the air near the ground has cooled overnight. Cloud formation in stable air is unlikely.

If the environmental lapse rate is between the moist and dry adiabatic lapse rates, the air is conditionally unstable — an unsaturated parcel of air does not have sufficient buoyancy to rise to the LCL or CCL, and it is stable to weak vertical displacements in either direction. If the parcel is saturated it is unstable and will rise to the LCL or CCL, and either be halted due to an [[Inversion (meteorology)|inversion layer]] of [[convective inhibition]], or if lifting continues, deep, moist convection (DMC) may ensue, as a parcel rises to the [[level of free convection]] (LFC), after which it enters the [[free convective layer]] (FCL) and usually rises to the [[equilibrium level]] (EL).

If the environmental lapse rate is larger than the dry adiabatic lapse rate, it has a superadiabatic lapse rate, the air is absolutely unstable — a parcel of air will gain buoyancy as it rises both below and above the lifting condensation level or convective condensation level. This often happens in the afternoon mainly over land masses. In these conditions, the likelihood of [[cumulus cloud]]s, showers or even [[thunderstorm]]s is increased.

Meteorologists use [[radiosonde]]s to measure the environmental lapse rate and compare it to the predicted adiabatic lapse rate to forecast the likelihood that air will rise. Charts of the environmental lapse rate are known as [[thermodynamic diagrams]], examples of which include [[Skew-T log-P diagram]]s and [[tephigram]]s. (See also [[Thermals]]).

The difference in moist adiabatic lapse rate and the dry rate is the cause of [[foehn wind]] phenomenon (also known as "[[Chinook wind]]s" in parts of North America). The phenomenon exists because warm moist air rises through [[orographic lifting]] up and over the top of a mountain range or large mountain. The temperature decreases with the dry adiabatic lapse rate, until it hits the dew point, where water vapor in the air begins to condense. Above that altitude, the adiabatic lapse rate decreases to the moist adiabatic lapse rate as the air continues to rise. Condensation is also commonly followed by [[precipitation (meteorology)|precipitation]] on the top and [[windward]] sides of the mountain. As the air descends on the leeward side, it is warmed by [[adiabatic compression]] at the dry adiabatic lapse rate. Thus, the foehn wind at a certain altitude is warmer than the corresponding altitude on the windward side of the mountain range. In addition, because the air has lost much of its original water vapor content, the descending air creates an [[arid]] region on the leeward side of the mountain.<ref name="Whiteman">{{cite book|last=Whiteman|first= C. David|title=Mountain Meteorology: Fundamentals and Applications |publisher=Oxford University Press|year=2000|isbn=978-0-19-513271-7}}</ref>