The dry adiabatic lapse rate (DALR) is the negative of the rate at which a rising parcel of dry or unsaturated air changes temperature with increasing height, under adiabatic conditions. Unsaturated air has less than 100% relative humidity, i.e. its temperature is higher than its dew point. The term adiabatic means that no heat transfer (energy transfer due to a temperature difference) occurs into or out of the parcel. Air has low thermal conductivity, and the bodies of air involved are very large, so transfer of heat by conduction is negligibly small.
Under these conditions, when the air is stirred (for instance, by convection), and a parcel of air rises, it expands, because the pressure is lower at higher altitudes. As the air parcel expands, it pushes on the air around it, doing work; since the parcel does work and gains no heat, it loses internal energy, and so its temperature decreases. (The reverse occurs for a sinking parcel of air.)
For an ideal gas, the equation relating temperature T and pressure p for an adiabatic process is
As unsaturated air rises, its temperature drops at the dry adiabatic rate. The dew point also drops, but much more slowly, typically about - 2 °C per 1000 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) absent mechanical lift, 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 8 °C per 1000 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 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 over many land masses. In these conditions, the likelihood of cumulus clouds, showers or even thunderstorms is increased.
Meteorologists use radiosondes 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 being Skew-T log-P diagrams and tephigrams. (See also Thermals).
where is the adiabatic lapse rate given in units of temperature divided by units of altitude, T = temperature, and z = altitude.
Note: In some cases, or can be used to represent the adiabatic lapse rate in order to avoid confusion with other terms symbolized by , such as the specific heat ratio or the psychrometric constant.
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