Lapse
rates are progression at which air temperature changes with
increasing / decreasing height in the atmosphere. The rate is considered positive when
temperature decreases with elevation, zero when temperature is constant with
elevation, and negative when temperature is increasing with elevation. While there are two different class of lapse
rates, normal and adiabatic. The difference between normal and adiabatic lapse rates
determine the vertical stability, or instability, of the atmosphere. That is, an
air parcel’s tendency to embrace or prohibit vertical motion.
Environmental Lapse Rate—non-rising air that is affected by radiation,
convection, and/or condensation. It averages about 6.5°C/km.
Dry Adiabatic Lapse Rate—rate of cooling with increasing
altitude. It is constant at about 10°C/km.
Moist Adiabatic Lapse Rate—air, saturated with water vapor, is not
constant but is determined by the combined effects of expansion cooling and
latent heating (LH) because saturated air cools slower than dry due to the
heating produced by condensing water vapor. Is always less than the dry adiabatic lapse
rate.
Due to the fact that density
differences are affected by the differences between the adiabatic lapse rates
and the environmental lapse rate, one may notice that absolute instability occurs when the environmental lapse rate (ГE) exceeds the dry adiabatic
lapse rate (ГD) [i.e. ГE > ГD]. Whereas, absolute stability occurs when the
environmental lapse rate (ГE) is less than the wet adiabatic lapse rate (ГW) [i.e. ГE < ГW]. However, when the environmental lapse
rate (ГE) falls between the wet adiabatic lapse rate (ГW) and the dry
adiabatic lapse rate (ГD) [i.e. ГW < ГE < ГD] the atmosphere is considered conditionally unstable, as you can see from the picture below.
