LAPSE RATES =} 
Poisson’s equation and the hydro- 
static equation (expressing the rela- 
tion between pressure, density, and 
height), it is possible to obtain the 
rate of cooling of a rising air parti- 
cle owing to its change in elevation. 
The result is the convenient rate of 
1 C deg. per 100 m. This rate is not 
strictly constant; it depends upon 
the amount of moisture within the 
unsaturated air particle as well as 
the temperature of the surrounding 
air through which it is displaced. 
However, these effects are relatively 
small and tend to counteract each 
other. Hence, for all practical pur- 
poses, they may be neglected, the adi- 
abatic rate of change of temperature 
being taken as 1 C deg. per 100 m. 
change in elevation. This is com- 
monly known as the dry adiabatic 
ALTITUDE km 
- ter condensed. 
lapse rate, or dry adiabat. (Lapse 
rate is defined as the rate of change 
in temperature with respect to height. 
Unless preceded by the qualifying 
term “adiabatic,” lapse rate refers to 
the existing difference of temperature 
per unit of height within a selected 
layer of the atmosphere.) 
Thus far -we have considered the 
vertical displacement of an unsatur- 
ated particle of air. Once the parti- 
cle becomes saturated the latent heat 
of condensation must be taken into 
account, for it supplies heat to the 
rising mass and therefore lessens the 
rate of cooling due to expansion. The 
lessening of the adiabatic cooling ef- 
fect depends upon the liberated heat 
of condensation, which in turn de- 
pends upon the amount of liquid wa- 
But as the particle 
to) 
TEMPERATURE C 
Gale 
TYPES OF LAPSE RATES 
