8 AIR MASS ANALYSIS 
a 
sample of air compressed to a pres- 
sure of 1000 mb. The equivalent-po- 
tential temperature may also be de- 
fined as the potential temperature of 
the equivalent temperature; that is, it 
can be determined by finding the 
equivalent temperature, then reduc- 
ing this adiabatically to a pressure of 
1000 mb. The equivalent-potential 
temperature combines the processes 
involved in the definition of the po- 
tential and the equivalent tempera- 
ture; hence it is independent of any 
effects due to expansion or compres- 
sion as well as condensation*. If 
we deal with the equivalent-potential 
temperature of a particle then the 
only processes which change its value 
are (1) conduction and mixing, (2) 
evaporation,* and (4) insolation and 
radiation. (1) and (4) obviously 
have their maximum effect in the sur- 
face layers, and are probably much 
less important at higher levels. There- 
fore the thermal properties of an air 
mass are far more conservative at 
upper levels than in the low layers. 
Consequently it is best to use upper 
air data rather than surface observa- 
tions as criteria for the determination 
and identification of air masses. 
B. LAPSE RATE. 
The lapse rate associated with an 
air mass is frequently a good index 
for identification purposes. As in the 
case of thermal quantities, the lapse 
rate is much more conservative at 
upper levels. In the surface layers 
the lapse rate will be found to vary 
appreciably from day to day, and 
from nighttime to daytime. These 
effects are mainly the result of radia- 
tion and turbulence. For example, in 
the early morning hours there is apt 
to be a ground inversion, while dur- 
ing the afternoon an adiabatic lapse 
rate may extend up to about 800 
meters. At higher levels surface ef- 
fects are comparatively small, but 
there are times when the lapse rate 
aloft changes because of extensive 
rising or sinking movements. In spite 
of variations in the lapse rate in the 
surface layers, the lapse rate is often 
indicative of the trajectory of the air 
current, for when moving over a cold 
surface the low layers of the current 
tend to become more stable, whereas 
when constantly moving over a warm- 
er surface the lapse rate becomes 
steeper. Characteristic types of 
clouds are commonly associated with 
certain lapse rates. 
C. HUMIDITY 
The use of aerological material is 
becoming increasingly important for 
the investigation of synoptic meteoro- 
logical problems, especially when air 
mass and frontal methods are applied. 
The correct identification of individ- 
ual air masses is greatly facilitated 
by the utilization of certain quanti- 
*The equivalent-potential temperature de- 
fined by Rossby is not perfectly conservative 
for an evaporating process, such as when fall- 
ing precipitation is evaporated into dry air 
or a fog is dissolved, though the effect does 
not change the equiv.-pot. temp. greatly. 
However, as Bleeker has pointed out (Q. Jn- 
Roy. Met. Soc., Oct. 1939), misleading con- 
clusions as to air-mass identity and move- 
ments can be made from the e.-p. temp. if 
evaporation takes place. The thermodynamic 
properties which are conservative for evapora- 
tion are not conservative for adiabatic changes 
and condensation; there is, in fact, no ele- 
ment which is conservative for all conceiv- 
able changes of the air. In Germany Rossby’s 
equivalent potential temperature is also known 
as the “pseudo-potentielle Temperatur”’ 
(Stiive). Another quantity differing from 
Rossby’s but sometimes referred to as the 
“equivalent potential temperature’? (Normand, 
1921) and ‘‘equi-potentielle Temperatur”’ (Ro- 
bitzsch, 1928), is conservative for evaporation 
but not for adiabatic processes and condensa- 
tion; it is defined as the temperature that 
would result by condensing out all the water 
vapor but at a constant pressure of 1000 
mb. This of course does not give the 
same result as when an air mass is raised 
adiabatically to lower temperatures (and low- 
er pressures), as in Rossby’s definition. 
Strictly speaking the Normand-Robitzsch 
quantity is an isobaric equivalent-potential 
temperature, whereas the Stiive-Rossby quan- 
tity is a dry-potential adiabatic equivalent 
temperature, and the corresponding equivalent 
temperatures should be distinguished likewise. 
Prof. Petterssen in his new book has intro- 
duced the terms pseudo-equivalent, and pseudo- 
wet-bulb potential temp., etc., for the adiab- 
atic quantities. Such terminology is too cum- 
bersome in practice but it is well to understand 
the differences, as the literature and practice 
are very loose and misleading about these con- 
cepts.—R. G. S 
