6 ATR MASS ANALYSIS 
II. CONSERVATIVE PROPERTIES OF AIR MASSES 
An air mass is defined as an exten- 
sive body of air which approximates 
horizontal homogeneity. The proper- 
ties of the air mass which are con- 
sidered in this homogeneity are mainly 
temperature and moisture. Thus over 
the earth’s surface one may observe 
large currents of air within which the 
temperature and moisture content re- 
main fairly constant at any given 
level. These large scale currents of 
air have their origin at a source re- 
gion—a large area characterized by 
sameness of surface conditions and 
evenly distributed insolation. Thus the 
northern part of Canada in winter 
may be considered as a source region 
in that it is practically snow-covered, 
and the amount of insolation received 
is almost evenly distributed over the 
entire area. A large body of air 
which remains over a source region a 
sufficient length of time assumes cer- 
tain definite properties in the vertical, 
particularly with respect to tempera- 
ture and moisture. Once these prop- 
erties have been attained, equilibrium 
with respect to the source region is 
reached, and any further stagnation 
or movement of the body of air over 
the source region will not appreciably 
affect the balanced distribution.* It is 
clear, however, that any movement of 
the air mass away from its source 
region will result in a modification of 
its properties. For example, if a body 
of air from northern Canada moves 
southeastward into the United States, 
there is bound to be some warming 
and moistening. Such modifications 
tend to destroy the homogeneity orig- 
inally established at the source re- 
gion. It is obvious that the modifica- 
tion will take place essentially within 
the lowest layers of the air mass, the 
upper layers being modified only grad- 
ually by means of the indirect pro- 
eesses of mixing with the modified 
low layers and by radiation chiefly 
from the surface of the new region 
over which the air mass is traveling. 
The theory of air masses as entities 
is based upon the fact that the varia- 
tions of any property in the horizon- 
tal in an air mass are small compared 
with the rapid change of properties 
observed at the boundary between two 
air masses which come from different 
source regions. This boundary zone 
of rapid transition is a front. 
From the definition of an air mass 
it is clear that in order to identify 
sections of a current as belonging to 
one and the same air mass we must 
not only know the properties at the 
source region and the modifications in- 
troduced, but we must also deal with 
observations which are most repre- 
sentative of these properties. The 
most representative observations are 
those made by means of upper air 
soundings; at the surface the most 
representative observations are those 
made at elevated and exposed sta- 
tions. Examples of non-representa- 
tive observations are those of valley 
stations, or those greatly affected by 
the proximity of a lake or perhaps a 
mountain barrier. In the latter case 
there might be an appreciable foehn 
effect. 
As an air mass progresses numer- 
ous changes take place, brought about 
by radiation, mixing (turbulent ex- 
change), adiabatic expansion or com- 
pression, and condensation or evapor- 
ation. Some meteorological elements 
will remain more constant than others 
as the air mass moves from point to 
*The processes by which air masses reach 
homogeneous equilibrium in their source re- 
gions are not yet well understood; further dis- 
cussion of this appears in the Appendix by 
Prof. Willett on the air mass properties; a 
recent study by Wexler (Mo. Wea. Rev., April, 
1936) discusses the origin of Polar conti- 
mental air, and J.-J. Jang has studied the 
formation of tropical marine air in the trade 
winds (see Bibliog.).—Ed. 
