CLASSIFICATION OF AIR MASSES 15 
lie almost entirely at high latitudes 
or at low latitudes. In middle lati- 
tudes, generally speaking, we find the 
zone of greatest atmospheric circula- 
tion or of most intense interaction 
between the warm and cold currents, 
i.e., air masses from the tropical and 
polar regions. Consequently, in 
middle latitudes the uniformity of 
conditions and the light air move- 
ment which must characterize a 
source region are generally lacking. 
Rather than the development of hori- 
zontally homogeneous air masses, we 
find here the rapid modification, in 
varied forms, with changing environ- 
ment, of the characteristic polar and 
tropical air mass types. Thus the 
basis of any comprehensive air mass 
classification must be the distinction 
between the polar and the tropical 
source types, with a further distinc- 
tion between the modified forms 
which these principal types acquire 
in middle latitudes during their later 
life history. 
The air mass classification may be 
earried further by the sub-division of 
the polar and the tropical source 
types into continental and maritime 
groups, according as the source in 
each case is a continental or an 
oceanic region. Since the uniform 
source regions are always entirely 
continental or entirely maritime and 
since this is the essential difference 
between source regions in the same 
latitude, this distinction furnishes 
a satisfactory basis for a general 
grouping of the air masses from each 
latitudinal zone. Consequently, for 
the investigation of the properties of 
the air masses which may appear in 
a given locality, the most significant 
designation of the different individual 
polar and tropical air mass types is 
that based on the particular geo- 
graphical area within which the air 
mass has its source. It is, of course, 
necessary to keep in mind the season 
of the year when considering the 
characteristics of any particular geo- 
graphical source region, as these char- 
acteristics, especially in the case of 
the continental areas, change greatly 
from the cold to the warm season. 
It is also necessary to consider the 
modifications of the original source 
properties of the air mass types, 
effects which become more _ pro- 
nounced with the increasing move- 
ment of the air mass from the source 
region. Eventually the properties of 
the air mass become so fundamentally 
modified from the source properties 
that the mass must be given a special 
transitional designation. 
Table II gives the complete classi- 
fication of the principal North Ameri- 
can air masses by geographical source 
regions, together with the principal 
transitional form for each air mass. 
The ordinary designation and the 
symbol entered for each air mass in 
the last two columns are those which 
appear on the Massachusetts Institute 
of Technology weather maps. 
It is with the purpose of making 
this discussion intelligible to meteo- 
rologists who are not familiar with 
our local classification, and of making 
the American air mass data more di- 
rectly comparable with the European 
data, that the general air mass classi- 
fication outlined by Bergeron is also 
introduced in this paper. 
In the general air mass classifica- 
tion which Bergeron*® has suggested 
for climatolegical and comparative 
purposes, and which, at least in its 
broader features, Moese*® and Schinze* 
2T. Bergeron: Rechtlinien einer dynam- 
ischen Klimatologie, Met. Zeit, 1930, pp. 
246-62. 
30. Moese und G. Schinge: Zur Analyse von 
Neubildungen, Ann. d. Hyd., March, 1929, pp. 
ee Schinze: Troposphdrischen Luftmassen 
und vertikaler Temperaturgradient, Beitr. z. 
Phys. d. fr. Atmos., Bd. 19, 1932, pp. 79-90; 
see also Met. Zeit., May 1932, pp. 169-79. 
