CLASSIFICATION OF AIR MASSES 17 
follow in their discussions of Euro- 
pean air masses, the essential dis- 
tinction is still that between the tropi- 
cal or the polar source of each air 
mass. However, Bergeron carries this 
zonal distinction one step further, dis- 
tinguishing between real Arctic (A) 
and sub-Arctic, or Polar (P) air mass 
sources in the north, and between 
sub-Tropical (T) and real Equatorial 
(FE) or Trade wind zone air mass 
sources in the south. Bergeron points 
out, however, that in northwestern 
Europe the Equatorial air masses 
play a negligible réle, appearing only 
at high levels in the atmosphere, if at 
all. In the case of the North Ameri- 
can air masses the distinction between 
Polar and Arctic air masses and that 
between Tropical and Equatorial air 
masses are both difficult to make and 
of little significance. 
The principal source air masses in 
Bergeron’s classification are the conti- 
nental Arctic (cA), maritime Arctic 
(mA), continental Polar (cP), and so 
on through the MP, cT, MT, cE and 
ME groups.* Of course such a designa- 
tion of air masses, while indicating 
very definitely the type of each air 
mass, is of necessity less precise in 
the information it gives to one tho- 
roughly familiar with the particular 
sources in question than is the local 
classification by direct specification of 
the source of each individual air mass. 
Air masses of Tropical and Polar 
origin are modified during their later 
history in either of two essentially dif- 
ferent ways. If the air mass moves 
over a surface warmer than its own 
temperature at the ground, the ten- 
dency is then towards a warming of 
the lower strata of the air mass, 1.e., 
an increasing thermal instability, and 
towards an increasing moisture con- 
tent of the lower strata of the air 
mass, caused by evaporation from the 
warm surface. If, on the other hand, 
the air mass moves over a surface 
colder than its own temperature at 
the ground, the tendency is towards a 
cooling of the lower strata of the air 
mass, i.e., an increasing thermal sta- 
bility, and towards a decreasing mois- 
ture content of the mass, caused by 
condensation from the cooled air 
strata. Evidently Polar air masses 
must normally undergo the first type 
of change when modified after leaving 
the source region, and Tropical air 
masses the second type. However, 
there may be exceptions in both cases, 
and any air mass may for a time be 
subjected first to the one and after- 
wards to the other type of influence. 
In Bergeron’s general air mass 
classification, modification of the 
source properties of the air mass, 
which in the local classification is in- 
dicated by the N (transitional) 
group, is indicated by a W (warm) or 
a K (cold, kalt) distinction according 
as the recent modification of the air 
mass has been of the second or the 
first type mentioned above. The warm 
(W) designation indicates that the 
air mass is warm relative to the sur- 
face it is moving over, the cold (K) 
designation indicates that it is cold 
relative to the surface it is moving 
over. Thus in the general classifica- 
tion of air masses the source designa- 
tions CP, MP, cT, and so forth, when 
applied to air masses which have left 
their source regions, appear in the 
modified forms CPW (continental Po- 
lar Warm), CPK (continental Polar 
Cold), MPW (maritime Polar Warm) 
and so forth, depending upon the type 
of modification which the air mass has 
undergone during its recent history. 
It should be stressed that this warm 
and cold designation has nothing to do 
with the evidence by the air mass of 
*This manner of designation was introduced 
into the practice of the U. S. Weather Bureau 
ein 1939; it has been in extensive use abroad 
and become understood internationally.—d. 
