32 AIR MASS ANALYSIS 
of these disturbances, which are cailed 
extratropical cyclones, is exceedingly 
complicated and is as yet unsolved. 
To account for these depressions of 
the mid-latitudes the Norwegian 
school of meteorologists, under the 
leadership of Professors V. and J. 
Bjerknes, have formulated what is 
now generally known as the Polar 
Front Theory. Since the inception of 
this theory (during the World War) 
secondary modifications have been 
added by the Norwegians themsecives 
and by meteorologists of other coun- 
tries. In fact, at present data are 
being collected and studied which, 
within the next few years, may lead 
to extremely important modifications 
of the fundamental ideas underlying 
the Polar Front Theory.* 
In the following article Dr. Haur- 
witz, an authority on the mathe- 
matical part of the Theory, interprets 
it for the general reader. But quite 
apart from their validity for a theory 
of the origin of cyclones (still not 
widely accepted), the basic concep- 
tions of the Norwegian school have 
been looked upon as extremely valu- 
able tools in the analysis of weather 
conditions. No impartial observer can 
doubt that the use of frontal ideas 
as a method of interpretation of me- 
teorological phenomena has served to 
objectify and clarify for the synoptic 
meteorologist the description of at- 
mospheric movements. Weather fore- 
casting should be considered as an 
attempt to give first a physical inter- 
pretation to what has taken place, 
*Prof. Rossby has recently seriously ques- 
tioned the Polar Front Theory in so far as it 
concerns the general circulation of the at- 
mosphere, the flow patterns in the upper air, 
and the significance of fronts and air masses. 
That many cyclones do form from waves on a 
Polar Front in the lower portion of the tro- 
posphere is not denied, however. See: Bull. 
Am. Met. Soc., 1937, pp. 201-209; Trans. 
Am. Geophys. Un., 
and On the Role of Isentropic Mixing in 
the General Circulation of the Atmosphere, 
Trans. 5th. Int. Congr. of Applied Mechanics, 
Cambridge, Mass., 1938, pp. 873-8.—Ed. 
1937, Pt. I, pp. 130-136; 
then, using this as a foundation, to 
extrapolate conditions and, what is 
of more importance, estimate the mo- 
difications which may occur. The 
method of fronts and air masses 
is particularly well adapted to both 
extrapolation and uetermination of 
modification. It thereby supplies a 
more quantitative basis for prognos- 
tication. 
The convergence of two air masses 
of different properties leads to a sur- 
face of discontinuity. There are, cn 
the earth’s surface, regions wkere 
large-scale air currents of appreciasly 
different properties generally con- 
verge. Bergeron has called these 
regions of frontogenesis. Regions of 
divergence, where fronts are not 
readily formed, and where they are 
generally destroyed, are called re- 
gions of frontolysis. For example, the 
region south of the Aleutian Islands 
may be considered as a_ breeding 
ground of fronts, because the Aleu- 
tian Low, a vast center of action 
formed and maintained chiefly by 
thermal differences between the rela- 
tively warm waters of the North 
Pacific and the cold snow-covered 
areas of Alaska and Siberia, serves 
to draw cold Polar Continental air 
into its western side and warm trop- 
ical air to its eastern side. The 
juxtaposition of these two air masses 
results in the Polar Front. 
If the front formed in the Aleutian 
region always remained stationary 
under the equilibrium outlined in 
Article V, it would have no dynamic 
significance. That is to say, it would 
simply represent the boundary zone 
between the cold polar and the warm 
tropical air, and, since there is a 
balance of forces, there could be no 
vertical motions—hence no precifi- 
tation. It may be shown, however, 
that this equilibrium cannot exist, 
for there must be an exchange of 
