CHARACTERISTIC AIR MASS PROPERTIES 
73 
Characteristic Properties of North American Air Masses’ 
H. C. WILLETT 
U. S. Weather Bureau, Boston, Mass. 
Associate Professor of Meteorology, Massachusetts Institute of Technology 
Lecturer in Meteorology, Harvard University 
I. INTRODUCTION 
In this discussion the term air mass 
is applied to an extensive portion of 
the earth’s atmosphere which approxi- 
mates horizental homogeneity. The 
formation of an air mass in this sense 
takes place on the earth’s surface 
wherever the atmosphere remains at 
rest over an extensive area of uni- 
form surface properties for a suffi- 
ciently long time so that the proper- 
ties of the atmosphere (vertical dis- 
tribution of temperature and moist- 
ure) reach approximate equilibrium 
with respect to the surface beneath. 
Such a region on the earth’s surface 
is referred to as a source region of 
air masses. As examples of source 
regions we might cite the uniformly 
snow and ice covered northern por- 
tion of the continent of North Amer- 
ica in winter, or the uniformly warm 
waters of the Gulf of Mexico and the 
Caribbean Sea. 
The concept of the air mass is of 
importance not in the source regions 
alone. Sooner or later a general 
movement of the air mass from the 
source region is certain to occur, as 
one of the large-scale air currents 
which we find continually moving 
across the synoptic charts. Because 
of the great extent of such currents 
and the conservatism of the air mass 
properties, it is usually easy to trace 
the movement of the air mass from 
day to day, while at the same time 
any modification of its properties by 
the new environment can be carefully 
noted. 
Since this modification is not likely 
to be uniform throughout the entire 
air mass, it may to a certain degree 
destroy the horizontal homogeneity 
of the mass. However, the horizontal 
differences produced within an air 
mass in this manner are small and 
continuous in comparison to the 
abrupt and discontinuous transition 
zones, or fronts, which mark the 
boundaries between different air 
masses. Frontal discontinuities are 
intensified wherever there is found in 
the atmosphere a convergent move- 
ment of air masses of different prop- 
erties. 
Since the air masses from partic- 
ular sources are found to possess, at 
any season, certain characteristic 
properties which undergo rather defi- 
nite modification, depending upon the 
trajectory of the air mass after leav- 
ing its source region, the investiga- 
tion of the characteristic properties 
of the principal air mass types can 
be of great assistance to the synoptic 
meteorologist and forecaster. We 
owe this modern analytical method of 
attack on the problems of synoptic 
meteorology and weather forecasting 
to the Norwegian School of Meteor- 
ologists, notably to J. Bjerknes and 
T. Bergeron. The analytical study 
of the synoptic weather map is based 
1An abstract and revision of American Air 
Mass Properties, Papers in Physical Oceano- 
graphy and Meteorology, published by Massa- 
chusetts Institute of Technology and Woods 
Hole Oceanographic Institution, Vol. 2, No. 2, 
1933 (now out of print). A part appeared in 
the Jn. Aeronaut. Sci., Vol. 1, No. 2, April, 
1934, pp. 78-87. The revisions herein concern 
chiefly the Tropical air masses; but it should 
be understood that this discussion of the 
specific properties of the American air masses 
was a preliminary and pioneer work and the 
results have required some modification as fur- 
ther accumulations of aerological data and ex- 
perience have been studied. A later study 
by Showalter is abstracted at the end of this 
article (pp. 109-113). The Bibliography lists 
many later papers which should be consulted 
by the serious student, as great advances are 
being made yearly.—Ed. 
