74 CLASSIFICATION OF AIR MASSES 
essentially on the identification and 
determination of the movement of air 
masses and fronts rather than of 
areas of high and low pressure as the 
entities of prime significance. The 
justification of this procedure is evi- 
dent from the fact that the weather 
which is experienced in a given region 
does not depend upon the prevalence 
of high or low barometric pressure. 
Even in the same locality a high or a 
low may be accompanied by widely 
varying meteorological conditions. 
The weather in a given locality de- 
pends upon the properties of the air 
mass which is present or upon the 
interaction which is taking place be- 
tween two air masses along a front 
in the near vicinity. The funda- 
mental concept is that of the air 
mass, for upon the properties and 
movements of the individual air 
masses appearing on the map depend 
not only the weather in the area 
covered by each air mass but also 
the formation and intensification of 
fronts and the genesis, development 
and movement of lows or disturb- 
ances on the fronts. But in order 
that the full advantage of a careful 
analysis of the weather map may be 
utilized in weather forecasting, it is 
absolutely necessary that the thermo- 
dynamic properties of the air masses 
(lapse-rate and vertical moisture dis- 
tribution) be approximately known. 
This is especially true of aviation 
forecasts, for just the meteorological 
elements which are of greatest in- 
terests to the pilot are those which 
are dependent upon the air mass 
properties. For good forecasting of 
convective turbulence, thunderstorms, 
horizontal visibility, fog and haze, 
cloud forms and ceiling, for just such 
forecasting full knowledge of the air 
mass properties is essential. Since it 
rarely happens that the forecaster 
has available for the current weather 
map aerological material sufficient for 
the satisfactory determination of the 
properties of the air masses present, 
an investigation of the characteristic 
properties of the typical American air 
masses should be of value both for 
practical weather forecasting and for 
a better understanding of the physi- 
cal processes underlying our usual 
weather sequences. It was with this 
thought in mind that the paper here 
under consideration was written. The 
discussion was definitely restricted to 
a consideration of the homogeneous 
air masses individually, the investiga- 
tion of the whole complicated frontal 
problem of converging air masses 
being left for later consideration in 
the light of more extensive American 
aerological data (see Bibliography). 
II. CLASSIFICATION OF AIR MASSES 
The study of synoptic weather 
maps indicates that air masses are 
entities having such definite charac- 
teristic properties that they may be 
classified and studied as_ distinct 
types. Since the characteristic prop- 
erties of an air mass at any point 
depend primarily upon the nature of 
its source region and _ secondarily 
upon the modifications of the source 
properties which the air mass has 
undergone en route to the point of 
observation, any classification of air 
mass types must be based funda- 
mentally on the air mass source 
regions, with perhaps a sub-classifica- 
tion based on later modifications of 
the source properties. 
The air mass sources fall naturally 
into two groups, the tropical or sub- 
tropical, and the polar or sub-polar. 
The large areas on the earth of uni- 
form surface conditions and compar- 
atively light atmospheric movement 
