718 AIR MASS ANALYSIS 
a high or a low temperature, but only 
as to the evidence of a temperature 
near the ground higher or lower than 
that of the surface beneath. This 
warm and cold distinction is not al- 
ways easy to make, as the passage of 
the air mass from ocean to continent 
or the transition from day to night 
may reverse the sign of the difference 
of the air temperature from that »f 
the surface beneath. In the present 
discussion the policy will be to consi- 
der only the general tendency in the 
change of properties from one day to 
the next in the history of the air mass 
when determining the warm or cold 
designation. Continued or increasing 
surface stability from day to day in- 
dicates a warm air mass (W), con- 
tinued or increasing instability from 
day to day a cold air mass (K). This 
thermodynamic classification of air 
masses into warm and cold groups is 
essentially differential in nature, de- 
pending as it does upon changes pro- 
duced in the air mass properties by 
boundary surface-temperature differ- 
ences. In contrast to the significance 
of the source classification which de- 
pends upon the conservatism of cer- 
tain of the air mass properties, the 
significance of the W and K classifi- 
eation lies in the modification of the 
non-conservative air mass properties. 
There are a number of conditions 
which are more or less frequently met 
with in the synoptic study of air 
masses which may locally or tempo- 
rarily render difficult the proper class- 
ification of an air mass. In particular 
the disturbing effect on the air mass 
properties of the surface over which 
the mass is moving and the consequent 
formation cf a ground layer with its 
own peculiar properties cannot be 
overlooked. When this influence is 
regular and continuous, it gradually 
affects the entire mass until it be- 
comes characteristic for the proper- 
ties of the mass. On just such influ- 
ences depend the thermodynamic 
“warm” and “cold” classification of air 
masses already mentioned. But mech- 
anical turbulence produced by surface 
irregularities, and the radiational ef- 
fects of a single night and insolational 
heating of a single day often produce 
ground layers with properties which 
may be neither permanent nor char- 
acteristic of the air mass, and which 
consequently must be allowed for in 
the discussion of the air mass proper- 
ties in particular cases. Especially 
troublesome are the large radiational- 
insolational effects at the surface in 
dry continental air masses during the 
warm season. In the central U. S. 
this diurnal surface temperature 
change may amount to more than 
20C°, may produce an afternoon un- 
stable layer more than 2 km thick, and 
may change the air mass from the 
warm to the cold type. Féhn and sub- 
sidence effects are also frequently to 
be noted, but they are usually defi- 
nitely characteristic of certain air 
masses under certain conditions, and 
belong as such to the characteristic 
air mass properties, not being to any 
great extent diurnally variable. 
Ill. SIGNIFICANCE OF THE PROPERTIES OF THE PRINCIPAL 
AIR MASS TYPES IN WINTER 
It is impossible in a short review 
to attempt a full summary of the 
seasonal properties of all of the air 
masses listed in Table II. Conse- 
auently, this discussion will emphasize 
the air mass types, Pc, Pp, and To, 
which are dominant in determining 
our winter weather in the United 
States. It is during the winter season 
that the air mass contrasts become 
most significant. The following dis- 
cussion will illustrate the applicability 
