CONSERVATIVE PROPERTIES OF AIR MASSES 9 
ties which remain about constant as 
the air mass moves from point to 
point. Chief among such conserva- 
tive quantities is the specific humid- 
ity—a term which is seldom defined 
in the elementary American text- 
books on meteorology, and which, 
prior to the adoption of air-mass 
analysis, was almost completely ig- 
nored by synoptic meteorologists. In 
consideration of this it will be of in- 
terest to review, qualitatively and in 
brief fashion, the definitions of hygro- 
metric terms and their relative degree 
of constancy when an unsaturated air 
varticle containing some water vapor 
is subjected to vertical displacement. 
The vertical displacement of an air 
particle in the atmosphere brings 
about changes in the pressure, since 
at any point the pressure within and 
surrounding the particle must be 
equal. If it rises work must be done 
in expanding against the decreasing 
pressure; if it descends work is done 
upon it. This work is realized as a 
change of temperature of the verti- 
eally moving particle. If no heat is 
added to or substracted from the mov- 
ing element, in other words if the 
particle remans thermally insulated, 
the process is termed adiabatic. The 
rate of temperature change with alti- 
tude of an unsaturated air particle 
due to these adiabatic changes is very 
nearly constant (about 1° C. per 100 
m.), although there are small varia- 
tions in this constant due to the water 
vapor content and the temperature of 
the surrounding air in which the par- 
ticle is moving. The first of these 
corrections, that for the presence of 
water vapor, becomes manifest when 
one considers the difference in the 
specific heats of dry air and of water 
*vapor. The correction, however, is 
very small, since the percentage of 
water vapor within a unit mass of 
air rarely exceeds a few percent of 
the total. The correction for the tem- 
perature of the surrounding air is also 
relatively small. Accordingly the dry 
air is the governing factor in the adi- 
abatic rate of cooling of unsaturated 
air. Practically this amounts to say- 
ing that the volume occupied by the 
vapor and its temperature at any 
stage are essentially the same as that 
of the dry air mass with which the 
vapor is associated. 
This facilitates a discussion of the 
variations of hygrometric quantities 
in the case of ascending and descend- 
ing motion. 
1. Vapor pressure (e). This term 
represents the partial pressure ex- 
erted by the water vapor molecules 
in the atmosphere. As an unsaturated 
particle of air is brought upward 
it expands. This expansion is ef- 
fected by a change in pressure 
and a change in temperature which 
it has been shown is due to the 
pressure change. The adiabatic tem- 
perature change acts volumetrically 
in the opposite sense to the pressure 
change, since decreasing temperature 
causes a contraction of volume. The 
net effect of these two opposing fac- 
tors is an increase of volume, so that 
the original amount of water vapor 
present within the sample of air must 
fill a larger space. Hence the vapor 
pressure (e€) decreases with adia- 
batic expansion. 
2. Relative humidity (f). This 
term is defined as the ratio of the ac- 
tual vapor pressure (¢) and the maxi- 
mum vapor pressure (én) possible at 
the same temperature. It should be 
noted that this maximum vapor pres- 
sure, €m, is a function of temperature 
only and has nothing to do with at- 
mospheric pressure. An unsaturated 
air particle rising rapidly through 
the atmosphere must cool nearly adi- 
abatically and therefore the maximum 
possible water vapor pressure, @m, 
