10 AIR MASS ANALYSIS 
must become smaller and _ smalier. 
Since the vapor pressure within the 
air particle (e) is also decreasing, the 
variation in the relative humidity 
(e/em) really depends upon the rate 
of change of both these quantities. It 
so happens that the rate of decrease 
in the denominator, that is, the falling 
off of ém due to adiabatic cooling, is 
much greater than the rate of de- 
crease in the vapor pressure, the nu- 
merator. Thus the percentual value 
of the fraction, defined as the relative 
humidity, must increase as the parti- 
cle rises.} 
8. Absolute humidity. This quan- 
tity is defined as the mass of water 
vapor present in a given volume, or in 
other words the density of the water 
vapor. Again subjecting the air par- 
ticle to adiabatic lifting it is obvious 
that the volume of air is increasing as 
the particle rises. Yet the number of 
water vapor molecules within the sam- 
ple of air remains constant, so that the 
density of the vapor, or the absolute 
humidity, must decrease with adia- 
batic expansion. 
4. Specific humidity (q). This 
quantity is defined as the mass of wa- 
ter vapor present in a unit mass of 
air. The unit mass of air is consid- 
ered as being made up of the usual 
gas mixture plus the water vapor. The 
amount of water vapor, however, is 
negligibly small when compared with 
the mass of dry air with which it is 
associated. Hence the mixing ratio, 
defined as the mass of water vapor in 
a unit mass of dry air, and conven- 
ient for certain theoretical calcula- 
tions is very nearly numerically 
equivalent to the specific humidity. In- 
deed errors in hygrometry in aerologi- 
cal soundings are generally much 
greater than the difference between 
the two quantities. 
As the unsaturated air particle as- 
cends adiabatically, both the mass of 
water vapor and the total mass of air 
remain constant. Hence the specific 
humidity, which depends on these two 
masses, also remains unchanged. 
Thus the air particle has the same 
specific humidity in spite of its rise. 
The significance of this constancy 
becomes apparent when one considers 
the active overrunning of warm air 
over a cold wedge (a warm front), 
forced vertical displacement by an 
advancing wedge of cold air, or the 
sinking of air layers within a cold air 
mass or in a stagnating anticyclone. 
In all these cases vertical movements 
bring about changes in vapor pres- 
sure, relative humidity, and absolute 
humidity of the vertically moving 
particle. The specific humidity, how- 
ever, does not change, providing no 
moisture is added to or subtracted 
from the air through precipitation, 
evaporation, or turbulent exchange. 
At the surface of the earth the 
moisture content depends upon the 
synoptic air mass present and the 
modification which it has undergone 
during its history. Since the modifi- 
cation is generally greatest in the 
lowest layers of the atmosphere it 
follows that the meteorological ele- 
ments within these layers are less 
conservative than at higher levels. 
Nevertheless, the proximity of our 
American air-mass source regions, and 
the marked difference in our air-mass 
properties* bring about considerable 
contrast in the elements even within 
the surface layers. The specific hu- 
+Note: In U. S. the dew-point temperature 
is reported in the airways hourly observations, 
and also in the six-hourly reports from first- 
order stations, for the humidity element; it is. 
not strictly conservative when changes or 
differences of pressure are involved, but is use- 
ful for local comparisons and for fog fore- 
easting.—R. G. S. : 
*For a thorough treatment of these proper- 
ties see ‘American Air Mass Properties’ by 
H. C. Willett, Papers in Physical Oceano- 
graphy and Meteorology, Mass. Inst. of Tech. 
and Woods Hole Oceanographic Institution. 
Vol. 2, No. 2, 19883 now out of print but 
largely reprinted in back of this booklet. Ar- 
ticles on air mass properties in other coun- 
tries are listed in the BIBLIOGRAPHY. 
