WINTER AIR MASSES 719 
of the analytical method to weather 
forecasting, without pretending to in- 
dicate more than a small fraction of 
the possibilities met with in actual 
practice. 
In Table III we find tabulated for 
each of the three principal air masses 
at two different aerological stations 
the mean values of the temperature, 
T, the specific humidity,* w, the 
relative humidity, R.H., and the 
equivalent-potential temperature, 0,7 
at the ground and at the successive 
km levels above sea level. These mean 
values are simply the averages of 
a number of ascents chosen as typical 
of each air mass type at each station. 
They represented the best observa- 
tional evaluation which could be made 
of the so-called characteristic prop- 
erties of each air mass type at the 
time this study was undertaken. 
[Since then a further analysis by 
Mr. Showalter of the 1935-36 aero- 
logical material has been published 
(Mon. Wea. Rev., July 1939) which de- 
erves careful study. At the end of this 
article some excerpts from his paper 
are reprinted, but the original gives a 
series of tables and cross-sect-ons of 
mean values and frequency diagrams 
for each air mass, which show that 
there is generally a large variation 
from case to case of the same air mass 
type even in the same season, so that 
mean values must not be accepted 
too literally. They do not necessarily 
give a typical picture because so many 
features which probably never all 
occur in any one sounding are averaged 
together. But the average picture is 
necessary and valuable as an orienta- 
tion for the student, forecaster, and 
researcher.—R. G. S.] 
An explanation of the full signifi- 
cance of 0, cannot be made here’ but it 
may be stated in a general way that 
turbulent mixing of an air stratum 
tends to effect isothermalcy of 4 ae 
whereas a rapid vertical change in @ ss 
indicates marked atmospheric stratifi- 
cation. If Oe increases with elevation, 
the possibility of thermal convection 
in the atmosphere is practically ex- 
cluded, whereas if ) decreases with 
elevation, the atmosphere is poten- 
tially unstable, an instability which 
becomes actual with sufficient vertical 
displacement of the affected stratum. 
Such is the displacement which may 
occur at a warm front. The amount 
of displacement necessary to effect 
actual instability of the affected 
stratum is less, the higher its relative 
humidity. 
In Table III the first station in- 
cluded for each air mass type is the 
one which gives the best indication of 
the characteristic properties of the 
air mass as it advances directly, fresh 
from the source region. The second 
station is chosen to indicate the most 
important modified form in which the 
air mass appears during its later his- 
tory. Thus for the Pc air mass, Ellen- 
dale indicates the characteristic cold 
wave type in the middle west, while 
Boston indicates the rather funda- 
mentally modified cold wave type in 
the northeastern United States. For 
the Pp air mass, Seattle indicates the 
characteristic properties as the air 
mass approaches the northwest coast 
of the United States fresh from the 
source, while Ellendale indicates the 
importantly modified form which the 
air mass assumes by the time it 
*The quantity w is not quite the specific 
humidity, defined by q=—0. 622e/p but the mass 
ratio of water vapor to dry air, defined ap- 
proximately by w=0. 622¢/ (p—e) where @¢é 
and p are water vapor and total atmospheric 
pressures. (See, Namias, article III.). 
5See C.-G. Rossby: Thermodynamics Ap- 
plied to Air Mass Analysis, M.I.T. Meteoro- 
logicul Papers, Vol. 1, No. 3; also Articles 
Ill and IV of Mr. Namias’ in this 
booklet. 
series, 
