18 AIR MASS ANALYSIS 
nomogram for q and @, which is shown 
in Fig. 4.—R. G.S. 
Note.—Beginning in the summer of 1938 the 
U.S. Weather Bureau ceased regularly trans- 
mitting over its telegraph and teletype circuits 
the potential temperature (@) for each signi- 
ficant level of the daily aerological soundings. 
It became necessary therefore to compute all 
the derived values needed for plotting various 
energy and thermodynamic diagrams or @py 
lapse-rates, etc. However, the Bureau at the 
same time published for its offices a new type 
of adiabatic chart base (“Upper Air Map C’’) 
which contains, besides the fundamental grid 
of ordinary temperature vs pressure, only the 
slanting curves of the wet (saturation) adi- 
abats; but on the margins are numbered 
marks to indicate where the lines of potential 
temperature (straight), of saturation specific 
humidity (also straight) and of kilometers 
of elevation above the sea, would intersect. 
By using a ruler, or better still, a celluloid 
or glass transparent plate with sets of lines 
for potential temperature and _ saturation 
specific-humidity drawn to scale on it, one can 
quickly evaluate these figures from the base 
chart once the sounding is plotted thereon in 
the ordinary terms of JT, p, and gq. Thus the 
new chart serves as a convenient nomogram. 
It supplants the temperatures vs height aero- 
logical plotting chart which has heretofore 
been used for years in the Weather Bureau 
forecast offices and elsewhere. Owing to the 
small size of this chart it is not suitable for 
accurate work and will be supplanted by a 
new and larger design. The large standard 
Stiive adiabatic chart has long been available 
but used chiefly at aerological stations; it is 
too large for ready comparison of many 
soundings at once, as the forecaster must do 
in some countries.—Editor. 
MopIFIED RossBy-DIAGRAMS 
A number of variations have been proposed 
and some are in use. They have some ad- 
vantages in convenience but little if any 
fundamental ones. Any diagram with two 
conservative elements among its coordinates 
would essentially still be a Rossby diagram. 
For gp Brunt (Phys. and Dyn. Met., 2nd ed., 
p. 92) has suggested substituting the (satu- 
rated) potential wet-bulb temperature and 
Hewson has analyzed situations in that manner 
(see Bibliography). However, Bleeker points 
out that this quantity is not conservative if 
any evaporation (of falling rain, or by subsi- 
dence) takes place, when it may lead to 
erroneous conclusions as to movement and 
identity of air masses. Rossby’s equivalent- 
potential temperature is somewhat less con- 
servative for an evaporation process than the 
adiabatic wet-bulb potential temperature. How- 
ever, there is no single index which is conserya- 
tive for both adiabatic processes and evapora- 
tion, and since evaporation does not often 
seriously invalidate the usual interpretation of 
the Rossby diagram the adiabatic wet-bulb and 
equivalent-potential temperatures are both 
equally the best conservative elements available. 
Clark of California Institute of Technology 
revised the Rossby-diagram by arranging the 
GE isotherms vertically and gD lines hori- 
zontally and plotting curved lines of constant 
mixing ratio and isobars of the condensation 
level upon the chart. The characteristic 
curves will vary in slope through wider 
angles than on the original diagram, so that 
different stability conditions are more easily 
recognized at a glance (described in Taylor’s 
Aeronautical Meteorology, p. 60.). 
Arakawa has published in Japan a new 
form of the Rossby-diagram having for 
ordinates the potential temperature on a 
linear scale instead of partial potential tem- 
perature on a logarithmic scale, and having 
equivalent-potential temperature lines which 
are calculated on the basis of convection in 
saturated air with the heat of fusion taken 
into account; also the diagram is extended to 
over 20 g/kg mixing ratio so that it can be 
used for extremely humid tropical conditions. 
The advantage of these coordinates is that the 
effect of mixing of two air masses can also 
be determined, since the potential tempera- 
ture of a mixture is equal to the mean of the 
potential temperatures of the original air 
masses (Bull. Amer. Met. Soc., March 1940, 
To% alalil)}. 
Finally, the Refsdal ‘‘Aerogram”’ should be 
mentioned here since it combines the proper- 
ties of the adiabatic chart and the Rossby 
diagram and the tephigram in one, though 
it is rather too complicated for the elemen- 
tary student (see:—Geofysiske Publik., Vol. XI, 
No. 13; Meteorol. Zeit., Jan. 1935, p. 1; Bull. 
Amer. Met. Soc., Jan. 1940, p. 1). 
(See also “A Note on Estimating Condi- 
tional and Convective Instability from the 
Wet-bulb Curve”, at the end of Article VIII 
in this booklet.) 
