24 AIR MASS ANALYSIS 
within one and the same current of 
air show no pronounced wedge-like 
curve, but instead a continued de- 
crease in the moisture content. 
The method of making use of the 
Rossby diagram in practical meteor- 
ological work is best grasped by 
studying synoptic discussions in 
which the diagrams are used, and if 
possible, making daily use of them 
in conjunction with the analyzed 
weather maps.* 
Note on present use of the Rossby and other 
diagrams.—The Rossby diagram is no longer 
so extensively used in daily routine weather 
analysis in the United States as some years 
ago. (The use of tephigrams and emagrams 
likewise seems to have decreased). The reason 
for this appears to be in part the lack of 
time available for plotting additional charts, 
but more importantly the increasing experi- 
ence of the analysts which permits them to 
recognize the air masses and stability condi- 
tions from an inspection of the surface map 
and of the aerological soundings plotted on 
adiabatic charts or against height. Time 
available to forecasters is so limited that the 
introduction of additional charts is resisted 
by them except where some very indispensable 
advantage can be readily demonstrated. Ali 
synoptic meteorologists agree, however, that 
some such diagram as Rossby’s should be a 
familiar tool and that it is invaluable for 
teaching principles to students and for re- 
search. The diagram still finds much use as 
a nomogram for finding 6> On» ete., and other 
thermodynamie computations. For identifying 
convective instability it is only necessary to 
have a table of the Oz lapse rates for the 
sounding. In the note on wet-buib tempera- 
ture appended to Article WIII is indicated 
how all types of stability and energy distribu- 
tion can be recognized on the tephigram or 
emagram alone, thus obviating the necessity 
of plotting two or three charts. Even the 
Sttive pseudo-adiabatic chart, which is usu- 
ally plotted anyway in all weather services, 
ean be adapted in a crude way for all the 
purposes attributed to other charts and since 
‘time is at a premium there is a tendency for 
American forecasters to depend on it almost 
exclusively. In a tropical country the tephi- 
gram is probably the most convenient all- 
purpose chart (see Article VIII). The Refsdal 
“Aerogram’”’ is designed to combine features 
of all other diagrams and is being used in 
some services. Its inconveniently small scale 
and the complications of many overlapping 
coordinate grid-lines do not commend it for 
students, nor for speed and _ simplicity.— 
Ra GS: 
V. ELEMENTS OF FRONTAL STRUCTURE—THE WARM FRONT 
Anyone who has worked witn a 
close network of surface observations 
clearly recognizes the existence of 
zones in which the rate of change of 
the commonly observed meteorolo- 
gical elements, e.g., temperature, is 
comparatively large. These zones of 
rapid transition of the elements are 
the ‘‘fronts” of the Norwegian system 
of weather analysis. In the general 
treatment of air masses it was pointed 
out that large-scale air currents, 
in spite of a long trajectory, tend 
to retain their original properties, 
this being particularly true at upper 
levels. It follows, then, that as two 
air masses, one from polar regions 
and the other from tropical regions, 
converge, there must be a zone of 
*See synoptic papers by Willett, Emmons, 
Namias, and Byers, 1932-1936, in Bibliography. 
separation between the two currents. 
The methods of air mass and frontal 
analysis are primarily based upon the 
postulate that the zone of separation 
between currents of air possessing 
different properties may be treated 
as a surface of discontinuity. Atmos- 
pheric discontinuities, to be sure, are 
not absolute mathematically abrupt 
transitions, but rather are zones in 
which the change in the elements is 
far more rapid than within the air 
masses on either side of the front. 
These discontinuities are not purely 
surface phenomena; they are sur- 
faces which generally slope back- 
ward (or forward) over the cold air. 
If an ideal case is assumed in which 
a warm current of air flows side by 
side but in opposite direction to a 
cold current the following formula 
