168 
AIR MASS ANALYSIS 
Analysis of the Rainfall Situation over the Western States 
May 6-7, 1938, by Means of Air Mass and Isentropic Charts 
The surface maps and isentropic 
chart below are from the article by 
Mr. R. H. Weightman in the Bull. 
Amer. Met. Soc., April 1939, and were 
analyzed at the U. S. Weather Bureau, 
Washington. The isentropic chart il- 
lustrates the use of condensation pres- 
sures practiced by the Bureau. The 
relation of the surface fronts and of 
the isentropic flow and moisture dis- 
tribution to the rainfall patterns can 
be readily interpreted in light of the 
principles outlined by Mr. Namias. 
The rain along the Gulf coast is ac- 
credited to prefrontal instability due 
L KPa ee | 
UE 34 
= 3 = = 
HinGale 
io Jee J 
as > pn C 
to convergence ahead of the cold front 
over Texas; the rain over the Missouri 
and Mississippi valleys is definitely 
frontal; the northern Rocky Mountain 
rain area is due to instability in a 
moist tongue ascending from the south 
(in the northwest the 303°-surface 
was above the tops of the soundings 
on May 7); the Colorado-New Mexico 
rain belt is related both to the surface 
fronts and to the ascent of the moist 
tongue. The eastward advance of the 
moist tongue brings rain over Iowa on 
the a.m. of the 7th.—R. G. S. 
| 7:30am. May 6 1938 ' 
—.L-LOW. H- HIGH, _ 
jsobars. ~~ 
yw Surface Cold Front 22 Upper Occluded Front. \ 
== Upper | Cold Front 
Bae Sucface Warm Front &o> Cold dL brontaerer es 
DOD Upper Clit Front PWS Rate al 
O88 Warm Frontegenesis. | 
Ed | 
Erol 
— a 
Cores 
SurFAcE Map For 7.30 A.M. (E. S. T.) May 6, 1938 (shaded area = 
rain falling at time of observation). 
i 
il 
