158 
In the discussion of the influence of 
convection on the moisture patterns on 
the isentropic chart it is convenient to 
distinguish between widespread con- 
vection within unstable air masses 
and convection associated with fronts. 
Convection that occurs along fronts 
(notably cold fronts) is usually re- 
stricted to a narrow zone which co- 
incides with a moist tongue on the 
isentropic chart. While the vertical 
currents transport moisture to higher 
levels, water is also precipitated from 
the frontal cloud system. The convec- 
tion which occurs along such fronts 
merely replenishes the moisture con- 
tent of the moist tongue, and it does 
not disrupt the continuity of the isen- 
tropic analysis. The same also applies 
to purely local convection (“pinpoint 
convection”). The matter may, how- 
ever, be different in cases of wide- 
AIR MASS ANALYSIS 
spread convection. The convective 
transfer of moisture may then at the 
beginning of this process radically 
change the moisture pattern aloft. 
This is particularly the case in Polar 
continental air when it moves over an 
ocean, because the convective currents 
will then transport much moisture to 
high levels. The convection that oc- 
curs in unstable Polar continental air 
moving over land in winter does not 
usually reach up to the representative 
isentropic surface; it therefore does 
not appreciably influence the moisture 
pattern aloft. 
From the above it follows that the 
occurrence of convective as well as 
frontal precipitation is closely related 
to the moist tongues, and that the 
isentropic charts afford the best 
means for analyzing the processes in 
the free atmosphere. 
SS \ f 
NORMAL SUMMER ISENTROPIC CHART— 
@=315°A SS 
Fic. 14. Tot NoRMAL FLOW PATTERN OVER UNITED STATES IN 
SUMMER. This chart is based upon aerological data for summer months 
from July, 1934, through August, 1939. 
It was constructed by aver- 
aging values interpolated at 5 degree intersections of longitude and 
latitude from analyzed monthly mean isentropic charts. 
The winds 
are normal resultant upper-air winds. 
