150 
a constant level chart.* In regions 
where pilot balloon wind data are 
lacking these isentropic stream lines 
are quite helpful. Moreover, by com- 
paring the patterns of stream lines 
from day to day, and noting the 
changes of the y values, it is possible 
to get a better idea as to the future 
AIR MASS ANALYSIS 
trajectory of dry and moist tongues 
of the isentropic chart.; There is also 
considerable advantage in construc- 
ting the stream function chart before 
completing the isentropic flow pattern. 
Wevancen of wy are now being transmitted 
daily over the airway and Weather Bureau 
teletype circuits in the United States. 
§ 7. THE RELATION OF ISENTROPIC FLOW TO PRECIPITATION 
Since one of the necessary condi- 
tions for the formation of precipita- 
tion is the presence of sufficient mois- 
ture content, it is not surprising that 
there is generally found some rela- 
tion between moist tongues and pre- 
cipitation areas. In winter, when the 
stratification is relatively stable over 
the continents, most of the precipita- 
tion over continental areas is caused 
by frontal action. The isentropic 
chart frequently indicates regions of 
ascent or descent of air through the 
relative configurations of the moisture 
lines. A frequent type of flow pat- 
tern is shown in fig. 10, where a 
moist tongue ascends the isentropic 
surface. 
4561 
716 5 4g/kg_ 
Fic. i0.—TIllustrating probable upslope mo- 
tion of a moist current as deduced from the 
relation of moisture lines to contour lines. 
It should be mentioned that the con- 
figuration of moisture and contour 
lines shown in fig. 10 does not always 
indicate upslope motion, because the 
shape of these lines is the result of 
a lengthy development, and the up- 
slope motion may have ceased by the 
time when the synoptic picture was 
obtained. 
Another indication of upslope mo- 
tion along the isentropic surface is ob- 
tained from the wind observations. 
If there are sizeable wind components 
normal to the contour lines, and if the 
contour patterns are uniquely defined 
by observations from a dense network 
of soundings, then it is probable that 
the air is ascending or descending the 
isentropic slope in the direction of 
the wind. However, when the con- 
tour lines themselves are displaced 
with the same speed as the wind com- 
ponent normal to them, the wind com- 
ponents normal to the contour lines 
are not indicative of up- or down- 
slope motion. If there is any doubt 
as to whether there is upslope or 
downslope motion the observed mixing 
ratios should be compared with the 
saturation mixing ratios (or the pres- 
sure should be compared with the 
condensation pressure) in order to 
find out how much lifting is necessary 
in order to make the air saturated. 
In addition, consecutive maps should 
be compared in order to determine 
whether the air is approaching satu- 
ration or not. A study of the closed 
systems and areas of precipitation 
will give additional information to 
this end. 
In regions where the heaviest fron- 
+See in this connection the interesting sug- 
gestions of Starr to show such changes by 
means of a “relative-motion isentropic chart’. 
(V. Starr, Bulletin Amer. Met. Soc., June, 
1940). 
