140 AIR 
cannot be completely satisfied. How- 
ever, it is in general possible to find 
an isentropic surface that satisfies 
these conditions well enough for a 
reasonably sound analysis. 
In North America suitable values 
of potential temperature for the in- 
dividual seasons are: 
Season Potential Temperature 
Winter 290°-295°A 
Spring 295°-300°A 
Summer 310°-315°A 
Fall 300°-305°A 
The abrupt increase from spring to 
summer values is due to the normally 
rapid increase of free air tempera- 
tures as summer convection sets in. 
During periods of unusual weather 
it is sometimes necessary to change 
to a different surface for a few days, 
and then to return to the normal for 
the season. It should be pointed out, 
however, that such changes carry 
with them a certain loss of contin- 
uity—and it cannot be emphasized too 
strongly that continuity is the prim- 
ary requirement of isentropic anal- 
ysis. For this reason it is most ad- 
vantageous to follow from day to day 
the flow patterns in a given isentropic 
surface, and when an abnormal period 
suggests change of surface, to con- 
struct an additional set of charts for 
a more representative surface for this 
particular period. 
The elements plotted along the isen- 
tropic surface are the mixing ratio, 
saturation mixing ratio, and atmos- 
§ 3. TECHNIQUE 
In order to enter all available pi- 
lot-balloon winds on the isentropic 
surface it is necessary to sketch a set 
of lines of constant pressure. We 
shall refer to these as contour lines. 
Usually it suffices to draw such lines 
for each 50th millibar. The number 
of aerological soundings is in general 
insufficient for drawing the contour 
lines in a purely mechanical fashion. 
MASS ANALYSIS 
pheric pressure’. The appropriate 
values for plotting on the isentropic 
chart are readily extracted from aero- 
logical soundings with the help of 
some convenient thermodynamic dia- 
gram (see Article III). The pilot-bal- 
loon wind observations are then en- 
tered along the isentropic surface for 
those stations where the height (or 
pressure) of the isentropic surface is 
already evaluated. In addition to the 
above data, it is helpful to indicate 
the form and motion of clouds as well 
as hydrometeors observed at the aero- 
logical sounding stations together 
with the levels in which these phe- 
nomena are reported. 
The atmospheric cross-section dia- 
grams now in use in United States 
have pressure (on a logarithmic scale, 
but p°®** scale is being introduced) 
as ordinate and horizontal distance as 
abscissa. The values of mixing ratio 
and potential temperature are plotted 
at the significant levels for each sta- 
tion. If time permits it is convenient 
to plot also the relative humidity and 
the temperature. By constructing lines 
of constant moisture and constant po- 
tential temperature in the cross sec- 
tions we obtain a picture of the mois- 
ture and temperature distribution 
along the vertical plane represented 
by the cross section, as well as a view 
of the moisture pattern in the isen- 
tropic surfaces. Pilot-balloon wind 
observations, clouds and hydrometeors 
are also plotted in the cross sections. 
oF ANALYSIS 
Contour lines may be drawn more ac- 
curately by taking into consideration 
the guiding factors mentioned below. 
It is, indeed, necessary to make use of 
these aids when one wishes to extend 
the analysis into regions in which the 
data are sparse. 
4Byers [10] has developed the use of con- 
densation pressure instead of mixing ratio, 
and that method has been adopted in the 
U. S. Weather Bureau. 
