52 AIR MASS ANALYSIS 
eee 
sloping downward from the left to 
right are lines of equal pressure. 
From Poisson’s equation it is readily 
seen that these isobars are defined by 
the rectangular coérdinates: potential 
temperature and temperature. In the 
diagram reproduced here (Fig. 13) 
the isobars are expressed in milli- 
meters of mercury; more frequently 
the unit used is the millibar. Broken 
lines sloping slightly to the right of 
the vertical are lines of equal specific 
humidity (on some diagrams the mix- 
ing ratio is used instead), the values 
being the saturated mass of water 
vapor in grams per kilogram of moist 
air which can exist under the ap- 
propriate temperatures and _pres- 
sures. The solid curved lines sloping 
upward from the left to right are 
pseudoadiabats — lines which repre- 
sent the path of a rising saturated 
particle of air precipitating its mois- 
ture as soon as it is condensed. Note 
how the slope of these curves ap- 
proaches the horizontal lines of equal 
potential temperature (the dry adia- 
bats) at low temperatures, because of 
the decreasing saturation humidity 
content as absolute zero temperature 
is approached. 
There are several ways to plot a 
tephigram, the most convenient de- 
pending upon the quantities one has 
at hand. For example, it is possible 
to use the rectangular co6érdinates, 
potential temperature and tempera- 
ture; then again one may plot tem- 
perature against pressure by using 
for coordinates the slanting isobars 
and vertical isotherms. In Fig. 13 is 
plotted the sounding for Oklahoma 
City on June 20, 1935; this is the 
solid line, the significant points of 
which are small circles, A to H. In 
conjunction with the tephigram it is 
also helpful to construct what is 
known as a depegram—a curve show- 
ing the variation of dew-point, and 
hence of moisture, with elevation. 
This is constructed by plotting speci- 
fic humidities at the significant levels 
against the corresponding pressures, 
which is more convenient than com- 
puting dew points and gives precisely 
the same result. The depegram for 
the Oklahoma ascent is represented 
in Fig. 13 by the dotted lines con- 
necting crosses. From the tephigram 
one may easily determine the stability 
of any given stratum, for the hori- 
zontal lines, being lines of constant 
potential temperature, are dry adi- 
abats; the vertical lines, isotherms; 
and the curves sloping upward from 
left to right are saturated adiabats. 
Thus the layer EF possesses a dry 
adiabatic lapse-rate, AD contains a 
temperature inversion (probably a 
ground radiation inversion), and the 
layer DE is in conditional equilibrium 
as the lapse-rate lies between the dry 
and the saturated adiabats. The depe- 
gram enables one to get a picture of 
the relative dryness of the various 
layers. For example, the surface 
layer at A is almost saturated, for 
here the dew-point is nearly equal to 
the temperature. Above the point H’ 
the air is very dry, a fact shown by 
the comparatively large distance be- 
tween the temperatures at HE, F, and 
above, and the dew-points at these 
levels shown at E’, F’, etc. 
The most important use of the 
tephigram lies in indicating the 
amount of potential energy, in the 
overlying air column, which may be 
converted into the kinetic energy of 
a thunderstorm. For this purpose it 
is necessary to choose some individual 
particle of air and follow by means 
of the diagram the path it would take 
if subjected to vertical displacement 
up through the entire air column. It 
is assumed that the surrounding air 
remains at rest while this displace- 
ment of a unit element of air is 
taking place. It might be assumed, 
for example, that the point A is car- 
ried upward. In this event the par- 
ticle of air represented by the point 
