THE TEPHIGRAM 55 
this case called positive area) en- 
closed between the path of the rising 
particle and the tephigram. 
When the path of a rising particle 
lies below the tephigram of the sound- 
ing, stability is indicated, and the 
area (called negative area) enclosed 
between the path of the rising particle 
and the tephigram represents the 
amount of energy which must be over- 
come if the particle is to penetrate 
the layer. 
From these considerations it is 
clear that large positive areas and 
small negative areas are most favor- 
able for the developmert of a 
thunderstorm. While there is no sub- 
titute for experience in working with 
the tephigram, it is possible to make 
some statements of a general nature 
which should assist the beginner in 
his use of the tephigram. Such a 
discussion, consisting of the indica- 
tions and limitations of the tephi-. 
gram, particularly when used in con- 
junction with a reliable weather map 
analysis, is taken up in the article on 
Thunderstorms, below. 
A Note on Estimating Conditional and Convective Instability 
From the Wet-bulb Curve 
Normand* has called attention to 
the fact that the usual method of in- 
dicating humidity by a depegram shows 
only the variation of humidity in a 
sounding, and that it cannot be used 
directly for visualizing the amount of 
energy available in a particle of air 
which rises from some particular layer. 
He pointed out further that in order 
to make reasonably correct deductions 
about the stability of such a particle of 
rising air, it is desirable to have a 
thermodynamic representation of hu- 
midity on the tephigram as well as one 
for temperature. In order to obtain 
such a representation he has advocated 
that the saturation (in practice, the 
wet-bulb) temperatures of a sounding 
be plotted on their appropriate isobars. 
This method is a natural sequence of 
the use of wet-bulb temperatures of 
the free air first advocated by him in 
1921. He argued that the “saturation 
temperature” curve (S.-T. gram; or 
estegram) of a sounding should 
be used because the tephigram and 
depegram alone do not consider the 
actual state of humidity, and because 
it is desirable to have a curve which 
* Normand, C. W. B.: Graphical indication 
of humidity in the upper air, Nature, 3rd 
October, 1931, Vol. 128, p. 583. 
can be compared with the saturation 
adiabats just as the temperature- 
height curve can be compared with the 
dry adiabats. Drawing conclusions con- 
cerning the “liability” to instability 
from a tephigram alone by comparison 
of the latter with saturation adiabats 
is undesirable, for if the air is dry at 
all heights it is a waste of time to 
consider it as if it were moist. When 
that method is used it is the same as 
assuming that it is possible by a mete- 
orological process to saturate dry air 
without altering its temperature! But 
it is well known that dry air which 
passes over a large lake or the ocean 
does not become saturated without a 
simultaneous change in its tempera- 
ture. It is necessary when estimating 
the probable effect of an increase in 
humidity to assume a decrease in dry- 
bulb temperature, an increase in the 
dew-point, and little change in the 
wet-bulb temperature. This is the most 
important justification for including a 
wet-bulb or saturation temperature 
curve (these two curves are practi- 
cally equivalent to one another) along- 
side the tephigram in estimating “la- 
bility” to instability. 
Normand has found the addition of 
the wet-bulb temperature curve on the 
