60 AIR MASS ANALYSIS 
mately become unstable if raised to the 
condensation level. Since energy is re- 
quired to raise the layer, however, and 
the amount which must be supplied for 
this purpose may exceed the energy 
which can be expected from lifting or 
heating, the energy of convective in- 
stability may be unrealizable. The only 
important types of convective instabil- 
ity are those in which the energy which 
has to be supplied in order to lift the 
layer to the condensation level is less 
than the amount which will be realized 
from the resulting instability. Swch a 
gain in energy is most likely to be real- 
ized when the layer of air under consi- 
deration is originally nearly saturated 
or when latent instability is present 
from the start. There may be convective 
instability with or without conditional 
instability at the start, of course. The 
use of the wet-bulb curve serves to 
classify the convective instability ac- 
cording to whether there is latent-, 
pseudo-, or no conditional-instability 
present at the same time, and hence 
the relative probability of its release 
by any trigger actions that can be 
foreseen on the weather map. In trop- 
ical regions and in summer in higher 
latitudes this simple method of esti- 
mating all types of instability on one 
diagram (tephigram, Neuhoff dia- 
gram, or emagram) will certainly 
appeal to practical meteorologists. 
. Even on the Stiive pseudo-adiabatic 
chart the wet-bulb curve will be quali- 
tatively helpful, in case there is in- 
sufficient time to plot an energy 
diagram. 
Bleeker has shown that the adia- 
batic wet-bulb temperature is not 
strictly a conservative element for 
an, evaporation process while the iso- 
baric wet-bulb is not conservative for 
wet or dry adiabatic processes, but for 
estimating instability this makes no 
practical difference. Petterssen has 
lately suggested drawing the wet-bulb 
temperature curve on the pseudo- 
adiabatic chart to indicate convective 
(or potential) instability (see Fig. 
16). This is a more convenient pro- 
cedure than plotting a Rossby dia- 
gram, but the deviation of the wet- 
bulb curves from the moist adiabats 
is usually so small that the layers 
with convective instability cannot be 
read off so accurately as on the 
Rossby diagram, though this is not 
serious except in borderline cases 
when the inaccuracy of aerological 
measurements would probably not 
permit a definite conclusion anyway. 
—R.G.S. 
IX. SYNOPTIC ASPECTS OF THE THUNDERSTORM}; 
In the preceding article the plot- 
ting of the tephigram and a partial 
interpretation of an individual sound- 
ing was outlined. This article will 
present briefly the probable causes 
of thunderstorms, indicating the use 
of aerological data in their analysis 
and forecasting.+ However, the origin 
and development of the thunder- 
storm from the synoptic viewpoint 
are by no means fully understood 
as yet, and therefore offer a fertile 
field for research. 
Perhaps the most convenient classi- 
fication of thunderstorms is based 
upon the physical factors which pre- 
sumably cause them: 
1. Air-mass thunderstorms: 
(a) From local convection. 
(6) In thermodynamically cold 
air masses. 
2. Frontal thunderstorms: 
(a) Associated with a _ cold 
front. 
(b) Associated with a warm 
front. 
{Further discussion of thunderstorm fore- 
easting will be found in Article X on Isen- 
tropic Analysis. 
