56 AIR MASS ANALYSIS 
tephigram permits a more clear-cut 
classification of tephigrams according 
to vertical stability. In tropical re- 
gions the state of conditional insta- 
bility generally prevails, and in middle 
latitudes it occurs rather frequently. 
Sometimes, however, it is associated 
with settled weather and sometimes 
with disturbed; in studies made in 
India the tephigram alone gives no 
criterion which allows a correlation 
with the type of weather. The Indian 
meteorologists found, however, that 
the important criterion in that connec- 
tion is the vertical distribution of 
water vapor, and that a wet-bulb curve 
drawn beside a tephigram gives a more 
definite picture of the amount of 
energy that is likely to become avail- 
able as a consequence of convection 
reaching the condensation level. 
The use of estegrams together with 
tephigrams makes it possible to clas- 
sify the conditions of particles of air 
with respect to their environment into 
three classes of conditional instab- 
lity :— 
1) Particles of air which if raised 
adiabatically will release more energy 
in the upper unstable portion of their 
ascent than must be supplied to them 
in their lower, stable portion, are in a 
state of (real) latent instability.+ 
2) A case in which a particle can be 
raised with a given supply of energy 
to a position where it is in an unstable 
environment and thus some of its 
energy can be liberated, but the energy 
so liberated is less than that supplied 
from below, is termed a state of 
pseudo-(latent) instability. 
3) In the third case, when the lowest 
saturation adiabat tangential to the 
tephigram does not intersect the este- 
gram, there is neither latent instability 
7 Normand first defined the concept of 
latent instability in an article on tropical 
storms in Gerlands Beit. z. Geophys., Vol. 
34, p. 234, 1931; the principle is examined in 
detail by him in the Q. Jn. Roy. Met. Soc., 
IGBs35 ioe Ley se 7 
nor pseudo-instability. In other words, 
there is no energy realizable ai all. 
A particle of air has latent insta- 
bility or wpseudo-instability, if the 
saturated adiabat through it (for ex- 
ample, the initial position of the 
particle under consideration) on the 
wet-bulb curve intersects the curve of 
the tephigram. The latent instability 
can be realized, however, only when 
the amount of work which must be 
done to raise a particle of air to the 
condensation level (from which it 
would thereafter rise freely) is less 
than the energy which will be realized 
during its further ascent (i.e., only in 
the case of real latent instability). If 
the saturated adiabat through the 
point of the initial wet-bulb tempera- 
ture of the particle under considera- 
tion does not cut across the line of the 
dry-bulb temperature, then no upward 
displacement of the particle can bring 
it to a level where it will be as warm 
as its environment, so then the atmos- 
phere is stable for all particle displace- 
ments—large or small. 
In order to release the energy in air 
which is in a state of latent or pseudo- 
instability a trigger action is neces- 
sary, notably surface heating, and 
surface evaporation, but also lifting 
by a cold front, a mountain, or by 
convergence; release by lifting how- 
ever, is strictly a case of convective 
instability in which of course whole- 
sale release of conditional instability 
is an intermediate process (see below). 
The recognition at a glance of the 
layers in which all particles have 
pseudo or latent instability is easy 
when the wet-bulb curve is entered 
beside the tephigram. For instance, 
in Figure 14 the layers with latent 
instability are determined by ex- 
tending the moist adiabat which is 
tangential to the portion of the tephi- 
gram which approaches the wet-bulb 
curve most closely down to the surface 
