GLOSSARY 
229 
compared with the ordinary tran- 
sitions in one and the same air 
mass. (Practically synonymous 
with front.) 
dry air (1)—air which is not satu- 
rated. 
emagram (VIII, I1X)—any adiabatic 
diagram with coordinates T, log p. 
Areas are proportional to energy as 
on the tephigram. 
energy diagram (iII, VIII, 1X)—any 
thermodynamic diagram on which 
area is proportional to energy; such 
as the tephigram or emagram 
(which see). 
equivalent-potential temperature (II) 
—the temperature a given air par- 
ticle would have if it were brought 
adiabatically to the top of the at- 
mosphere (i.e., to zero pressure) so 
that along its route all the mois- 
ture were condensed (and pre- 
cipitated), the latent heat of con- 
densation being given to the air, 
and then the remaining dry sample 
of air compressed adiabatically 
to a pressure of 1000 millibars. 
equivalent-potential temperature dia- 
gram—see Rossby diagram. 
equivalent temperature (I11)—the tem- 
perature a particle of air would 
have if it were made to rise adia- 
batically to the top of the atmos- 
phere (i. e., to zero pressure) in 
such a manner that all the heat 
of condensation of the water va- 
por were added to the air and 
the sample of dry air were then 
brought back adiabatically to its 
original pressure. 
estegram (VIII)—the characteristic 
curve of wet bulb temperatures of 
a sounding plotted on a tephigram 
or pseudo-adiabatie chart. 
front (V, VI and VI1)—the discon- 
tinuity between two juxtaposed cur- 
rents of air possessing different 
densities. Most frequently fronts 
represent the boundary between 
different air masses. The so-called 
“wind-shift line” is usually a well- 
marked front. 
frontogenesis (VII)—the creation of 
fronts generally brought about 
through the horizontal convergence 
of air currents possessing widely 
different properties. 
frontolysis (VI1)—the destruction of 
fronts generally brought about by 
horizontal divergence at the dis- 
continuity zone. 
instability (1)—the opposite of stabil- 
ity; a lapse rate in which particles 
will be readily displaced vertically 
upon small impulse. (Also called 
lability.) See: conditional, latent, 
pseudo-, mechanical and absolute in- 
stability. 
instability showers (V)—showers 
caused by steepening of the lapse- 
rate in any way, such as the rapid 
warming of the lower layers of 
a cold current as it moves over 
a relatively warm surface. In most 
eases there is an appreciable ad- 
dition of moisture to the lower 
layers, as for example, when a polar 
continental current moves over a 
body of warm water. 
lapse rate (1)—the existing rate of 
change of an element, commonly 
temperature, with height in a given 
layer of the atmosphere. 
latent instability (VIIL) —on the 
tephigram, when the area of posi- 
tive energy is greater than the 
negative energy area (this is more 
properly called real latent instabil- 
aty). In general latent instability 
refers to the energy that can be re- 
leased after the convection reaches 
the condensation level. It is the 
case of conditional instability where 
the air is moist enough for convec- 
tion to form clouds. (See also pseu- 
do-instability.) 
loop (or bent) back occlusion (VII)— 
an occluded front which has bent 
back in the rear of the cyclone so 
that it appears in the meteorologi- 
cal field as another front behind the 
cold front. In most cases these oc- 
clusions are of the cold front type. 
That is, the air behind is colder 
than that preceding them. 
mechanical instability (1)—a lapse 
rate such that the air density de- 
creases with elevation; for this con- 
dition the lapse rate must be greater 
