228 
AIR MASS ANALYSIS 
Glossary of Elementary Terms Used in Articles I to IX 
absolute humidity (I1)*—the mass of 
water vapor present in a unit vol- 
ume of air, or the density of the 
water vapor. 
absolutely stable (1)—a‘ vertical dis- 
tribution of temperature such that, 
whether the air be dry or saturated, 
particles will tend to remain at 
their original level. In this case the 
lapse rate must be less than the 
saturation adiabat at the prevailing 
temperatures. 
absolutely unstable (1)—a lapse rate 
greater than the dry adiabatic; 
both dry and saturated air are un- 
stable. Also called a super-adiab- 
atic lapse rate. 
adiabat (1)—a curve along which a 
thermodynamic change takes place 
without the addition or subtraction 
of heat. In the case of the atmos- 
phere a dry adiabat is generally 
considered a temperature-height or 
temperature-pressure curve along 
which a rising or sinking air par- 
ticle will fall providing no satura- 
tion occurs and providing, of course, 
that no heat is given to or taken 
from the particle in its path. Simi- 
larly a wet adiabat (saturation 
adiabat, condensation adiabat, or 
pseudo-adiabat) is a temperature- 
height or temperature-pressure 
curve along which the saturated ris- 
ing particle will fall. 
adiabatic chart (1I1)—a thermo- 
dynamic diagram in which tempera- 
ture is plotted against pressure 
(either on a _ logarithmic seale, 
or pressure to the 0.288 power) and 
in which dry adiabats are con- 
structed. The chief use of this 
chart is for evaluation of aero- 
logical soundings. 
adiabatic process (1)—a thermo- 
dynamic process in which no heat 
is transferred from the working 
substance to the exterior or vice 
versa; a thermally insulated pro- 
cess. 
adiabatic rate of cooling with ascent 
for dry air (1)—very nearly con- 
stant in the troposphere at 1 de- 
gree Centigrade per 100 meters (see 
adiabat) . 
adiabatic rate of cooling with ascent 
for saturated air (1)—a rate which 
varies chiefly with the temperature 
and hence has no fixed value. 
air ‘mass (II)—an extensive body of 
air which approximates horizontal 
homogeneity. 
characteristic curve (1V)—the curve 
joining the significant points of an 
aerological sounding when plotted 
on the Rossby diagram. 
cold front (V and VII)—the discon- 
tinuity in front of a wedge of cold 
air which is displacing warmer air 
in its path. 
conditional equilibrium (1)—a_ ver- 
tical distribution of temperature 
such that the layer is stable for dry 
air but unstable for saturated air. 
In this case the lapse rate hes 
between the dry and the saturated 
adiabat. Also called conditional in- 
stability, or conditional stability, 
and moist labile equilibriwm. (See 
latent instability.) 
conservatism (I11)—the degree of con- 
stancy of a meteorological element 
when the given air mass is subject 
to modifying factors. 
convective instability (1V)—a verti- 
cal distribution of temperature and 
moisture such that lifting of the 
entire layer will eventually lead to 
instability with respect to dry air. 
In convective instability the equiva- 
jent-potential temperature decreases 
with elevation. Also called “poten- 
tial instability”. “Convective equi- 
librium,’ however, merely means an 
unstable (adiabatic) lapse rate. 
depegram (VIII)—-a curve repre- 
senting the behavior of the dew- 
point with pressure changes for a 
given sounding, drawn on the tephi- 
gram. 
discontinuity (1)—a zone of compar- 
atively rapid transition of the me- 
_teorological elements. These dis- 
continuities are not mathematically 
abrupt, but are rapid transitions 
1Roman numerals following each term refer 
to the article in which the topic is discussed 
in some detail. 
