230 
— 
than 3.42 degrees C. per 100 m; also 
called “auto-convection”, since no 
initial impulse is needed to set it 
off; “‘self-starting”’. 
mixing ratio (111)—the mass of water 
vapor per unit mass of perfectly 
dry (absence of water vapor) air. 
w = 622 e/(p—e) grams per 
kilogram (see symbols) 
modification of air mass properties 
(I1)—the change in the values of 
the meteorological elements within 
an air mass due to such influences 
as radiation, turbulence, subsidence, 
convergence, and so on. These modi- 
fying influences tend to destroy the 
original horizontal homogeneity of 
the air mass. 
negative area (VIII)—the area on a 
tephigram enclosed between the 
path of the rising particle and the 
surrounding air when the rising 
particle is at every stage in its 
ascent colder than the environment. 
neutral equilibrium (1)—a_ vertical 
distribution of temperature such 
that a particle of air displaced 
from its level neither assists nor 
resists the displacement; that is, at 
every level the density of the dis- 
placed particle is equal to that of 
the surrounding air. In the case 
of dry air the corresponding lapse 
rate is that of the dry adiabat; in 
the case of saturated air, the satu- 
ration adiabat. 
occluded front (VII) or occlusion— 
the front formed when and where 
the cold front overtakes the warm 
front of a cyclone. This front 
marks the position of an upper 
trough of warm air, originally 
from the warm sector, which has 
been forced aloft by the action 
of the converging cold and warm 
fronts. Occlusions may be of the 
warm front type in which the air 
in advance of the front is colder 
than that behind, or of the cold 
front type, in which the air in ad- 
vance is the warmer. 
Occlusion is also the term used to 
dencte the process whereby the 
warm air of the cyclone is forced 
from the surface to higher levels. 
AIR MASS ANALYSIS 
partial potential temperature (I11)— 
the temperature a given air par- 
ticle would have if it were reduced 
adiabatically from the pressure 
exerted solely by the dry air to a 
pressure of 1000 mb. 
©, = T [1000/ (p—e) ]©-288 
(see symbols) 
penetrative convection (I1V)—small 
convective up-currents locally pen- 
etrating an overlying more stable 
layer without generally or greatly 
altering the existing atmospheric 
stratification. 
Poisson’s equation (1)—the relation 
between temperature and pressure 
in dry air which is undergoing 
adiabatic transformation. 
if Ro S (p:/ po) ° 7 
polar front (VII)—the frontal zone 
between air masses of polar and 
those of tropical origin. 
positive area (VIII)—the area on a 
tephigram enclosed between the 
path of the rising particle and the 
surrounding air when the rising 
particle is at every stage in its 
ascent warmer than the environ- 
ment. 
potential temperature (I1)—the tem- 
perature a given particle of air 
would have if it were reduced adia- 
batically to a pressure of 1000 mb. 
© = T(1000/p)°™™ 
pseudo-adiabatic (1V)—the process 
wherein a saturated air particle 
undergoes adiabatic transforma- 
tions, the liquid water being as- 
sumed to fall out as it is con- 
densed. 
pseudo-adiabatic chart (III, VIIT)— 
an adiabatic chart on which wet 
adiabats are also drawn; lines of 
saturation specific humidity are 
usually added too. Used for analyz- 
ing stability conditions in a sound- 
ing (see emagram). 
pseudo-(latent) instability (VII1)—on 
the tephigram, when the area of 
positive energy is less than the area 
of negative energy. 
relative humidity (II)—the ratio of 
the actual vapor pressure and the 
maximum vapor pressure possible 
at the same temperature. 
f= e/e (see symbols) 
