AIR MASS ANALYSIS 
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unstable (1)—a vertical distribution 
of temperature such that particles 
of air, because of their lesser or 
greater density than the surround- 
ing air, will rise or sink of their 
own accord once given an initial 
impetus up or down. For dry air 
the unstable lapse rate is greater 
than the dry adiabat; in the case 
of saturated air, greater than the 
saturation adiabat. 
upper front (VII)—a front whose 
principal development and evidence 
is in the upper air, usually the 
active wpper cold front of a warm 
front type occlusion. 
vapor pressure (I11)—the partial pres- 
sure of the air exerted solely by 
the water vapor molecules. 
warm front (V)—the discontinuity at 
the front of a warmer air mass 
which is displacing a retreating 
eolder air mass. 
warm sector (VII)—the air enclosed 
between the cold and warm fronts 
of a cyclone. t 
wave disturbance (VII)—a deforma- 
tion produced along a front. These 
waves travel along the discontinuity 
surface often producing new cy- 
clones. 
wet-bulb temperature (III, VIIL1)—the 
lowest temperature to which a 
wetted ventilated thermometer (as 
of a psychrometer, e.g.) can be 
cooled, i.e., by evaporation. while 
not strictly a temperature of the 
air, it is a function of dry bulb, 
relative humidity and barometric 
pressure. On the adiabatic chart 
or tephigram: find the intersection 
of the dry adiabat and specific hu- 
midity line and then follow down 
the wet adiabat to the original pres- 
sure level of the point in question. 
(See: estegram; latent instability; 
pseudo-instability.) The wet adiab- 
ats are isotherms of equal wet-bulb 
potential temperature (also equiva- 
lent-potential temperature). 
