54 AIR MASS ANALYSIS 
large inversion in the surface layers 
is for the most part a radiation in- 
version. Under the effect of insola- 
tion as the day progresses the air 
near the ground may be expected to 
warm up considerably. Therefore the 
point A will be transferred to higher 
and higher temperatures while the 
pressure remains essentially the same. 
Thus A moves along the 720 mm iso- 
bar until the maximum temperature 
is reached. It is for the forecaster 
to decide what this value is most 
likely to be. In this case let us say 
that the temperature will rise to 
35°C (95°F). A is thereby shifted 
along its isobar to J. Let us con- 
sider that there is no addition or 
subtraction of moisture so that the 
specific humidity remains constant at 
about 14¢/ke. Then as J rises its 
path is given by the dashed line; it 
follows the dry adiabat until its tem- 
perature falls to the value where the 
specific humidity (14g/kg) is the 
saturation quantity. Thus J is moved 
horizontalty until it intercepts the 
14 g/kg moisture line at L. Beyond 
L the path of a rising particle is 
given by the pseudoadiabat LN. 
The lapse-rate in the surface layer 
has been materially changed since the 
morning hours, and in view of con- 
vective mixing it is safe to assume 
that a dry adiabatic lapse-rate has 
been established up to the point K. 
Along JK there is neutral equilibrium 
with respect to dry air. That is, a 
particle of air along JK that is ver- 
tically displaced is neither assisted 
nor resisted by the density distribu- 
tion. But while this adiabatic lapse- 
rate is built up, the structure of the 
air aloft, barring any change of air- 
mass properties, remains essentially 
unchanged. Thus above K, the point 
of intersection of the isentropic line 
and the original sounding, it is 
assumed that there are no major 
changes in air-mass properties. Con- 
sequently there still remains in the 
layer KD a portion of the original 
ground inversion. Now if a particle 
at J is forced to rise, following the 
path KL, its temperature at each level 
(defined by a particular isobar) 
would be lower than that of its sur- 
roundings. In fact, even after satu- 
ration at L the rising particle would 
remain colder than its environment 
until the point M when the tempera- 
tures of the rising air and its sur- 
roundings would become just equal. 
Throughout the layer from K to M 
work is required to lift a particle 
forced upward from K. The amount. 
of energy required for this purpose 
may be shown to be equal to the 
vertically hatched area KLMEDK. On 
the original scale of the diagram from 
which Fig. 13 is reproduced, 1 square 
cm is equivalent to 2x10° ergs per 
gram; in Fig. 138, 1 square cm is. 
equivalent to 3.3 x 10* ergs per gram. 
Beyond M, a rising particle, follow- 
ing the pseudoadiabat MN at every 
stage in its ascent, would be warmer 
than the surrounding air. Im this. 
manner, after reaching M. it will rise 
of its own accord, so to speak, for 
it is now less dense than the air com- 
posing its environment. From energy 
considerations it may be shown that 
the energy liberated by a unit ele- 
ment of air rising from M to N is 
given by the horizontally hatched 
area MFGHNM. 
(In practice it is customary to color 
in red the horizontally hatched area 
and in’ blue the vertically hatched 
area.) 
To generalize: 
Where the path of a rising particle 
lies above the tephigram of the sound- 
ing, energy is available for producing 
overturning, which may result in 
thundershowers, and the amount of 
this energy is given by the area (in 
