66 AIR MASS ANALYSIS 
rately the position of the warm front 
at the surface, and obtain a fairly 
good idea of its structure. A sound- 
ing made some distance in advance of 
a warm front, if subjected to the 
ordinary analysis for local con- 
vective showers, will rarely show 
thunderstorm indications, because 
here the erroneous assumption is 
made that the surface cold air, in 
reality belonging to the cold wedge, 
will rise through the upper strata. 
The proper procedure in these cases 
is first to identify the discontinuity 
surface separating the two air masses 
aloft, then instead of a surface parti- 
cle assumed to rise take some element 
in the warm sector, ascribing to it a 
probable maximum temperature as in 
the case of air-mass_ convective 
showers, and then plot the ascent of 
this element on the original tephi- 
gram. It is best to choose for the 
rising particle the air at the base 
of the warm current. 
C. Thunderstorms associated with 
an occluded front. Perhaps the larg- 
est number of summer thunderstorms 
are associated with fronts previously 
occluded, which, through their action 
on the field of flow, have regenerated 
into active cold fronts. It is im- 
portant in summer for the analyst 
to follow closely all occluded fronts, 
for it is the rule rather than the 
exception for bent-back occlusions, 
weak as they may appear, to develop 
into surprisingly active cold fronts. 
It is largely the activity of these 
occlusions that makes summer-map 
analysis so interesting. They form, 
as do ordinary cold fronts, a 
pronounced field of convergence, 
making the front sharper and con- 
ditioning the preceding warm air to 
the point where it becomes rich in 
convective energy. In general, for 
purposes of thunderstorm forecast- 
ing, these cold-front occlusions may 
be treated in the same manner as 
cold fronts. 
The occluded warm front is inter- 
esting in that it represents an upper 
cold front marching into the field 
aloft, well in advance of the surface 
warm front; the warm air originally 
above the warm front surface is thus 
displaced by a current of colder air. 
In this way, the lapse-rate above the 
lowest part of the warm front is 
made steeper. These warm-front oc- 
clusions seem to be characteristic 
over the southwestern plains, where 
the TG air is colder than other warm 
air masses (often NTP) which there- 
fore override it, while NprP air, 
warmer at the surface than the Tec, 
but colder than the Ntp, rapidly over- 
runs the Te and displaces the NTP; 
the warmest air is then pocketed be- 
tween the TG wedge and the Npp. 
wedge.’ Thunderstorms are at times 
associated with this structure, though 
their exact mode of formation is 
not as yet definitely known; but it 
appears that the steep lapse-rates 
observed aloft in the NppP air mass 
play an important part once some 
particular body of air starts to rise. 
It is possible that insolational heat- 
ing in the cool wedge (TG) may 
give the initial impulse, while the 
stability of the frontal boundary may 
be insufficient to prevent its penetra- 
tion into the more unstable air aloft. 
In making use of the tephigram in 
these situations one must consider the 
possibility of rapid change in struc- 
ture of the upper air and must be 
familiar with the characteristics of all 
the interacting air masses. 
III. ORoGRAPHIC Z,THUNDERSTORMS 
Practically all types of thunder- 
storms are more frequent over hilly 
and particularly over mountainous 
5Cf. H. Wexler: Analysis of a Warm-front- 
type Occlusion, Mo. Wea. Rev., July, 1935, 
pp. 213-221. 
