64 AIR MASS ANALYSIS 
most interesting and generally the 
most violent of all. In the older 
terminology they are the “‘line squall’ 
thunderstorms. The predominating 
eause appears to be the mechanical 
upthrusting of the warm air by the 
advancing cold wedge. The indica- 
tions for thundershowers with the 
passage of a cold front are to be 
found largely in the upper air char- 
acteristics of the warm air mass and 
in the structure and movement of 
the cold front. The tephigram is 
well-adapted to these cases. The 
soundings used should naturally be 
those in the warm sector. As before, 
a sequence of soundings at one sta- 
tion is best, but if this is not pos- 
sible it is advisable to study several 
tephigrams that show the trend of the 
change in the characteristics of the 
warm air mass. Here again, increas- 
ing positive areas and decreasing 
negative areas are most favorable 
indications for thunderstorms, par- 
ticularly if the current tephigram 
shows appreciable positive areas. The 
most indicative lapse-rates are con- 
ditionally unstable. Convective in- 
stability is also generally indicated, 
for it may be that the lifting of entire 
layers is responsible for the genesis 
of the showers. 
In forecasting cold-front thunder- 
showers it is important to consider 
the diurnal change in lapse-rate. In 
the early morning hours there is 
usually a ground inversion present, 
while in the afternoon the surface 
layers are characterized by a super- 
adiabatic lapse-rate (Fig. 19). Then 
the air mass is in its most favorable 
state for producing thunderstorms once 
the “trigger action” of a cold front 
is supplied. It is for this reason that 
cold fronts passing during the night 
are frequently accompanied by no 
thunderstorms, while the same fronts, 
passing during the afternoon or early 
evening, are characterized by thun- 
derstorms all along their length.* 
The cold air immediately behind 
the front is cooled by evaporation of 
the falling rain; in this manner the 
sharpness of the front is maintained. 
The air masses preceding the front 
on which cold-front thunderstorms 
may be observed hardly seem to be 
restricted to any individual types, 
although they occur with a pro- 
nounced maximum frequency in TG 
eurrents, and only rarely (if ever) 
in the dry currents which move from 
the southwestern U. S. and Mexico. 
Cold-front thunderstorms may be 
further classified as prefrontal, front- 
al and postfrontal, according as they 
occur appreciably before, at, or after 
the front passage. We need spend 
little time here on the fine points of 
this classification. Prefrontal types 
may be due to mechanical uplifting 
of the warm air some distance ahead 
of the cold front, or to the entrap-. 
ping of the warm, moist air below the 
overrunning squall head. In the 
latter case it is conceivable that ex- 
tremely steep lapse-rates and violent 
storms may be brought about. Those 
thunderstorms occurring immediately 
at the front are presumably the nor- 
mal type due to uplifting. The post- 
frontal thunderstorm probably be- 
longs, for the most part, to the class 
discussed as “thunderstorms in 
thermodynamically cold air masses.” 
An interesting situation occurs 
when a cold front slows up in its 
movement and becomes quasi-station- 
ary. Then thunderstorms may be 
occurring at various points along the 
front and persisting for a relatively 
long time. These are associated with 
small wave disturbances moving 
4This is brought out clearly in the statistical 
frontal study of Thunderstorms in Ohio during 
1917, by W. H. Alexander, C. F. Brooks and 
G. H- Burnham, Mo. Weuther Rev., July, 1924, 
Vol. 52, pp. 343-348. 
