THUNDERSTORMS 65 
along the front and supplying the 
necessary “trigger action” to release 
the energy in the warm air. The 
showers apparently form both in 
advance of and behind the front, but 
the structure and action of these 
waves is not fully understood. Ex- 
cellent examples of these quasi-sta- 
tionary frontal thunderstorms can be 
observed in summer over New Eng- 
land. 
B. Thunderstorms associated with 
a warm front. Thundershowers as- 
sociated with a warm front are not 
nearly so common as those with a 
cold front, the reason being that the 
vertical motions brought about by a 
warm-front surface are not as pro- 
nounced as with a cold front. Never- 
theless, in favorable circumstances 
one may observe on the surface 
weather map the outbreak of a large 
number of thunderstorms definitely 
associated with a warm front. It 
appears that this type is almost 
entirely confined to fronts where Te 
is concerned, particularly in autumn, 
winter and spring; there are occa- 
sional summer cases, where the 
warmer air mass is a transitional 
polar current whose temperature and 
moisture content have been increased 
by stagnation over the southeastern 
U. S. almost to the characteristic 
values for tropical air. These warm- 
sector air masses capable of produc- 
ing warm-front thundershowers are 
well supplied with potential energy, 
which is on the verge of being re- 
leased. The releasing agent or 
“trigger action” necessary for the 
transformation of this potential 
energy into the kinetic energy of a 
thunderstorm is the upward deflec- 
tion of the warm air over the cold 
wedge.+ 
Another important factor is the 
horizontal convergence of air in the 
warm sector; this serves to steepen 
‘perature are 
the lapse-rate in the warm air, for 
the surfaces of equal potential tem- 
stretched vertically.* 
This process may be considered the 
reverse of subsidence, wherein hori- 
zontal divergence occurs and the 
lapse-rate becomes more stable. With 
horizontal convergence in the warm 
sector, the air involved is so trans- 
formed that the relatively small 
vertical motions supplied by ascent 
over the cold wedge are sufficient to 
develop thunderstorms. It is obvious 
that this same conditioning process is 
also important in setting the stage 
for other types of thundershowers. 
The major action in warm-front 
thundershowers takes place aloft. 
The storms are usually less violent 
than other types. Furthermore, the 
characteristic cloud display may be 
obscured by the warm-front cloud 
deck. 
The diurnal variation in frequency 
of these showers is not as pronounced 
as with the other types, particularly 
in the non-summer months, when 
they seem to be about as frequent 
at night as by day. 
Just as with quasi-stationary cold 
fronts, waves may form on a slowly 
moving warm front and may at times 
be responsible for the development 
of thunderstorms along it. It is not 
rare to find a comparatively sudden 
outbreak of violent showers along an 
east-west front separating Ta from 
Pc air, and usually this can be traced 
to some wave action which has re- 
leased large amounts of indicated (on 
the tephigram) energy in the TG air. 
In using the indications of the 
tephigram for forecasting Wwarm- 
front showers one must locate accu- 
+In India it has been shown that the cold 
air descending out from under one thunder- 
storm often spreads out along the surface as 
a local cold wedge and acts as a trigger to 
set off new thunderstorms nearby.—Ed. 
*This is true only in case the original lapse 
rate was stable, otherwise convergence would 
tend to stabilize the air.—Kd. 
