THUNDERSTORMS 63 
favorable while flat Cu (Cu. humilis) 
covering most of the sky are contra- 
indicative of thunderstorm activity. 
There is, however, a limit to the nar- 
rowness of Cu, for tall narrow clouds 
are apt to be torn apart and dissi- 
pated by turbulence and wind shear. 
Thus there is an optimum size and 
space distribution for Cu in order for 
Cb to develop. 
Although the following rule is more 
or less obvious, it is important enough 
to italicize: Before attempting to 
use the indications of a_ tephi- 
gram in forecasting local convective 
showers make certain that no fronts 
will enter the field within the period 
for which you are forecasting. Thus 
the intelligent use of upper air data 
goes hand in hand with a reliable 
analysis of the synoptic chart. 
B. Thunderstorms in thermody- 
namically cold air masses. In this 
type of thundershower the steep 
lapse-rate in the surface layers is 
established chiefly by the transport of 
a cold (usually Pc air) current over 
a relatively warm land surface. The 
Pc air mass is considered cold in a 
thermodynamic sense, because it re- 
mains continually colder than the sur- 
face over which it is travelling. Con- 
sequently, the lapse-rate in its lowest 
layer must be steep. The situation is 
characterized in winter by instability 
snow flurries, in spring and early 
summer by instability showers which 
infrequently develop into thunder- 
showers, and then usually mild, for 
the necessary ingredients of a well 
developed thunderstorm, air with high 
moisture and heat content, are lack- 
ing. 
In summer time this kind of 
thunderstorm is not common, its for- 
mation apparently being hindered by 
the gradual transformation of the 
Pc properties as the air mass moves 
southward over warmer surfaces. In 
the spring, on the other hand, the 
Pe air may be transported from its 
snow-covered source region to the 
warmer bare-land surface in such a 
short time that the upper layers have 
little time to warm, and consequently 
very steep lapse-rates are established. 
Another restraining influence on 
the formation of this type of thunder- 
storm is the presence of surfaces of 
subsidence within the cold air, which 
act as “lids” that stop the upward 
growth of the Cu clouds in the cold 
air; it requires considerable energy 
for the rising air to penetrate these 
stable surfaces. The limiting sub- 
sidence layers generally lie within the 
cold air in the shape of a vast, flat 
dome—the top being near the center 
of maximum pressure rise (anallo- 
bar), the edges becoming lower as 
the front boundary of the cold air 
mass is approached. Consequently, 
for some distance behind a cold front 
conditions are unfavorable for the 
type of shower in question, for here 
the surface of subsidence is lowest. 
Moreover, near the front the stability 
of the frontal (vertical) transition 
zone helps to check the convection in 
the cold air. (See Figs. 17 and 18.) 
It is difficult to apply the tephi- 
gram to these situations. One cannot 
make certain of the extent of the 
warming or humidification of the low- 
est layers of the air mass, or for that 
matter, the changes in structure 
which may go on aloft. A sequence 
of tephigrams constructed from 
flights at fairly short intervals is the 
most helpful. Lacking these, perhaps 
the best indications are expressed in 
the history of the cold air mass traced 
through analysis of the surface 
charts. 
II. FRONTAL THUNDERSTORMS 
A. Associated with a cold front. 
Cold-front thundershowers are the 
