THUNDERSTORMS 67 
country than over flat terrain. Be- 
sides there are many situations in 
which thunderstorms form only in 
that part of an air mass overlying 
certain mountain regions. The reason 
for this seems to be that rough ter- 
rain increases vertical turbulence, and 
slopes force marked ascent of the air. 
These upward deflections may be 
enough to set free any large stores 
of energy available in the air masses 
(especially the conditionally un- 
stable). Solar radiation on moun- 
tains is more intense, and it is also 
conceivable that the higher angles of 
incidence of the solar rays on some 
mountain slopes may favor convec- 
tive thundershowers. 
A characteristic of many of these 
showers in mountainous regions is 
their tendency to remain almost sta- 
tionary, presumably because the up- 
ward deflection of the air flow over 
the mountain is operative in only 
certain localities (windward sides) 
while dissipating forces are at work 
in others (leeward). 
Another type of orographic thun- 
derstorm is the coastal shower in 
conditionally unstable maritime air 
masses, frequent on the Pacific north- 
west coast of the U. S., where the 
ascent of fresh PP air up the coastal 
slope and ranges supplies the “trigger 
action.”’ A similar phenomenon 
occurs in conditionally unstable (or 
convectively unstable) Te air masses 
invading the country along the Gulf 
coast, where the slight elevation of 
the land and initial heating over land 
appears to be sufficient to produce 
showers. 
IV. THUNDERSTORMS IN HoRIZON- 
TALLY CONVERGING AIR CURRENTS 
It has been pointed out that hori- 
zontal convergence of air, through its 
vertical spreading action, makes the 
lapse-rate steeper. This process is 
undoubtedly present in most open 
warm sectors. There are many cases 
in which a bent-back occlusion in 
some manner establishes behind it a 
strong convergent field, so that the 
front appears to be elongating itself. 
In the U. S. this convergent zone is 
generally accompanied not only by 
an abrupt wind-shift, but also by 
typical cold-front characteristics; and 
there presumably are cases of such 
zones of convergence in which there 
is a pronounced wind-shift but no 
discontinuity in temperature. Such 
a zone cannot be called a front. 
The Norwegian meteorologists have 
found several cases of this kind over 
Europe.’ The characteristics of such 
a trough of low pressure are similar 
to a front. Owing to the converg- 
ence there is frequently precipita- 
tion. As far as I am aware, no one 
has published an analysis of such a 
case in the U. S. It seems probable 
that bent-back occlusions here almost 
invariably regenerate into cold fronts, 
because the air masses concerned 
(usually Pc or NPP) seem to undergo 
- modification along their trajectory 
more rapidly than the air masses over 
Europe (moist maritime-polar cur- 
rents) (see Petterssen: Contribution 
to the theory of Frontogenesis). 
Another type of convergence, which 
might have been discussed under the 
heading of occlusions, is that forcibly 
brought about by the rapid occlusion 
of a cyclone at its center. This 
occlusion may eventually lead to 
thunderstorms but only when the cold 
front rapidly cuts off the warm sector 
and the warm air is not very stable. 
This kind of thundershower is often 
observed over southern New Eng- 
6J. Bjerknes: Investigations of Selected Eu- 
ropean Cyclones by Means of Serial Ascents 
(Case 3: Dec. 30-31, 1930), Geofysiske Pub- 
likasjoner, Vol. XI, No. 4, 1985 (2:00 Kr.). 
