24 TOR BERGERON 
assume that this rain maximum was 
caused by a stationary front along the Ohio 
Valley, but as far as I can see from the weather- 
maps the front was not stationary. Instead, I 
venture to say that what we have here may be 
a special orographie effect that is very compli- 
cated. Time will not allow me to enter in detail 
upon this problem, but I want to direct your 
attention to the possible existence of even such 
queer things as this one in our atmosphere. I 
think that when the bulk of the rain fell, the 
gradient wind was southwesterly and very moist; 
and then there was an overflow of air from the 
south or southeast across the Appalachians be- 
cause of friction, causing a convergence on the 
lee side of the mountain range. In other words: 
a convergence might occur between a direct SW 
flow (corresponding to a SE-NW pressure gra- 
dient) over the Ohio Valley and a SSE flow 
through the Appalachian region (following the 
gradient). Naturally it would be important that 
also the moisture and lability conditions prevail- 
seems to 
ing at the occasion were favorable for some 
precipitation mechanism. There may, of course, 
also be other explanations. 
Figure 16 shows two weather maps from the 
period in question, taken from the Report cited. 
Those two days the general flow was an easterly 
to southwesterly, and there was no stationary 
front over the Ohio Valley, since in this case the 
Polar front had come down to the Appalachians 
and even further south. I sincerely hope that 
some American colleague will collaborate with me 
on this very interesting case, which I have had 
in my mind ever since I found it. 
Here is another case, from Sweden, with a 
stationary cold front extending N-S over cen- 
tral Sweden and a postfrontal precipitation area 
(Fig. 17). That is not unexpected in itself, but 
the rain also extends far west, to the lee of the 
Scandinavian mountain range, and it stayed 
there for three days, giving flooding rains on the 
lee side of that mountain range. How would you 
explain that? In fact, the stream lines of Figure 
aoe 
Scandes 
raa777 Coast 
station 
——» 3 Cold air flow 
— 
L 
= _ warm air flow 
Fic. 19—Tentative schematic map and vertical cross section of rain mechanisms over Scandinavia, 
July 26-28, 1950 
