156 
RAINFALL mm. One ni 
DISCUSSION 
e Rainfall station 
=== Shore-line 
133m alt. (400 ft.) 
266m alt. (800 ft.) 
aN 
Fig. 5—Stability pattern of rainfall (warm-front with SE wind aloft) in the same region as Fig. 4 
Another situation from the same year, October 
19 (not shown here), also with a warm-front 
rain and south-southeast gradient wind, showed 
practically the same rainfall distribution. From 
these examples you may see what forecasters 
have to cope with! They should be able to tell 
that, in such a situation, there will be heavy 
precipitation (25-35 mm) during 12 hours near 
the shore of Lake Vattern, but only a few milli- 
meters at the shore of Lake Vianern, that is, one 
fifth as much. This whole area is as big as the 
station ring on an American weather map, at the 
scale of one to ten millions. So, you see what we 
are up against; and you may take my word that 
these differences are even stationary with cer- 
tain wind directions. 
Discussion 
Mr. Jerome Namias—You described the al- 
most local character of regional differences in 
precipitation. If I remember correctly you had 
two very low precipitation areas and also rather 
high zones, the high being over a forest. You 
mentioned friction and some other factor as 
causes. I wonder if you could repeat your ex- 
planation. 
Dr. Tor Bergeron—Well, first of all you no- 
ticed in those first cases that there was a maxi- 
mum to the lee of Uppsala, and people hearing 
of or seeing these maps have suggested that nu- 
cleation from Uppsala might have something to 
do with it, and also convection raised by the 
heating. Uppsala is rather small, 70,000 mhabi- 
tants and some industry; therefore I think the 
main cause will be the friction. I think these 
great differences which are constant would prob- 
ably correspond to different kinds of precipita- 
tion efficiency; that is to say, over the plains 
you will have rather great evaporation of the 
from the 
whereas over the forest regions you will have 
perhaps a precipitation efficiency of nearly 100 
percent and then in addition also the water con- 
precipitation falling Nimbostratus, 
tent of those local cloud caps. That might ex- 
plain the very great differences in the amount 
