DISCUSSION 29 
for that five-day period, but also for the rest 
of the month, was so closely patterned within 
the Ohio Valley that it would be very difficult to 
assume that there could be a large-scale phe- 
nomenon that should, just by chance, direct the 
whole thing, so that you got the maximum Just 
in that valley. I cannot get away from feeling 
that there must be some rather local orographic 
effect. Moreover, the Appalachians have an aver- 
age height of, say, 1000 meters. Clouds in that 
region will miss the lowest thousand meters of 
air, which contain the bulk of humidity, and 
thereby the best opportunity of getting abun- 
dant convective rain is lost. It might start better 
above the mountains, but once it has got started 
it grows better on the plains, as far as I can see. 
There may be other orographie effects also; but 
I want especially to direct your attention to the 
fact that the average maximum precipitation 
in January also falls near the Ohio Valley. 
Dr. C. W. Newton—I studied an example (J. 
Met. Soc. Japan, 75th Anniversary Vol., pp. 
243-245, 1957) that looked very much like the 
one in January 1937, and I just want to describe 
what the precipitation looked like on an hourly 
basis. In that case there was a cyclone that 
looked very much like this one, moving regularly 
eastward as in this ease. The long streak of 
heavy total precipitation in that case was due to 
repeated movements of four or five heavy con- 
vective rainstorms over the same region, which 
I imagine is true also in your example. These 
storms formed every four hours, and the peculiar 
thing was that they moved successively over 
nearly the same track. It was not so surprising 
that their paths were almost in the same diree- 
tion since the upper wind direction did not 
change much, but the odd thing is the rain- 
storms all started in nearly the same place. This 
was down in the southwestern corner of Louisi- 
ana, and at the time I saw this, I thought of 
your suggestion on convergence near the coast 
caused by differences in frictional effects, be- 
cause I cannot see anything else that can pos- 
sibly account for it. There is no great mountain 
chain in southwestern Louisiana so I think it was 
an orographic effect of the kind that you dis- 
cussed, not due to mountains, but to differences 
in friction. 
Dr. Donald M. Swingle—I had oceasion to ob- 
serve the release of rainfall in the vertical for a 
year and a half, and the contrast between the 
fairly orderly release of rainfall in either con- 
tinuous rain or light showers, and the explosive 
periodic mechanism in the thunder showers is 
very clear in the data we have. This also stands 
out in the size distribution of raindrops, which is 
almost the same for shower rain and continuous 
rain. Therefore, relationships between rainfall 
and radar echo intensities are also almost the 
same. I think there is quite a contrast between 
the somewhat constrained semicontinuous or- 
derly released precipitation as you had in these 
orographic cases, and the rather sporadic re- 
lease one has in the strongly convective precipi- 
tation process. 
Dr. Bergeron—So you mean if one knows the 
character of the precipitation, one would be able 
to evaluate radar pictures into rainfall amounts? 
Dr. Swingle—Yes. 
Dr. R. Wexler—I had oceasion to do it by 
radar. It is very difficult to ascertain in advance 
where new storms are going to appear. Quite 
frequently, with a cyclonic system moving into 
New England, conditions appear to be the same 
with regard to instability all over the area. 
Nevertheless, the thunderstorms are bred in a 
few favored areas which do not seem to be too 
dependent on orography, but vary from day 
to day depending on the other conditions. One 
frequently finds that the outbreak of these thun- 
derstorms is associated with mesoscale pressure 
features which are not detected on large-scale 
maps; but I find it difficult to ascribe these small- 
scale features entirely to orography. It appears 
as if the convergence in the area was channeled 
into a few favored Cumulus which then devel- 
oped into thunderstorms. 
Dr. Bergeron—l have now ordered my 
thoughts a little more concerning that Ohio flood 
problem, taking into consideration all that has 
been said here, by Dr. Wexler and others. I 
agree with Mr. Namias that the stage was set 
on the whole by a large-scale arrangement. In 
fact, there is a map showing the average pressure 
distribution for January 1937, and also the wind 
and temperature distribution. All those maps 
show that the trough and convergence between 
the Bermuda high and another high in the west- 
ern United States did not le over the Ohio re- 
gion, but on an average further to the west. The 
stage for the activity was set in another way in 
the east. First of all, during those five days 
frontal cloud areas moved constantly across the 
Ohio region; thus, the upper air was loaded with 
any amount of releaser cloud, so that any 
spender cloud would have been released. Sec- 
ondly, there was so much moisture coming in 
