382 DISCUSSION 
tive lapse rate. There are some cases where this 
is not so simple, and I am sure there are errors 
in trying to decide where the tropopause was. 
Major Currie S. Downie—Tropopauses in gen- 
eral are not like a wall or curtain that stay in 
one position from one sounding to the next. 
These things can vary considerably over a 12- 
hour period; time and space variations may be 
appreciable. 
Mr. Donaldson—I am sure we can have a 
possible six-hour error or a 150-mi error from 
the time it is measured. But the time and space 
errors probably average out over hundreds of 
items of data. 
Dr. R. Wexler—I enjoyed the paper very 
much, but I want to put in this suggestion about 
the use of statistics. I have noted that down in 
the tropics, for example, it is very difficult to 
tell where rain will develop on the basis of sta- 
bility alone; even with a very unstable sound- 
ing no rain may develop. Whenever anybody 
determines the frequency of hailstorms asso- 
ciated with large positive energy, I would lke 
to ask the frequency of clear weather also asso- 
ciated with large positive energy. 
Mr. Boynton Beckwith—Forecasters know 
better than the dynamic meteorologists, that 
thermodynamic curves are very poor forecast- 
ing tools by themselves. 
Dr. Hitschfeld—I agree with you. Positive 
energy areas are a very poor indicator for hail 
occurrences both in Denver and Alberta. I 
nevertheless believe that the Fawbush and Mil- 
ler criteria are perfectly valid for the region 
that they are worked out for. I think they have 
a physical basis and have been verified empiri- 
cally, but they do not seem to be exportable to 
Canada or the Rockies. 
Major Downie—Recently I made a study con- 
cerning the frequency of occurrence of two-inch 
hailstones, and find in this country the maxi- 
mum frequency occurs in the Great Plains re- 
gion; they almost never occur along the sea 
coast and only occasionally in the Middle West. 
I would like to find out if there is an explana- 
tion for this distribution. How can we explain 
the fact that practically all the two-inch hail 
in the country occurs over the land with an ele- 
vation of 3000 to S000 ft, whereas in southern 
United States practically no large hailstones are 
reported. 
Mr. Jerome Namias—Actually we are here 
dealing with large-scale dynamic climatology, 
and a very complex problem. However, there 
are some points which are fairly straightforward. 
In the first place very deep layers of unstable 
air are required for hail and tornadoes whatever 
way you look at it. These are manufactured 
especially im spring and early summer in a 
manner that favors the Middle West. Usually, 
the deep moist layers are produced this way: 
As the spring sun begins to warm southern 
United States and the Bermuda High anticy- 
clone begins to build, a shallow layer of warm 
moist air flows northward (see warm moist 
tongue in Fig. 13). At the same time mid- 
Fra. 13—Schematic differential flow pattern in spring leading to severe 
thunderstorms, hail and tornadoes, especially over shaded area 
