92 DISCUSSION 
tion, persisted for another two to three hours, 
and then began to dissipate. Individual showers 
within the band were on the order of ten miles 
in dimension and lasted about an hour. There 
appear to be certain local regions in which 
showers tend to develop or intensify. 
Dimensions, durations and development of 
areas of heavy rain have been considered in order 
to determine the scale of atmospheric circulation 
which is especially important in the development 
of precipitation. It is hoped that the analysis of 
more cases and different types of storms may 
provide information regarding the dynamics of 
this scale of atmospheric motion, especially con- 
cerning its relation to the larger scale environ- 
ment and to local topography. 
Acknowledgment—The author wishes to ex- 
press appreciation to Searle G. Swisher, Captain, 
U.S. Air Force, who is carrying out much of the 
analysis of the squall lines. The research reported 
on has been carried out under the sponsorship 
of U.S. Army Signal Research and Development 
Laboratory, Contract DA 36-039 SC-75030. 
REFERENCE 
Kopatra, N., An iso-echo contouring device, Proc. 
Sixth Weather Radar Conference, Amer. Met. 
Soc., pp. 307-314, 1957. 
Discussion 
Dr. Helmut Weickmann—What is your opin- 
ion as to the mechanisms for the development 
and propagation of these rain areas? 
Dr. Pauline M. Austin—The fact that we find 
areas of recurrent development suggests topo- 
graphical reasons. If we compare the wind field 
with the areas of development, we can draw 
conclusions regarding the influence of conver- 
gence. On the other hand, although a heavy rain 
area usually develops in a particularly preferred 
region, it then moves along with the wind and 
keeps on pouring out heavy rain for about three 
hours, moving right across the area represented 
on the scope. Clearly there is some dynamic effect 
continuing to produce rain, so I feel sure both 
factors are involved; but we have not analyzed 
enough different cases to provide the answer to 
your question. 
Mr. Jerome Namias—I was very fascinated by 
this. Of course, there are very good reasons, I 
think, for getting recurrence of synoptic scale 
events against a planetary scale background; 
but the amazing part of it is that there are small 
places in a given situation that keep getting 
battered by the same type of thing, and it is dif- 
ficult to attempt to explain it. I do not think it 
comes out of the synoptic pattern itself, and as 
you indicate there is a possibility that the synop- 
tic pattern sets the stage for different orographic 
influences. Some work done by Charles F. Brooks 
in Texas about 1926 consisted in studying the 
distribution of thunderstorms. He indicated that 
those areas which had thunderstorms one day 
would be unlikely targets the succeeding day. He 
explained that it was due to the fact that the 
ground was wet and the heating would not be so 
pronounced. Here is something which is highly 
speculative, but I think it should be considered. 
The second thing is something that I think Dr. 
Squires is going to talk about. There is a possi- 
bility, of course, that variations of the soil and 
ground-cover complex affected by rain or dry- 
ness could induce variations in nuclei kind 
and content and thus in the concentration of 
droplets, which might have some effect. But the 
complexity of these recurrence problems sug- 
gests that we have not only to go deeper into 
the synoptic physies, but that we may also have 
to take into account other factors as well. 
Dr. R. Wexler—The motion of a line depends 
on: (1) the motion of the individual cells, and 
(2) the rate of development of new cells which 
later become part of a new line. In squall line 
situations it is frequently observed that new cells 
develop some 10 miles ahead of the existing line. 
About half an hour later, these new cells become 
the dominant line while the cells of the former 
line are dissipated. On other occasions, the new 
cells develop at the forward edge of the existing 
line evidently by a mushrooming effect. In such 
cases, the question arises as to how it can be 
determined whether a high intensity echo occurs 
in a previously existing cell or in a newly de- 
veloping cell. 
Dr. Austin—The six lines that we have were 
all observed in the summer of 1958. Some cells 
formed ahead of the squall lines and some within 
them. None of the storms was intensely severe. 
What we did in tracking individual cells was first 
to overlay successive maps and select a dark 
