DISCUSSION 59 
structure the cloud physicists have been at- 
tempting to determine what the differences are 
which decided whether a cloud will precipitate 
or not. 
Synoptic meteorologists had been puzzled for 
many years as to why apparently similar situa- 
tions under some circumstances gave rain and 
under other circumstances did not. Then Profes- 
sor Bergeron came forward with his very re- 
markable explanation in terms of the ice-crystal 
theory. That handled a good many of the prob- 
lems. People began to look at the temperatures 
and degree of supercooling of clouds, and to 
consider the possibility that the supply of ice- 
crystal nuclei might, under some conditions, be 
inadequate. 
Then we became aware that there are many 
cases of rain when clouds are not supercooled; 
in these cases the ice-crystal theory could not 
explain the precipitation. There apparently are 
significant differences even in the micro-structure 
of warm clouds, and we are now trying to see 
what those differences are. Far from concluding 
that every cloud has the same structure, liquid 
content, or nuclei count, we are seeking to 
find out what are the critical differences in them. 
Tn all this we are aware that it is necessary 
to seek the key to the differences in micro- 
structure, and that this key may be found in 
the larger scale air mass properties and flow pat- 
terns. Our hope is that by knowing what dif- 
ferences in micro-structure are involved, the 
synoptic and dynamic meteorologist will know 
what to look for in the large scale structure. 
Dr. Henri Dessens—This is very interesting. 
Especially the organization of the cloud groups 
is striking. They have been observed and de- 
scribed in the equatorial region by V. Schaefer 
(Transactions, New York Academy of Science, 
Oceanography and Meteorology, pp. 535-540, 
1958). In the Congo Basin the organization of 
clouds is one of the most remarkable facts. Ob- 
servations regarding the vertical organization 
of the atmospheric layers in which these clouds 
form have recently been made at Lukoléla (1°S, 
17°E) in the Belgian Congo. The basie very 
persistent fact in rainy or dry seasons is the 
existence of a moist layer at the ground. This 
layer is called the ‘monsoon’ and is in motion 
from WNW to ESE. It is topped by the trade- 
wind layer which moves from ESE to WNW. 
The trade-wind layer is very dry in the dry 
season and sometimes humid in the rainy season. 
During fine weather the humid layer is invariable 
in thickness at about 1200 meters; but during 
the rainy weather great variations are observed. 
Within a few hours one finds successively: 600 
meters, 3200 meters, 600 meters, 2200 meters, 
ete. Deepening of the humid layer permits the 
formation of cumulus-type clouds. If Cumu- 
lonimbus clouds form, the layer may be as thick 
as 7000 meters. The word ‘deepening’ has been 
selected instead of ‘wave’ in order not to an- 
ticipate a wave mechanism for this phenomenon 
which appears so important in the equatorial 
region. 
Mr. Richard G. Semonin—Incidentally, it also 
exists elsewhere; Dr. Fujita and I for instance, 
observed from the aircraft several of these 
streaks over central Michigan, so they are not 
limited to tropical regions. 
Dr. Tor Bergeron—With respect to what Dr. 
Dessens said I wonder if he knows that British 
Colonel Keeling as early as 1908 made a pilot 
balloon expedition to Khartoum and Mongalla 
in Sudan. Durmg a dry week in August at Mon- 
galla (5°N) the monsoon formed a very shallow 
layer, only one to two kilometers thick, but other- 
wise it was six to seven kilometers deep. I have 
worked a little with these data and I have formed 
some ideas as to the structure of the southwest 
monsoon over Africa, and maybe I shall have an 
opportunity to come back to this thing during 
this conference. (See Dr. Bergeron’s comments, 
pages 3899-401—Ed.) 
Dr. Malkus'—The skewed distribution of trop- 
ical rainfall is well known. The question re- 
mained in the minds of some investigators, how- 
ever, as to whether this concentration of rainfall 
into disturbances would be equally pronounced 
over the much vaster tropical oceans as com- 
pared with that over the relatively restricted 
island and continental masses. The opportunity 
to make a fairly conclusive test was offered by 
the Pacifie Marshall area data collected and 
distributed under ‘Operation Redwing.’ Our 
study was begun using the radiosonde and six- 
hourly rainfall data from the atoll of Kwajalein 
(S°43’N, 167°44’E) which extended in time 
from April 15 through July 24, 1956. It is in- 
teresting to note that the ‘equatorial trough’ 
had a mean position in July running right 
*The above comments on tropical rainfall were 
omitted by Dr. Malkus during the conference be- 
cause of the limited discussion period. As they are 
most fitting, Dr. Malkus has kindly contributed 
them in written form from a manuscript: Studies 
of tropical rainfall by J. Malkus and M. C. Thayer 
—Kd. 
