210 
variation with time of the contribution to the 
liquid content by the various droplet groups in 
addition to the liquid content for the whole cloud. 
The droplet group just a little larger than the 
modal size is the one that contributes most to the 
liquid content in most cases. 
Returning to the question of initial conditions, 
what I said previously applies as well to the verti- 
cal velocity as to the nucleus distribution. I would 
like to do the computations with a case in which 
the vertical velocities were actually observed si- 
multaneously with observations of the nucleus size 
distribution, rather than the one we assumed as 
‘representative’. In arriving at the choice we made 
Mr. Chien plotted from the tables in Dr. Malkus’ 
papers the vertical velocities for various positions 
with respect to the cloud base, drew some lines 
through them, and showed the result to me, and 
I said, ‘‘Let’s use this line as a sort of compromise 
among them.” Now, there must have been one 
or more measurements of a meter and a half or 
so per second, or our curve would not have been 
drawn so far over down at the cloud base; but it 
may be that most of the measurements really gave 
lower values, and I am inclined to agree that this 
may have been an unfortunate choice. 
Dr. Tor Bergeron—I would like to connect the 
results especially of the papers by Dr. Neiburger 
and Mr. Mordy with certain things which are 
known to synopticians and those who observe 
clouds. Especially I would like to connect it with 
visibility observations that I have made during 25 
summers up in the Swedish mountains, at latitude 
of 63 1/2° N. I am very pleased to see that gap in 
the size distribution, that gap between the inac- 
tivated nuclei as Dr. Neiburger calls them, and 
DISCUSSION 
the nuclei that really grow. Because in those lo- 
calities of which I spoke, from April to September 
in different months and different years, I had al- 
ways a gap in the spectrum of visibility ranges. 
Visibility could be less than one or two kilometers 
because of fog or mist; but it could never be be- 
tween two and twenty kilometers. Remember that 
we were rather far from the origin of the nuclei, 
that is, from the ocean, and from any industrial 
pollution also. So we had evidently very little 
amount of hygroscopic nuclei. Then we have had 
the visibility from 20 up to 500 km, the latter cor- 
responding to practically pure air. And those 
visibilities, of course, occurred in the presence of 
inactivated nuclei; that is nuclei with sizes gener- 
ally less than 0.1 micron. At those sizes, the 
amount of scattered light will be roughly propor- 
tional to the fourth power of the diameter. In 
order to get a haze by inactivated nuclei of that 
size whose visual range could be comparable with 
that of fog their number should be between 10° 
and 10° times as numerous as the activated parti- 
cles. According to Dr. Neiburger’s diagram, as 
far as I can see, they were more numerous, but 
only 10 to 100,000 times perhaps. This then will 
explain that gap in the visibility spectrum; a gap 
the existence of which has been absolutely im- 
possible for me to get the continental colleagues 
in Europe to understand. Therefore any discus- 
sion in the International Synoptic Commission, 
and other places, of scales of visibility and such 
things was always doomed to fail, because Central 
Europe, England, Eastern United States, and 
even the coasts of the ocean, would all have rather 
plenty of big hygroscopic nuclei that grow with 
increasing relative humidity. 
