DISCUSSION 
with maritime clouds, which are so ready to do 
so. 
Dr. W. E. Howell—There seems to be a real 
difference here between conclusions as to the 
effect of vertical velocity. Computations by 
Neiburger and myself, among others, seem to 
show that different nuclei spectra produce clouds 
in which the difference is much less marked be- 
cause of the compensating effect that, if there 
are few nuclei, the supersaturation is forced 
higher, and a larger proportion of them is ac- 
tivated. These computations in general led to 
clouds more uniform than are found in nature, 
while your figures show the computed droplet 
concentration more disperse than the natural 
ones. I am led to wonder whether new classes 
of nuclei were brought into your computations 
at close enough intervals to follow the natural 
process. 
Dr. Squires—The compensating effect noted 
by Dr. Howell certainly exists; with a maritime 
spectrum of nuclei, the supersaturation maxi- 
mum is twice or three times that computed for 
a continental spectrum. However, the observed 
nucleus spectra show a much slower increase in 
accumulated number with increasing supersatu- 
ration than has usually been assumed in ecaleula- 
tions of the condensation process. As a result, 
an increase in the supersaturation consequent 
upon an increase in the assumed updraft speed 
does not result in so large an increase in the 
number of activated nuclei. The form of the ob- 
served nucleus spectrum is such that the com- 
puted cloud droplet concentration increases only 
as about one-fifth power of the updraft speed. 
As regards the last point, the droplet concentra- 
tions were estimated by an analytical method 
which gives an upper and a lower bound. The 
starting point was, of course, a curve fitted to 
the observed nucleus spectra. 
Dr. Howell—I would also like to question 
further your assumption that vertical velocities 
are roughly the same in maritime and continen- 
tal Cumulus. Malkus has shown that maritime 
Cumulus are generally rootless, without convec- 
tive currents extending below the cloud base, the 
energy for the convection being supplied by the 
condensation within the cloud and the convec- 
tive currents becoming stronger as they rise 
farther above the cloud base; whereas with con- 
219 
tinental Cumulus the convective currents origi- 
nate near the ground and, especially in small 
Cumulus, often deerease in intensity above the 
cloud base. Even within the cloud, convective 
currents in maritime Cumulus are generally cited 
as being weaker than in continental Cumulus. 
Finally it is my experience that the character- 
istic differences between maritime and continen- 
tal clouds appear even within one and the same air 
mass between clouds that form over the sea and 
those that form nearby over small tropical is- 
lands, where vertical velocities are perhaps the 
principal differences involved. 
At Mount Washington we have found nearly 
the same differences in droplet concentration be- 
tween maritime and continental trajectories that 
you cite, namely, about 300 per em* in west 
winds and 60 per em* in east winds; but meas- 
urements of cloud thickness and liquid water 
content seem to show that the high concentra- 
tions occur in clouds formed by slow warm- 
front lifting. And so I do not think that vertical 
velocity differences should be ruled out yet as a 
major factor in determining the droplet concen- 
tration in clouds. 
Dr. Squires—It is probably true that up- 
drafts are somewhat stronger in continental 
Cumuli than in maritime clouds of the same ver- 
tical extent. Our maritime clouds were, on the 
average, a little deeper than the continental ones. 
In any case, a comparison of subjectively clas- 
sified turbulence is an extremely crude way of 
comparing updraft speeds. However, because of 
the slight dependence of droplet concentration 
on updraft speed, this matter is not one of 
great significance. Thus, if the average updraft 
speed were ten times greater in continental Cu- 
muli, the nucleus spectra being the same, the 
ratio of droplet concentrations (continental/ 
maritime) should be about 1.5; the observed 
ratio is about 5. 
Concerning Dr. Howell’s observations of clouds 
over and near tropical islands, the appearance 
of a cloud and its chance of producing rain both 
depend, no doubt, on the pattern of convection 
below and within it, as well as on its micro- 
structure. It is relevant to point out that, with 
the exception of one point, all the data of Figures 
4 and 5 were obtained one to two hundred miles 
inland. 
