SECTIONAL TRANSACTIONS .—Af. 463 
Discuss1on on Condensation of water in the atmosphere :— 
Dr. G. C. Stvpson, C.B., F.R.S.—Problems of the condensation of 
water in the atmosphere (11.20). 
A simple straightforward description of the physical processes involved 
in the condensation of water in the atmosphere leading up to precipitation 
would appear to be as follows : 
The nuclei of condensation are hygroscopic particles, mainly sea salt. 
These collect water and grow in size with increase of relative humidity, 
but remain invisible even in saturated air except as a haze. When air is 
cooled below the dewpoint, water is deposited on the nuclei, and a mist, 
fog, or cloud is produced. Continued cooling (within ascending air currents) 
causes growth of cloud particles, until their increased size, augmented by 
collisions, results in rain. 
This simple statement will be considered in the light of the Pole wine 
difficulties or problems : 
(a) Kohler’s statement that if p is the concentration of salt in water 
derived from clouds (grams per litre), then p = p. 2” in which pois aconstant 
and 7 is an integer. 
(6) Kéhler’s similar but quite unrelated statement, that if v is the volume 
of a cloud particle, then v = vo2” in which ¥o is a constant and n is an integer. 
(c) Why do some clouds rain and others not ? 
(d) Is coagulation caused by (1) the relative motion of drops of different 
size, (2) the turbulent motion of the atmospheres, (3) electrical charges ? 
(e) Bergeron’s statement that no rain (other than fine drizzle) occurs 
without the presence of snow in the upper part of the cloud—the melting 
snow-flakes being the origin of the raindrops. 
(f) Do certain sizes of raindrops occur more frequently than others, as 
first suggested by Defant, and later by K6hler and Niederdorfer ? 
(g) The part played by radiation in fog. 
(h) The diurnal variation of fog. Is Entwistle’s explanation of the high 
frequency of fog just after sunrise satisfactory? (Jour. Roy. Aeronautical 
Soc., 1928). 
(t) Optical phenomena show that spherical cloud particles occur at very 
low temperature—e.g. cirro-cumulus clouds, fogs in polar regions, etc. 
What is the physical state of these particles ? They cannot be supercooled 
water because they appear in air masses, the temperature of which is always 
far below the freezing-point, and it is difficult to see why sublimation should 
not build up crystals at such low temperatures. 
Mr. H. L. Green.—A critical study of direct methods for determining 
the number and size-frequency of particles in aerosols (11.40). 
The ranges of sizes of particles found amongst atmospheric nuclei, dusts, 
fogs, clouds and other aerial disperse systems are considered, and direct 
methods for determining the number and size-frequency of such particles 
are critically examined. The study is confined mainly to condensation 
(Aitken), photographic, microscopic, ultramicroscopic and other optical 
methods, particular attention being paid to the accuracy and limitations of 
each method. 
Prof. J. J. Notan and Mr. J. P. Ryan.—Discharge from a raindrop 
in an intense electric field (12.0). 
When a drop is exposed to an intense electric field, it becomes pulled 
out and begins to discharge. Negative discharge is greater than positive 
