166 
supports Aitken’s conclusion that the hygroscopic nuclei 
are the important cloud producers and that neutral 
dust is of lesser importance. Boylan [5] found in Dublin 
that the number of kerns determined with the Aitken 
instrument averaged fifteen times the number of dust 
particles determined with the Owens impact dust 
counter. 
Aitken and many others have established that flames 
and burning materials form tremendous numbers of 
nuclei and also that heat alone, as from a heated plat- 
inum filament, will form nuclei in natural air. Aitken 
and also Coste and Wright [6] showed that nuclei could 
be formed by spraying sea water. The latter authors 
also found that fuming sulfuric acid was an active kern 
producer. Although flames produce a variety of products 
it has been established that substances containing sulfur 
are the most effective fuels. This is significant because 
of the sulfur content of coal. When sulfur is burned the 
nonhygroscopic dioxide is formed. This does not easily 
oxidize to the hygroscopic trioxide in air at normal 
temperatures. Aitken found that both ozone and hydro- 
gen peroxide are effective oxidizers of sulfur dioxide. 
He also claimed that ultraviolet solar radiation oxidized 
the sulfur dioxide. Coste and Wright [6] suggested that 
at temperatures above 620C, and in the presence of 
water vapor, hygroscopic nitrous acid is formed. This 
would explain the production of nuclei by hot filaments. 
They also suggested that the nitrous acid is the oxi- 
dizing agent for sulfur dioxide. 
There is general agreement that sea salts are effective 
as condensation nuclei. Sea-salt crystals have been 
observed in the atmosphere by Owens [43], Dessens 
[8, 9], and Woodcock and Gifford [57]. Kohler [27, 28] 
and others have demonstrated the presence of chloride 
ion and of other sea-salt anions in water from clouds, 
fogs, and rain. If the amount of chloride ion, for 
example, as measured in a bulk sample is divided by 
the number of drops, it yields a nucleus of reasonable 
size. Any object which has not been carefully cleaned 
exhibits the ubiquitous sodium flame when it is heated. 
On the other hand there is evidence which suggests 
that the sea is not the principal source of condensation 
nuclei. The number of nuclei is much smaller over the 
oceans than over the land. A minimum kern count of 
nearly zero has been observed over the ocean and the 
usual count is from several hundred to several thousand 
per cubic centimeter. Typical values over land range up 
to 100,000 in rural, and a million or more in urban areas. 
Simpson [49] shows that if all active nuclei come from 
the sea surface, the rate of nucleus production must be 
about 1250 sec em. This appears to be unreasonably 
large if the nuclei are formed by the evaporation of the 
spray resulting from wave action. A large proportion 
of the spray drops will be so large that they will fall 
back to the surface. Findeisen [12] holds that the sea 
salt indubitably present in the atmosphere is in the 
form of say 10-20 large particles of about 10—!° grams 
mass per liter of air. Although these are probably all 
active nuclei, their number is small compared to the 
total number of nuclei. Their presence would serve to 
explain the observed chloride content of cloud and 
CLOUD PHYSICS 
rain water. There are isolated observations that the 
evaporation of sea water (without visible spray) pro- 
duces kerns, and Aitken found that nuclei were formed 
by the action of the sun on the foreshore at ebb tide. 
Attempts to identify the nucleus in an evaporating 
cloud drop under the microscope have failed. Dessens 
[8] has caught nuclei from the air on spider webs and 
caused them to grow into drops or to form crystals by 
varying the humidity. These are probably the relatively 
large sea-salt particles referred to by Findeisen [12]. 
Wright [58] has shown that visibility near the seacoast 
is a function of the relative humidity, which can be 
explained by assuming that the nuclei are hygroscopic. 
Wright felt that he had shown inthis way that the nuclei 
in question were sea salts but Simpson [49] argued that 
his procedure did not permit the identification of the 
hygroscopic agent. 
It is generally concluded that most active conden- 
sation nuclei are hygroscopic particles and that the 
nonhygroscopic nuclei are unimportant. More infor- 
mation is required to explain this strong preference for 
hygroscopic nuclei. The hygroscopic and nonhygro- 
scopic kerns are presumably of about the same size; if 
anything, the hygroscopic nuclei are somewhat smaller. | 
It can be shown that the lowering of the vapor pressure 
by the salt is not of major importance, since a slightly 
larger nonhygroscopic particle will support condensation 
at the same degree of supersaturation. It appears from 
Volmer’s work [52] that the wettability of the substance 
plays an important role. On a surface which is not 
wetted by water, condensation occurs in the form of 
small lens-shaped drops. The work required is greater 
than when the surface is wetted and increases with the 
contact angle between the water and the surface. 
Greater work for the phase change implies a higher 
supersaturation. Hygroscopic nuclei are liquid drops 
before cloudy condensation occurs and are therefore 
perfectly wetted. All other nonhygroscopic surfaces are 
less easily wetted and it may be that the nonhygro- 
scopic atmospheric dust is largely hydrophobic. Junge’s 
finding [26] that paraffin spheres will serve as con- 
densation nuclei does not contradict this explanation 
since Volmer shows only that hydrophobic nuclei re- 
quire a greater supersaturation than hydrophilic nuclei. 
Junge did not report on the supersaturations used, but 
since he employed an Aitken‘instrument it may be 
assumed that they were from 200 to 300 per cent. 
An extension of Junge’s work with provisions for vary- 
ing the supersaturation would probably shed some 
light on this problem. 
The available evidence suggests that the hygroscopic 
nuclei are formed of sea salts and nitrous and sulfuric 
acids. Other hygroscopic materials may also be im- 
portant but it has yet to be shown that other such 
substances regularly exist in the atmosphere in suf- 
ficient quantity and in finely divided form. If it be 
assumed that sea-salt nuclei are formed only by the 
evaporation of spray, it must be conceded that the 
source is insufficient to supply a major fraction of 
atmospheric kerns. Further study on other possible 
mechanisms for the formation of sea-salt nuclei is 
