ON THE PHYSICS OF CLOUDS AND PRECIPITATION 
desirable. Perhaps rapid evaporation of sea water in the 
absence of spray forms nuclei. Aitken’s observation that 
nuclei are formed by the action of sunlight on salt- 
water beaches should be followed up. Dessens [9] found 
that salt droplets become supersaturated when the 
relative humidity is decreased and finally crystallize 
explosively, occasionally resulting in rupture. He ten- 
tatively offered this as a possible mechanism for the 
production of small condensation nuclei from the rela- 
tively large crystals formed when sea spray evaporates, 
but later expressed the opinion that the fracture 
mechanism is too rare to be of major importance. 
The sulfuric acid i the atmosphere must come pri- 
marily from the sulfur in coal, although volcanic activity 
may contribute a small amount. Much of the research 
on nuclei has been done in industrial areas such as the 
British Isles and Germany, so that the importance of 
sulfuric acid nuclei may have been overemphasized. 
Although the high concentrations of nuclei in industrial 
areas is almost certainly due to the products of man- 
made combustion, no one believes that the cloud and 
precipitation regimes of the world have been greatly 
altered by industrialization. It appears that nitrous 
acid nuclei are also produced by combustion but here it 
is the high temperature which acts as a catalyst to form 
the nuclei from the natural constituents of the air. 
Forest fires are as effective for this as man’s furnaces. 
It is also known that nitrous acid is formed by the 
action of lightning discharges. It may be that nitrous 
acid is the principal kern material and that the high 
kern-concentrations in urban areas are due to the 
products of combustion and are of only local impor- 
tance. Adel [1] and Shaw and collaborators [48] have 
obtained spectroscopic evidence of the presence of about 
1 cm of nitrous oxide at NTP in the atmosphere. 
There is no information on the vertical distribution of 
the nitrous oxide but these observations suggest that it 
is a universal constituent of the atmosphere. There 
may well be other substances, even nonhygroscopic 
ones, obscured by the abundance of other nuclei in 
industrial areas, which are the really important nuclei of 
the free atmosphere. 
Size and Size Distribution of Nuclei. The upper 
limit of nucleus size is set by the settling rate and also 
in some cases by the method of formation. As an 
example, the largest sea-salt nucleus probably has a 
diameter of the order of 5 X 10-* cm. Because the work 
of subdivision of a solid or a liquid increases rapidly 
with the amount of new surface formed, the diameter 
of the smallest sea-spray nucleus or dust particle pro- 
duced by erosion or grinding is of the order of 10-° em. 
Diameters of nuclei formed from gases such as sulfuric 
and nitrous acids probably range from 10-* to 10-° cm. 
Such nuclei are not apt to be as large as sea-salt nuclei 
because of the small concentration of the gases from 
which they are formed. All sizes within these rather 
wide limits are to be expected. (For completeness it may 
be mentioned that small ions with dimensions of the 
order of 10-7 em will serve as nuclei only at four to five 
fold supersaturations and cannot play any role in 
natural atmospheric condensation.) 
167 
Only the larger nuclei can be measured with the visual 
microscope. The electron microscope opens the way for 
the measurement of smaller nuclei if a means can be 
found for the collection of the nuclei on the stage of this 
instrument. Most measurements of the size of nuclei 
have been made by indirect means. The large or Lan- 
gevin ions of the atmosphere are generally believed to 
be condensation nuclei which have captured a small 
ion or an electron. Boylan [5] states that about 60 
per cent of the nuclei carry a single electronic charge 
and are identical with the large ions. The mobility 
(velocity in unit electric field) of such ions may be 
measured and their size computed from Stokes’ law. 
Such measurements yield diameters in the neighbor- 
hood of 10-> em. 
As already mentioned, the size of the nucleus may be 
determined by assaying a bulk sample of cloud or fog 
water for the assumed constituent and dividing by the 
number of drops represented in the sample. This was 
done first by K6hler [27], who collected rime at a 
mountain observatory and determined the chloride con- 
tent by titration. The mean drop size was measured 
by the corona method. By assuming that the other 
anions of sea salt were present in the same proportion as 
in the sea, he computed the average mass of the nuclei 
to be 1.847 & 10-™“ g, equivalent to a diameter of about 
5 X 10->cm. This procedure is open to the criticism of 
Findeisen [12] that the salt might be in the form of a 
few relatively large particles. 
Direct measurements of the size of nuclei have been 
made by Dessens [9] and by Woodcock and Gifford 
[57]. Dessens caught the nuclei on spider threads and 
examined them with a visual microscope. He reported 
radu of nuclei ranging from 0.3 to 0.5 uw. The nuclei 
were in the form of drops at a relative humidity of 60 
per cent. There were others too small to measure, and 
also some larger ones. Woodcock and Gifford collected 
nuclei on glass slides 1 mm by 15 mm in size from an 
airplane flymg over the ocean. The counts were cor- 
rected for the collection efficiency of the slides. They 
identified the nuclei as sea salts by determining the 
relative humidity at the transition between crystal and 
solution. They present their data in form of the mass 
distribution of the nuclei. The largest nuclei had a mass 
of about 2 X 10 g and the smallest a mass of near 
5 X 10-“ g. These weights correspond to diameters of 
24 and 0.7 pw respectively at a relative humidity of 
80 per cent. When plotted on logarithmic paper, Wood- 
cock and Gifford’s distribution curves of nucleus mass 
versus number are nearly linear with negative slopes. 
Their method did not permit the collection of nuclei 
of mass less than 5 X 10-“ g. The total number of 
nuclei in a cubic centimeter of air was found to range 
from less than one to about thirty and to decrease 
rapidly with elevation in the first 300 m above sea 
level. Although no simultaneous measurements with an 
Aitken counter are reported, it is probable that the 
Aitken count would be large compared with that of 
Woodcock and Gifford. There is no evidence that these 
unmeasured nuclei are composed of sea salts. 
The methods outlined above of determining the size 
