The Nucleation and Growth of Ice Crystals 
B. J. Mason 
Imperial College, London, England 
Abstract—Experiments to test the ice-nucleating ability of a wide variety of natural 
mineral dusts suggest that kaolinite is probably the main source of atmospheric ice 
nuclei, and that a number of silicate nuclei may be preactivated. The nucleating prop- 
erties of artificial ice nuclei, mamly morganic compounds, are discussed in relation to 
their solubility, crystal structure and, especially, their surface structure. It is shown that 
nucleation occurs preferentially at steps and other special sites on the substrate surface. 
A particular nucleating substance has a critical temperature above which it may act 
only above water saturation but below which it may act as a sublimation nucleus pro- 
vided the air is supersaturated relative to ice by a well-defined critical value. 
Ice crystals growing as hexagonal plates on the surface of covellite show interference 
colors which give a measure of their thickness. Colored growth layers may be seen 
spreading across the crystal surface, careful measurement of which provides new infor- 
mation on the mechanism of crystal growth. 
The growth of snow crystals in a diffusion-cloud chamber shows that, over the tem- 
perature range 0 to —50°C, the crystal habit undergoes five changes, and these are con- 
trolled primarily by the temperature and not the supersaturation of the vapor. Their 
habit changes are not affected by the pressure and nature of the carrier gas, or by the 
presence of aerosols, as reported by Japanese workers. 
Natural and artificial ice nuclei—In an at- 
tempt to discover the nature and origin of at- 
mospheric ice nuclei, we have tested the ice- 
nucleating ability of various types of soil 
particles and mineral dusts. Of the 80 substances 
tested, 16, mainly silicate minerals of the clay 
and mica groups, were found to produce ice 
crystals in supercooled clouds at temperatures 
of —15°C or above, and of these, seven were 
active above —10°C (see Table 1). The most 
abundant of these is kaolinite with a threshold 
temperature of —9°C. Ten natural substances, 
again mainly silicates, were found to become 
more efficient ice nuclei having once been in- 
volved in ice-crystal formation, that is, they 
can be pre-activated or ‘trained.’ Thus, ice 
crystals grown on kaolinite nuclei, which are 
initially active at —9°C, when evaporated and 
warmed to near 0°C in a dry atmosphere, leave 
behind nuclei which are thereafter effective at 
—4°C. Particles of montmorillonite, another im- 
portant constituent of some clays, and which 
are initially inactive even at —25°C, may be 
pre-activated to serve as ice nuclei at tempera- 
tures as high as —10°C. It is suggested that 
although such particles can initially form ice 
crystals only at Cirrus levels, when the ice erys- 
tals evaporate they will leave behind some 
‘trained’ nuclei which may later seed lower clouds 
226 
at temperatures only a few degrees below 0°C. 
On this hypothesis, the fact that efficient nuclei 
are occasionally more abundant at higher levels 
would not necessarily imply that they originate 
from outer space. Indeed, in view of our tests 
on particles of stony meteorites, produced both 
by grinding and by vaporization, which show 
them to be ineffective at temperatures above 
—17°C, it appears that atmospheric ice nuclei 
are predominantly of terrestrial origin, with the 
clay minerals, particularly kaolinite, being a 
major source. This work is described m greater 
detail by Mason and Maybank [1958]. 
Although a good deal of work has been ear- 
ried out in different laboratories on the ice- 
nucleating ability of a wide variety of chemical 
compounds there has been little agreement in 
the results. Careful tests in our laboratory indi- 
cate that many of the published results are 
spurious because of the presence, in the air or 
the chemicals, of small traces of silver and free 
iodine, leading to the formation of silver iodide. 
If all such trace impurities are removed, many 
of the substances that have been suggested are 
found to be quite ineffective. 
The first six substances listed as artificial 
nuclei in Table 1, all being practically insoluble, 
are active to the extent of about one particle in 
10* producing an ice erystal at the indicated 
