174 
mental technique. Schaefer [46] has found that a super- 
cooled fog may be converted to an ice-crystal fog by 
cooling a portion of it to —39C. Hollstein, quoted by 
Weickmann [55], studied the freezing of drops of sodium 
chloride solution of various concentrations. She found 
that the freezing point of the solution was equal to the 
freezing point of the water solvent minus the computed 
lowering of the freezing point due to the solute. There 
was some suggestion that the lowest possible freezing 
point of a sodium chloride solution lies in the neigh- 
borhood of —35C at which point the salt may act as a 
freezing nucleus. 
Condensation and Sublimation Below OC. The ice 
phase may appear in the atmosphere either by the 
freezing of the liquid or by the direct sublimation of the 
vapor to the solid phase. Wegener [54] first suggested 
that the atmosphere contained sublimation nuclei which 
would act in a fashion analogous to condensation nuclei 
to promote sublimation in the vicinity of ice saturation. 
Findeisen [13] expanded on this concept and based his 
precipitation theories, in part, on the existence of such 
nuclei. It was assumed that sublimation nuclei were 
small solid particles of a shape similar to an ice crystal. 
Sublimation should occur on a nucleus truly isomorphic 
with ice at ice-saturation. 
The search for sublimation nuclei has been conducted 
by two experimental techniques. The first of these 
employs the expansion chamber and the second the 
dew-method in which the processes are observed on a 
chilled surface. Cwilong [7] used an expansion chamber 
of the Wilson-type which could be refrigerated. After 
the air was cleaned by repeated expansions, he found 
that a cloud of ice crystals was formed when the mini- 
mum temperature during the expansion fell below 
—41.2C. He felt that small ions were acting as sub- 
limation nuclei below this temperature since the ice 
cloud formed at smaller expansions than those used to 
clean the air. In uncleaned outdoor air the transition 
temperature rose to —32.2C and to —27C when tobacco 
smoke was added. Cwilong also reported that at —70C 
there was a distinct change, in that a small shower of 
quite large grains of ice accompanied the cloud of 
crystalline dust. Fournier d’Albe [17] repeated and ex- 
tended Cwilong’s experiments with similar apparatus. 
He was unable to get the dense ice cloud in cleaned air 
below —41C reported by Cwilong but confirmed the 
latter’s conclusion that only liquid drops are formed 
above this temperature. Fournier d’Albe also found 
that no ice crystals were formed until water-saturation 
was reached or exceeded. He concluded that the ice 
phase was attained via the liquid phase and suggested 
that the particles be called freezing nuclei rather than 
sublimation nuclei. He also investigated the action of 
several types of artificial nuclei, including silver io- 
dide, which Vonnegut [53] had reported as causing 
the crystallization of supercooled clouds. Fournier 
d’Albe found that silver iodide nuclei caused ice erys- 
tals to appear at —7C, but only at water-saturation. 
Other artificial nuclei such as sodium chloride, sodium 
nitrate, caestum iodide, and cadmium iodide were found 
to have no effect on the water-ice transition temper- 
CLOUD PHYSICS 
ature. It is to be noted that the latter two substances 
form crystals with lattice constants similar to ice. 
It was on this basis that Vonnegut selected silver io- 
dide. On the other hand a water-drop cloud formed on 
cadmium todide and then evaporated left nuclei on which 
ice formed at ice-saturation. This suggests that the 
nuclei of silver, caesium, and cadmium iodides used by 
Fournier d’Albe may not have been in crystalline form. 
Findeisen and Schulz [16] used a much larger ex- 
pansion chamber with a volume of 2 m’, which was 
arranged to permit expansions at rates comparable to 
those in nature. Their results were similar to those of 
Cwilong and Fournier d’Albe in that the clouds formed 
by steady expansions were invariably water or mixed 
water-ice clouds even at a temperature of —40C. On 
some occasions when the expansion was imterrupted 
just prior to the attainment of water-saturation, ice 
crystals were formed in the absence of a water cloud. 
At an expansion equivalent to a vertical velocity of 
5 m sec ice crystals were first observed at about 
—7C and their number increased with decreasing tem- 
perature. In the neighborhood of —35C a very large 
increase occurred. At more rapid expansions the first ice 
crystals appeared at lower temperatures but the sudden 
increase occurred at a temperature somewhat higher 
than —35C. All of these experiments were performed 
in uncleaned surface air. Very similar results were ob- 
tained by Palmer [44], who observed the formation of a 
few ice crystals at —22C and at a relative humidity 
of 97 per cent with respect to water. He also observed 
a rapid increase in ice crystals at —32C in natural 
surface air. In airplane flights, with a smaller expansion 
chamber, Palmer found the —32C nuclei only below 
the haze inversion; at higher altitudes the first crystals 
appeared at from —41C to —44C. 
The most extensive investigations of condensation, 
freezing, and sublimation on a chilled surface have been 
made by Weickmann [55]. The advantages of this 
technique are that the mdividual particles may be 
viewed with a high-power microscope, that the tem- 
perature and rate of cooling may be controlled pre- 
cisely, and that the supersaturation with respect to ice 
is known at all times. The disadvantage is that the 
condensation occurs on a surface rather than in the air. 
Weickmann showed rather conclusively that the effect 
of a properly cleaned surface was small. Weickmann 
worked mostly at or near —40C. He found that ice 
crystals formed only when water-saturation was ap- 
proached. In one series of ten tests, using nuclei from 
a heated room, ice crystals formed at an average water 
relative humidity of 97 per cent, the range being from 
93 to 104 per cent. Using the residue from evaporated 
drops as nuclei, a few crystals formed after 33 minutes 
at relative humidities of from 85 to 90 per cent with 
respect to water or from 120 to 130 per cent with respect 
to ice. At or above 100 per cent water relative humidity 
thousands of ice crystals formed at once. A few sub- 
stances were found which favored the formation of ice 
at higher temperatures or lower ice-supersaturations, 
but in no case were crystals formed near ice-saturation. 
