. Compendium of meteorology. Meteorology. THE PHYSICS OF ICE CLOUDS AND MIXED CLOUDS 193 small ice-supersatiirations). Fournier d'Albe [9] pro- duced these particles bj'- evaporating crystals which had been grown at —41C in the presence of solution droplets of cadmium iodide. It seems likely that the particles remaining consisted of cadmiimi iodide bearing microfilms of water which retained an icelike structure such as cannot be imposed upon films adsorbed from the vapour, even by cooling to temperatures in the neigh- bourhood of — 70C. This suggests that the only sublima- tion nuclei are in f
. Compendium of meteorology. Meteorology. THE PHYSICS OF ICE CLOUDS AND MIXED CLOUDS 193 small ice-supersatiirations). Fournier d'Albe [9] pro- duced these particles bj'- evaporating crystals which had been grown at —41C in the presence of solution droplets of cadmium iodide. It seems likely that the particles remaining consisted of cadmiimi iodide bearing microfilms of water which retained an icelike structure such as cannot be imposed upon films adsorbed from the vapour, even by cooling to temperatures in the neigh- bourhood of — 70C. This suggests that the only sublima- tion nuclei are in fact particles of ice, and raises the possibility that after the evaporation of ice clouds at low temperatures some freezing nuclei may be left in a condition which enables them to act as sublimation nuclei in a future cloud formation. However, nuclei having the efficiency of those made by Fournier d'Albe cannot be expected to occur naturally in the atmos- phere, where even the poor ice nuclei available are greatly outnumbered by nuclei capable of aiding the formation of liquid droplets. This scarcity of ice nuclei, which may be even more marked in the high troposphere than near the ground, is an important factor which in association with the speed of atmospheric condensation processes results in the frequent appearance of droplets during cloud formation at temperatures down to — 50C or even lower. If the humidity is increased very slowly some ice particles will form some time before water- saturation is reached; their subsequent growth may then remove enough vapour to prevent a further increase in humidity, or the process responsible for the rise of humidity may cease. In this way very thin pure ice clouds may arise without any droplet formation. Their occurrence must be very infrequent in view of the great scarcity of ice nuclei which act before water-saturation is almost reached, and the slowness and duration of the condensation mechanism which would be necessary, but t
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