Sublimation in a Wilson chamber

Abstract

About fifty years ago Professor C. T. R. Wilson began a classical series of experiments (Wilson 1895 to 1933) on the condensation of water vapour in both dust-free and ordinary air. The only reference to ice throughout that work is as follows (Wilson 1897 c , p. 299): ‘It is assumed here that the cloud-particles are actually liquid drops and not ice crystals, in spite of the fact that the condensation begins at temperatures much below the freezing-point and that the temperature when the particles are fully grown is, as we shall see, also slightly below the freezing-point.’ In the present paper experiments are described in which the final temperatures after expansion are much lower. These confirm that down to surprisingly low temperatures only liquid water droplets are formed, but they show that after a threshold low temperature is passed ice particles begin to be formed. These ice particles are not formed by the freezing of water droplets but by the direct sublimation of water vapour to ice crystals, and the process has some essential differences from the condensation of water vapour at higher temperatures. The experiments also indicate that in ordinary outdoor air there are no sublimation nuclei on which the ice crystals form when the air is supersaturated at temperatures between 0 and — 32° C.