AbstractDuring the last decade many experiments have been designed to investigate the initial processes of ice formation in the atmosphere. The results of these experiments are examined for clues as to the identity of the responsible nuclei and the following tentative conclusions are reached: (a) The nuclei which cause freezing between 0°C and −32°C are mainly solid, insoluble particles which are wetted by water and produce ice particles through the freezing of water drops.(b) The nuclei which become operative in the range −32° to −11°C consist of droplets of sea salt solution, ice and salt crystallising out on contaminating foreign particles at a temperature depending on the efficiency of these particles and the rate of expansion of the ambient air.(c) The nuclei effective at just below −41°C consist of droplets ofpuresalt solution and possibly of “ gaseous” nuclei formed industrially or by the action of ultra‐violet light on the gases of th