AbstractCalculations are made from which it appears that the rate of loss of heat from ice particles growing by coagulation in dense supercooled clouds may be insufficient to allow all the acquired water to freeze. The larger coagulation‐elements within cumulonimbus clouds may therefore have liquid skins at levels where the temperature has fallen to −20° or −30°C. The rain‐drop multiplication process envisaged by Langmuir may thus proceed also at temperatures well below 0°C, and the effect may also be important in the production of ice nuclei by‘splinter’ formation and in the generation of thundersto