Abstract
Langmuir's chain-reaction theory concerning the formation of precipitation in seeded cloud at Molokai is discussed. For this chain reaction, an upward wind of about 10 m set−1, taking place in a column with a diameter of about 4 per cent of that of the cloud, is necessary and this wind has not been observed. It is possible that the dry-ice pellets, falling through the cloud, cause a relative humidity of less than 100 per cent so that the cloud drops evaporate in their track. Attractive forces exerted by evaporating drops on each other cause coalescence of these drops, diminishing thereby their number and increasing their size. In this way, numerous drops of large size will be available for the formation of rain drops by falling down and coalescing with the cloud drops they meet. A similar explanation is given for the process of rain formation in tropical clouds below the freezing level and the vanishing and “raining out” of stratus layers in moderate latitudes.
