Turbulence in clouds as a factor in precipitation

Visual and radar observations of the height of formation of precipitation in shower clouds in relation to temperature, and measurements of droplet population, show that when cumulus clouds produce showers it is often by coalescence, rather than by an ice‐phase process. Langmuir's work gave the conditions necessary for collisions to take place between droplets. The droplets of young cumulus clouds are less than 20 microns in radius and calculation shows that they are too small and settle too slowly by gravity alone to grow at an appreciable rate by colliding with each other. The updraught in a cumulus cloud is highly turbulent so that the air containing the droplets is being perpetually accelerated in a random way. An analysis of random acceleration in one dimension shows that it would produce collisions if violent enough, and in conjunction with gravity would start droplets into growth. Once their radii reached about 20 microns they would continue to grow with or without turbulence. Such a mechanism is consistent with the observed changes in droplet size distribution. Provided that sufficient energy were available in a suitable frequency range, turbulence would initiate a shower in a cumulus cloud. Numerical confirmation of the mechanism awaits suitable measurements of small‐scale turbulence in cloud, but meanwhile an alternative approach through macroscopic energy considerations might prove fruitful.