Influence of the Environment on the Initiation of Precipitation in Tropical Cumuli over the Ocean

An analysis has been made of the day-to-day variation of the likelihood of precipitation in cumulus clouds observed over the ocean in the vicinity of Puerto Rico. It is shown that the probability of rain in a cloud having a particular height depends on environmental conditions, and that the principal factors governing precipitation initiation are stability, vertical wind shear and depth of the moist layer. On the west side of polar troughs moving into the tropics, these parameters have values which make precipitation likelihood small.