Is rain formed by the condensation of water vapor or by the melting of ice?
This important question is still under investigation. Much of the rain is produced by clouds whose tops do not extend to temperatures colder than 0° C. The mechanism responsible for rain formation in these "warm" clouds is merging or "coalescence" among cloud droplets, which are first formed by vapor condensation. Coalescence is probably the dominant rain-forming mechanism in the tropics. It is also effective in some mid-latitude clouds whose tops may extend to subfreezing temperatures. However, once a cloud extends to altitudes where the temperature is colder than 0° C, ice crystals can form and "ice-phase" processes become important. In favorable conditions, ice-involving processes can initiate precipitation in half the amount of time water-only processes would need. Hence, at mid-latitudes, cumulus cloud rain is probably initiated by ice-processes and melting of ice. Observations have shown, however, that precipitation can first appear at levels warmer that 0° C, where vapor condensation and coalescence are the main rain producers. Thus, precipitation may be initiated by either process.