The collision-coalescence
 process is an important
 mechanism in forming
 raindrops in warmer clouds
 (those with tops warmer than
 -15°C = 5°F).  In warm clouds
 (those that are above freezing
 throughout) raindrops form
 exclusively by this process.
 Most tropical rain is formed in
 this way.  The collision-
 coalescence process is of
 relatively little importance in
 middle and high latitudes
 where, even in the summer,
 precipitation begins high in the
 clouds where temperatures are
 well below freezing and the
 dominant precipitation-
 producing mechanism is the
 so-called ice-crystal or
 Bergeron process.