Which one of the following is applicable to collision-coalescence process of precipitation?

The correct answer is A, Clouds which do not extend beyond the freezing level. Collision-coalescence is the way rain forms in warm clouds, whose tops stay below the level where water freezes.

In such clouds all droplets are liquid. Larger droplets fall faster than small ones, collide with them and merge, growing until they are heavy enough to fall as rain. This warm-rain process is common in the tropics, over the oceans and in low clouds. In cold clouds that rise above the freezing level, rain forms mainly by the Bergeron-Findeisen process, in which ice crystals grow at the expense of supercooled droplets and melt as they fall.

B is wrong because clouds that extend above the freezing level produce precipitation mainly through the ice-crystal or Bergeron process. C is wrong because the process is not general to all clouds; it is the typical mechanism only of warm clouds. D is wrong because cirrocumulus is a high, thin cloud made of ice crystals that seldom gives any precipitation.

Exam tip: warm cloud rain comes from collision-coalescence; cold cloud rain comes from the Bergeron ice-crystal process.