CSIR NET Dec 2015 Q130: Meteorology

Precipitation efficiency in warm (liquid-phase) clouds depends strongly on the concentration of cloud condensation nuclei (CCN) available for droplet formation. In a polluted urban/continental environment, abundant CCN (from anthropogenic aerosols) cause cloud water to be distributed among a very large number of small droplets, which grow slowly by condensation and collision-coalescence, delaying or suppressing precipitation (the well-known aerosol indirect/Twomey effect). In a relatively pristine marine environment, far fewer CCN are available, so the same amount of cloud liquid water is distributed among fewer, larger droplets, which collide and coalesce into raindrop-sized particles much more efficiently and quickly, making marine warm clouds precipitate more readily than urban warm clouds under otherwise similar conditions. This matches option 4 (fewer cloud condensation nuclei in the marine cloud).

Option 1 (higher liquid water content in the marine cloud) is not the primary distinguishing factor here; while liquid water content matters, the key contrast highlighted by this classic comparison is aerosol/CCN loading, not liquid water content per se. Option 2 (higher updraft velocity in the marine cloud) is not generally true or the defining reason; updraft strength is more related to atmospheric instability and convective dynamics than to the marine-versus-urban distinction being tested. Option 3 (air temperature inside the marine cloud is more conducive to precipitation) is not the standard explanation; temperature differences are not the primary driver of the marine-continental precipitation efficiency contrast. Option 4 is correct: the markedly lower CCN concentration in marine air (compared to aerosol-rich urban air) allows cloud droplets to grow larger and coalesce into rain more efficiently, explaining the higher precipitation likelihood of marine warm clouds.