Raindrops develop in warm clouds through collisions of larger with smaller cloud droplets. The process is sometimes called accretion or collection. It works best when some droplets are much larger than the others, as in marine clouds. Then the larger droplets or drops fall onto the smaller and collect them, whereas drops of equal size have the same terminal velocity as they fall, so that few overtake and collide with others, and, if they do collide, they bounce off rather than coalesce. In any case, very small droplets cannot be collected easily because they tend to dodge away.
The slow agglomeration of droplets by collection in warm clouds means that they usually yield only light rainfall, despite there being more liquid in warm clouds than in clouds at higher levels (Table 8.3). Warm clouds yield heavy rainfalls only when there is a sustained strong updraught, as on coastal hills in the tropics facing prevailing onshore winds (e.g. northeastern Queensland).
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