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When the water droplets in clouds get too heavy to stay suspended, they fall as rain. Temperature plays a significant role in that.

But why does it feel like a chain reaction when it starts to rain? We don't see a few droplets here and there over a long period. We see a lot of droplets in a relatively short time period

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    $\begingroup$ The fact that droplets falls simultaneously does not mean that it is a chain reaction or a domino effect. Imagine a bus that suddenly breaks: all passengers fall simultaneously. If some fall on others - this is domino, if one pushes two, them push two others each, etc. - this would be a chain reaction. I think you need to develop the question more, to explain why you believe that rain is a chain reaction. $\endgroup$ Jun 9 at 12:49
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    $\begingroup$ @RogerVadim True, but there are definitely examples of positive feedback in cloud/rain formation. One example is the interplay between heat released by condensation, uplift of warmer air, and the relationship between dew point and pressure: this has to do with why thunderheads get tall. Another is the relationship between thunderclaps, nucleation of liquid droplets, the overall conductivity of the air, and lightning formation. $\endgroup$
    – rob
    Jun 9 at 13:48
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    $\begingroup$ Might Earth Science be a better home for this question? We can migrate it if you like. $\endgroup$
    – rob
    Jun 9 at 13:49
  • $\begingroup$ When rain comes on suddenly, the explanation often is that a moving storm cloud, which already was producing copious rain, has suddenly passed over your head. If you had been travelling with the cloud, hovering beneath it in an airship since before the rain began to fall, you might have had an entirely different experience. $\endgroup$ Jun 9 at 14:48
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    $\begingroup$ P.S., I removed the nuclear-physics tag from your question. Even if some kind of a "chain reaction" inside a cloud can trigger rain to start falling, that has nothing to do with nuclear chain reactions. $\endgroup$ Jun 9 at 14:57

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I believe in the phenomenon you are describing, rain starts from the cloud tops where the temperatures are cooler and fall through the cloud. As the cooler raindrops fall through the cloud, they take away heat which causes more drops to condense. The effect is analogous to a chain reaction and creates an initial burst. Please note this is one explanation. Storms are notoriously complicated and I've seen drizzles that fade out and do nothing

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