KentuckyFC writes: Rain is so closely studied that it’s easy to imagine that it long ago gave up its secrets to science. In fact, rain drops still offer one great mystery: the droplets grow in clouds much more rapidly than anyone can explain. Meteorologists know the general details of course. Water vapour cools as it rises and condenses on tiny aerosol particles to form a fine mist of droplets about 1 micrometre in diameter. These drops are too small and light to fall as rain and have to grow to at least 50 micrometres before gravity takes over. This takes about 15 minutes or so, even though the rate of condensation is more or less matched by the rate of evaporation and the density of this mist is too low for the droplets to grow by colliding and coalescing. Indeed, the best theoretical models suggest that this kind of growth ought to take hours not minutes. Now one group of researchers say the fact that turbulence consists of many tiny vortices explains the difference. As the tiny droplets get swept along in these eddies, centripetal forces push them to the boundaries of the vortices where they become trapped. It's here that the density of the droplets becomes high enough for them to grow rapidly by coalescing, finally explaining the last great mystery of rain.
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