July 19th, 2012

coalescence cascades

The other day I was wondering… Why does rain not accumulate (‘coalesce’) in the sky, given the distance it falls, to form deadly sheets or lakes of rainwater that could fall in one place as one large mass? I found the explanation that there is a maximum size of droplet that is reached before the droplet begins to break apart again in freefall.

Picture a huge room full of tiny droplets milling around. If one droplet bumps into another droplet, the bigger droplet will “eat” the smaller droplet. This new bigger droplet will bump into other smaller droplets and become even bigger–this is called coalescence. Soon the droplet is so heavy that the cloud (or the room) can no longer hold it up and it starts falling. As it falls it eats up even more droplets. We can call the growing droplet a raindrop as soon as it reaches the size of 0.5mm in diameter or bigger. If it gets any larger than 4 millimeters, however, it will usually split into two separate drops.

On the topic of water coalescence I coincidentally discovered this uncanny video that shows a mind-boggling dance that occurs when a drop of water meets another body of water. The drop only becomes assimilated after a strange interaction that happens too quickly for us to see without high-speed cameras.

Video via 3qd, text above via US Geological Survey.


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