# Sheets of Rain

Man, it’s coming down in sheets outside. Just in time for my trek across the parking lot.

Wait a minute, why does rain come down in sheets? I’ve seen it happen all my life, and today was the first time I ever wondered why. I have no idea. Shouldn’t rain just come down in a random, uniform way? Why does it organize itself like that?

So, meterological Dopers, why does it happen?

Among other reasons, there’s this:

Imagine a group of raindrops falling at perfectly even spacing; a perfectly uniform gust of wind blosw sideways toward them - the ones in the middle of the formation are sheltered by those on the edge, so the pattern (which was a uniformly spaced grid) gets ‘squished’ on one side. it can’t be squished into a single large ball of water, because the surface tension isn’t sufficient to maintain it as it falls through the air.

So it’s marginally easier for raindrops to ‘clump’ together than it is for them to be spread apart, at least under some conditions.

This is probably only one of many effects; others might include a tendency for an existing raindrop falling through a saturated area of cloud to trigger the precipitation of more droplets. I think it’s a fair bet that chaos theory will be in there somewhere too.

All these year I thought it was raining CATS and DOGS!

“Black Sheets of Rain” is a song!

I always presumed that it’s the leading edge of a gust which ‘gathers’ the drops. (How can you have a uniform gust? Isn’t it a contradiction?)

I was supposed to be a simplified example (to eliminate things like turbulence); a uniformly-spaced group of falling raindrops will not all be moved together by a coherently-moving ‘block’ of air, because those drops in the middle are sheltered by those at the edge. This is pretty much the same thing as the wind gathering them, but it is important to note that the wind ‘spends’ itself on moving the first layer of droplets it enoucnters and has nothing left to move those behind; the effect is that the pattern develops into a ‘clump’.

Now I think about it, what we’re basically talking about is the formation of waves, isn’t it? And the perception of waves of droplets moving horizontally, to a human observer, would be the sheets of rain in question.

I suppose it is, yes.