Bad Meteorology

What does a raindrop look like as it falls? Close your eyes and picture it. Are you coming up with anything?

The shape below is the traditional, iconic shape of the raindrop as depicted in art and TV weather reports.

Here’s a familiar Weather Channel image:

And here are enlarged cross-sections of real raindrop shapes.

From Bad Meteorology:

The artistic representation of raindrop as presented by popular culture is that of a teardrop. Actually, real raindrops bear scant resemblance to this popular fantasy (except after they have ceased to be raindrops by splattering on a window, say). It may seem too easy a target to single out the Weather Channel for criticism for their shoddy representation when virtually everyone from advertisers to illustrators of children’s books do likewise. Yet, I would like to think they could be held to a higher standard as they attempt to convey the image of purveyors of accurate information.

Small raindrops (radius < 1 mm) are spherical; larger ones assume a shape more like that of a hamburger bun. When they get larger than a radius of about 4.5 mm they rapidly become distorted into a shape rather like a parachute with a tube of water around the base — and then they break up into smaller drops.

This remarkable evolution results from a tug-of-war between two forces: the surface tension of the water and the pressure of the air pushing up against the bottom of the drop as it falls. When the drop is small, surface tension wins and pulls the drop into a spherical shape. With increasing size, the fall velocity increases and the pressure on the bottom increases causing the raindrop to flatten and even develop a depression. Finally, when the radius exceeds about 4 mm or so, the depression grows almost explosively to form a bag with an annular ring of water and then it breaks up into smaller drops.

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