Raindrops

Alistair B. Fraser was so irritated by the teardrop-shaped depiction of raindrops in popular culture that he uses as his symbol for his 'bad science' website.

This is how he explains it:

Real raindrops bear scant resemblance to this popular fantasy (except after they have ceased to be raindrops by splattering on a window, say).

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 4mm, the depression grows almost explosively to form a bag with an annular ring of water and then it breaks up into smaller drops.

I was quite happy with the shape of these ones dangling from our rosebush this morning.