My wife asked me this question yesterday. I thought about it and then guessed spheres, because I figured surface tension would hold things together.
Happily for my ego, I was right—but only for raindrops less than one millimeter in diameter. After that, things get interesting. Larger raindrops, it turns out, are either shaped like hamburger buns or little headphones.
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).
[…] Small raindrops (radius of less than 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.
And that is your “I didn’t know that!” factoid for the day. (If you did know that, give your ego a pat on the back.)