What makes rain drops tear shaped?

Category: Earth Science
Published: December 17, 2012

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University

Raindrops are not tear shaped when falling through the air. They are approximately round. As captured by James E. McDonald in a Journal of Meteorology article titled "The Shape and Aerodynamics of Large Raindrops", drops can even take on hamburger shapes when they get big enough. You might think that the resistance of the air flowing passed the falling drop would smear it backwards to form a tail. But the air resistance instead just flattens the front of the drop. The smaller the rain drop, the smaller the effect of air resistance and the more spherical the raindrop becomes. Perhaps this misconception arises from the fact that raindrops streaking down a window are tear shaped for the same reason tears streaking down your face are tear shaped: friction. The window, or your skin, has enough friction to drag some of the water back into a tail. The air does not. Tailed drops streaking down a transparent window seem to be falling in air, but are not.

Large raindrops are flattened spheres. Public Domain Image, source: Christopher S. Baird.

Small raindrops are spherical, not tear shaped. Public Domain Image, source: USGS.

Topics: air resistance, droplet, drops, precipitation, rain, rain drops, rain shape, tear drops, weather