What is the shape of a raindrop?
—Bobby Merritt, Lyons
It’s probably not what you think it is. The actual shape of a raindrop is the result of two forces constantly acting on the drop and acting in opposition to each other: surface tension and air pressure (drag) pushing up against the bottom of the drop as it falls through the air. Surface tension pulls the drop inward equally in all directions, causing it to be spherical. It is the dominant force in smaller raindrops (0.08 inch or less in diameter). But as drops grow larger and fall faster, upward air pressure increases, causing drops to flatten ( at ~0.16 inch in diameter), bulge upward in the middle (~0.24 inches in diameter) and assume the shape of a kidney bean (~0.31-inch diameter) before splitting apart.