This is not the shape of a falling raindrop

The above teardrop shape is commonly how raindrops are represented in the media, weather reports and throughout our culture. That teardrop shape is actually not a possible shape for a raindrop. What is the actual shape? It depends on the size of the raindrop and is a function of the interplay of water surface tension and air resistance.

Water surface tension. Water molecules are made of one larger atom of oxygen and two smaller hydrogen atoms. The oxygen atom sits in the middle and the hydrogen atoms are stuck on either side in a sort of a V-shape. The force of attraction between hydrogen and oxygen is very strong and this causes water molecules to pull strongly on the other water molecules around them, pulling the whole mass of water together. This is called “hydrogen bonding.” Molecules within a body of water are being pulled in all directions, while the molecules at the surface are only being pulled by the water molecules beneath them and beside them – so they are being pulled downward and sideways, but not up.  This is surface tension and it causes water to assume the shape that has the least surface area.

What surface tension means to a raindrop is that a drop of water will be a sphere unless some other force distorts it because a sphere is the smallest surface area that can be achieved by liquid. Thus, small raindrops up in clouds are spheres. As these drops collide into each other they combine and become heavy enough to fall. As they fall they collide with nearby drops and continue to gain size.  As they get bigger raindrops are affected by the air resistance of falling which distorts the spherical shape and they then look like a hamburger bun.

As the drop grows it continues to flatten and at some point the surface tension can no longer hold the drop together and it splits into two or more spherical drops.

Time lapse video of raindrop falling

None of the above pictures of a raindrop falling look anything like a teardrop. So, where did the thought that raindrops look like teardrops come from? Probably from looking at drops of water from a faucet – the surface tension of water will cause the a single drop of water to extend into a teardrop shape just before releasing from the faucet.

Here’s an illustration from a presentation of raindrop shape: