Did you know that the shape of freely falling raindrops is not like a tear drop? You wont guess what it is shaped like either. Think of the image of a raindrop and the shape you picture will most likely be that of a tear shaped drop. We come about shaping this image from our life experiences. We see drops from taps as tear shaped, and every night on the television the news stations will deliver the prediction of rain with a tear dropped shape. But what if we told you that everything we think about raindrop shapes is wrong? The simple reason is that they are not tear shaped at all.
What’s the shape of freely falling raindrops?
As rain is formed high up in the clouds it has a long way to fall. They start their life as spherical balls of water, then build in size. Once the weight of the water exceeds that of the updraft of wind holding it in the air it will begin to fall. The determining factor on its shape is surface tension and the pressure of the air pushing up against the bottom of the drop. If the drop is small the surface tension of the water will maintain its shape, but when the drop builds in size the pressure will win the battle.
As the raindrop gains speed falling freely towards the ground the pressure on the bottom of it will force the drop to flatten out. It may even form a depression on the bottom side, a bit like the top half of a hamburger bun. Are you ready for a bigger surprise? Once the radius of the drop exceeds about 4mm the depression in the bottom will grow to form a bag with an annular ring of water. It will then break up to form smaller drops of rain.