What is the Shape of Raindrops?

We are all used to the idea that raindrops are shaped like tears. Science books, magazine illustrators, advertisers, and even TV news channels show raindrops as tear-shaped. It is unknown where this idea originated, but this has been accepted. Whenever we see tears, we imagine rain. Yet, raindrops are not actually shaped like tears. So what is the true shape of raindrops? What factors influence their shape?

Raindrops begin as rounded or spherical. High in the atmosphere, water collects on particulates like dust in the clouds. Due to its weight, it falls down as raindrop and will eventually lose its shape.

The raindrop now takes a roughly spherical structure, much like the shape of the top half of a hamburger bun. It is flattened on the bottom and with a curve dome top. The slight change in its shape is due to the surface tension of water, its speed and the pressure of the air pushing up against its bottom as it falls through the atmosphere. 

Surface tension can be viewed as the bonding force that hold rain together or that makes water molecules stick together. This is a weak molecular force created by the hydrogen bonds in water. For smaller raindrop, the surface tension is greater than in larger drops.

As a raindrop falls along with an increase in air flow, the surface tension weakens. Further, air flow on the bottom is greater than at the top. The less turbulent air at the top creates less air pressure. Hence, surface tension will be greater because of the presence of lesser force to counteract it. The aerodynamic resistance that falling raindrop provides to push through the atmosphere is known as drag.  In effect, the top part of the raindrop will likely to keep its spherical shape while the bottom, with less surface tension and more exposure to pressure, will get more flattened out.

The size of raindrop will also affect its shape. A larger size means larger surface area, lesser surface tension and greater tendency to change its shape. Small raindrops with radius of less than 1 millimeter (mm) are spherical. Larger ones, say 2 to 3 mm, take on a shape that is more like a hamburger bun. Some even tend to develop depressions at the bottom. But for very large ones with radius greater than 4.5 mm, they assume a more distorted shape. They will be like a parachute with a tube of water around the base, which will consequently break into smaller drops. Some references describe them as sagging dumb-bells.

A raindrop can also increase in size as it collides with one another. Yet, when it size gets too large, it will eventually break up apart back into smaller drops.

Hence, raindrops are not tear-shaped but sphere. So next time you illustrate raindrops in your drawing books, better think twice. Will you draw tear-shaped drops? Or spherical droplets of water?

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