The answer to that lies in a simple phenomenon known as surface tension, the attractive force exerted upon the surface molecules of a liquid by the molecules beneath, which tends to draw the surface molecules into the bulk of the liquid and makes the liquid assume the shape that has the least surface area. In the case of raindrops, the smallest area that can be achieved while falling is that of a sphere. An isolated drop that isn’t distorted by external forces is pulled by its surface tension into a spherical shape.
This difference of pressures tends to flatten the bottom part of the drop, i.e drive the water molecules from its top and center to the sides, thereby flattening the drop and increasing its horizontal diameter.
What’s more intriguing is that the resultant modified surface of curvature of the drop restores the pressure equilibrium in it. This proves that the best examples of science are found in nature.
The above adjustments only make up for the pressure differences caused by external pressures and aerodynamic forces. However, there’s another force that comes into picture and it needs to be overcome in order to achieve complete equilibrium. Obviously, we’re talking about the gravitational force!
If the raindrop becomes too large in size, it splits in two and reacquires its original spherical shape. This condition is satisfied when the drop achieves terminal velocity (the speed at which the drop falls without any further acceleration) and when the internal pressure isn’t uniform throughout. This pressure gradient is usually small, since terminal velocity is quickly achieved.
On the more technical side, the combination of hydrostatic and aerodynamic principles should result in a flatter top and a curvier bottom. In reality, however, it is the exact opposite.
This surprising revelation is because of another phenomenon known as viscosity (the resistance offered to the flow of any liquid). Up to this point, we assumed that the drops fall through a perfectly non-viscous fluid. However, air has some viscosity and it is enough to influence the shape of the drops. When moving around a large raindrop, air behaves in the same manner as that of an airplane wing. The layer of air surrounding the drop forms an unsteady region at the top surface. In this region, the air pressure is significantly lower than that of the base of the drop. This results in a greater curvature at the top surface than the base.
Whew, that’s a whole lot of physics, but we made it through! The common myth of teardrop-shaped raindrops has been finally debunked. Even though we know the truth, artists and illustrators will continue drawing raindrops however they want, teardrops and all!
