Why water in the sink follow a curved path?
The difference between rain and water in the sink is that rain is simply falling, while water in the sink is being drawn into a center from a distance away, and the water in the sink is not perfectly still. It is rotating, if only a little bit. As it is drawn to the center, the rotation becomes more rapid.
The principle is Conservation of Angular Momentum.
Here, Ms. Kawaguti speeds up her rotation by pulling in her arms:
In basic principle, both could do the same thing.
Pragmatically, water in a drain has the resistance of the sink/drain walls to influence the effect. (This is a hairpin vortex regime.) Basically, vortices differ per sink.
Surface tension of a rain drop exceeds wind friction. Coriolis forces still exist within the rain drop, and could produce a toroidal-like vortex flow therein.
The vortex is a cascade phenomenon influenced by
- molecular dynamics,
- boundary conditions, and
- environmental forces