Why does falling rain drops are almost spherical?

As a rain drop falls it is subjected to the forces of the air it is falling through, the most important being drag. The drag of the air on the surface of a drop of water is trying to hold it back as it falls. As the air circles around the back of the water drop, the slight vacuum that is formed by the falling pulls the upper part of the drop up into a cone shape. Eventually, these actions result in the very streamlined shape that a raindrop has before it hits the ground.