Because of the water's surface tension, which results from the propensity of water molecules to stay together, raindrops acquire their spherical shape.
The spherical shape has the smallest surface area possible, which allows it to withstand any atmospheric external force.
Surface tension is caused by the forces that hold liquid molecules together.
The water molecules on the surface don't have similar molecules surrounding them.
Thus due to surface tension, water has a tendency to form spherical droplets.
