The whole journey of the raindrop can be explained as follows:

Firstly, the raindrop departs from the cloud towards the Earth with zero initial velocity. There exist two forces, buoyancy force and gravitational force conflicting with each other. The gravitational force of Earth accelerates the raindrop downward, and the buoyancy force is very small (as the density of water is much greater than air). So the speed of drop increases.

As speed increases, the raindrop will experience a new force acting upwards: air resistance. This is a frictional force. Unlike friction between rigid bodies, this fluid friction depends on the relative velocity between the surfaces. That means as the speed of raindrops increases, the fluid friction also increases. So the forces on the raindrop now are buoyancy and gravitational force which are constant and fluid friction which is increasing.

When the speed of raindrop increases so high that the fluid friction is equal to the gravitational force-buoyancy force, the net force on the raindrop isfinally becomes zero. Now the drop does not accelerate anymore, hence the speed of the drop is saturated.

The whole journey of the raindrop can be explained as follows:

Firstly, the raindrop departs from the cloud towards the Earth with zero initial velocity. There exist two forces, buoyancy force and gravitational force conflicting with each other. The gravitational force of Earth accelerates the raindrop downward, and the buoyancy force is very small (as the density of water is much greater than air). So the speed of drop increases. As speed increases, the raindrop will experience a new force acting upwards. This is a frictional force. Unlike friction between rigid bodies, this fluid friction depends on the relative velocity between the surfaces. That means as the speed of raindrops increases, the fluid friction also increases. So the forces on the raindrop now are buoyancy and gravitational force which are constant and fluid friction which is increasing.

When the speed of raindrop increases so high that the fluid friction is equal to the gravitational force-buoyancy force, the net force on the raindrop is zero. Now the drop does not accelerate anymore, hence the speed of the drop is saturated.