Why Don’t Raindrops Keep Accelerating as They Fall?

When a raindrop begins to fall, gravity accelerates it. As its speed increases, however, the resistance from the air also grows. Once the downward force of gravity and the upward drag are roughly balanced, the net force is close to zero. The drop then falls at an approximately steady speed. This is terminal velocity: nothing locks the speed in place; the forces have reached a dynamic balance.

Terminal velocity is not one number shared by every drop. A larger drop has more weight relative to the drag it encounters and can generally reach a higher speed. NASA's precipitation material says that, in still air, the largest raindrops reach about 10 metres per second. Tiny cloud droplets are much more strongly affected by air resistance, fall very slowly and may be carried upwards by rising air.

Real rain is more complicated than a textbook sketch. Wind changes a drop's speed relative to the ground. After drops collide, merge or break apart, they may temporarily move faster or slower than the terminal speed expected for their new size. Larger drops are also flattened by airflow and are usually not tear-shaped. So “raindrops do not accelerate forever” is a sound central approximation, but terminal velocity is not a fixed constant independent of the surrounding air.

https://gpm.nasa.gov/resources/faq/how-fast-do-raindrops-fall
https://www.noaa.gov/jetstream/global/learning-lesson-atmospheric-collisions