| MadSci Network: Physics |
Raindrops typically hit the ground at velocities of around a meter per second. Since the raindrop usually falls from a cloud at a height of at least one kilometer, it must fall for many minutes before reaching the ground. Thus it has lots of time to reach terminal velocity. The terminal velocity depends on the size of the raindrop, being larger for larger raindrops. This is because the terminal velocity results from a balance between the force of gravity and the aerodynamic drag force. The gravitational force is proportional to the mass of the drop, and therefore its volume. The aerodynamic drag is proportional to the radius times the velocity. (This is called Stokes' Law.) Fog consists of very small drops, which fall at speeds of less than a meter per minute. A large hailstone can reach a velocity of several tens of meters per second, and thus can cause considerable damage when it hits the ground!