Raindrop Size and Speed


Dear Mr Jones

What causes different types of rain? Sometimes it comes down in "stair rods"--lengthened droplets that fall at great speed and bounce high after hitting the ground. Other times there's just a misty drizzle that blows aimlessly in the breeze. How can rain fall so heavily that it can cause physical pain, or so lightly that it is just a soaking mist. And how do you get the types in between?


Hi Daniel,

All rain falls at a speed called its terminal velocity. That is the speed where the force of air resistance on the drop equals its weight. The weight is proportional to the cube of the drop radius. The air resistance is proportional to the square of the drop radius and the square of its speed. Each particular drop then has its own terminal velocity which increases with drop size. Big drops fall fast and hard. Small drops more slowly. Some drops are so small that they appear not to fall at all, as in clouds and fog.

Drops form from the moisture in the air, usually starting around a bit of dust or pollen or some other impurity. In some clouds the heat released by the condensing droplets cause an updraft that may actually carry drops upward. The longer the drop hangs around in the cloud the bigger it gets until it finally falls out of the updraft and plummets to the ground. Weak updrafts lead to small drops, strong ones to bigger drops.

This information is brought to you by M. Casco Associates, a company dedicated to helping humankind reach the stars through understanding how the universe works. My name is James D. Jones. If I can be of more help, please let me know.