Question

If you stand motionless under an umbrella in a rainstorm where the drops fall vertically, you remain relatively dry. However, if you start running, the rain begins to hit your legs even if they remain under the umbrella. Why?

Short answer

If you start running, the rain will hit your legs because of its horizontal component of velocity.

Step-by-step solution

Step 1. Motionless in a rainstorm

In a rainstorm, if a person is standing still in a cylindrical shape in which there is no rain, the horizontal component of the rain is zero, and rain cannot move from outside the cylinder.

Sometimes the rain hits the legs even if a person is standing still in a rainstorm because, in this situation, the rain will have some horizontal component of velocity towards the person.

Step 2. Determine why the rain hits your legs while you are running

When any person is standing under an umbrella, they remain dry, but if the person is running with an umbrella, they are moving with a forward horizontal velocity. The rain will have a backward horizontal velocity relative to the person. So, the rain begins to hit your legs even if they remain under the umbrella.