Pedestrian with an Umbrella

Vertical rain appears slanted because the walker moves beneath it.

Ground view: rain follows its true path while the observer moves through it.

Rain & Moving Observer

Discover why motion changes the apparent direction of rain.

Real-world example

View from

−2.0apparent vx

−8.0apparent vy

14°tilt from vertical

rain relative observer = rain relative ground − observer relative ground

Tilt umbrella 14° forward.

Observer velocity2.0 m/s →

Rain downward speed8.0 m/s

Wind-driven rain velocity0.0 m/s

Simulation speed1×

Blue is rain velocity on the ground, green is observer velocity, and purple is the apparent rain velocity.

Teacher / Demo mode

The rain does not need to change its ground path. The observer's motion changes the measured velocity.

Wind gives rain a horizontal velocity. Gravity gives it a downward velocity. The street observer sees both components directly.

Subtract the observer's velocity from the rain's ground velocity.

apparent horizontal velocity = rain horizontal velocity − observer velocity

apparent vertical velocity = rain vertical velocity

If the observer moves right through vertical rain, the relative horizontal component is left. The drops therefore appear to come from ahead.

The umbrella shaft should lean toward the direction the rain appears to come from. Faster motion usually requires a larger lean.

Score: 0 / 7