When moving forward in the rain, vertical surfaces such as a person’s body will be hit by more raindrops as speed increases. From the walker’s perspective, the drops appear to fall at an angle, with a horizontal velocity equal to their own walking speed.

While walking faster means encountering more drops per second, it also reduces the time spent in the rain. As a result, the two effects balance each other out: more drops per unit of time, but less time in the rain overall.

When the walker is stationary, rain only falls on horizontal surfaces – the top of the head and shoulders. As the walker begins to move, she or he receives raindrops that would have fallen in front, while missing the drops that now fall behind. This creates a balance, and ultimately, the amount of rain received on horizontal surfaces remains unchanged, regardless of the walking speed.

However, since walking faster reduces the total time spent in the rain, the overall amount of water collected on horizontal surfaces will be less.