Is it worth running or walking in the rain if you want to avoid getting wet?

Are outside, and it starts pouring down. What to do?

Often we start to rush to shelter, then a small drizzle or water pours down from the sky.

If you take shelter quickly, you won’t get wet as much. This is how we think with common sense. Is that even worth justifying?

Physicists think it’s worth it, and they’ve approached the issue from many angles over the years.

French physicist Jacques Treiner has now made his own arguments public. He present them on The Conversation.

We tend to believe that going fast means getting less wet, Treiner writes.

However, hurrying can mean that we will encounter more rain as the speed of the movement increases and we lean forward. A person leans forward when accelerating.

Paris Treiner working at Cité University using a simple model to find out if moving faster actually reduces how wet you get.

In other words: does the amount of water hitting a person depend on how fast they are moving?

Let’s imagine that it drips evenly in a straight vertical direction.

As a physicist, Treiner divides the body into two surfaces in different directions. The front and back of the body are on the vertical surface. The horizontal plane includes the head and shoulders.

If you move forward faster in the rain, more of the vertical surface is exposed to the rain.

Accelerated when walking, the pedestrian encounters more droplets per second.

On the other hand, fast walking also reduces the time spent in the rain, because you can get to shelter faster.

These two effects balance each other. A person encounters more drops per unit of time but spends less time in the rain.

When the walker is stationary, the body rains mostly on horizontal surfaces, i.e. on the top of the head and shoulders.

When the walker starts to move, he receives raindrops that would have fallen in front of him.

At the same time, however, he can avoid certain drops that now fall behind.

This balances the situation. The amount of rain falling on horizontal surfaces remains the same regardless of how fast you walk.

However, brisk walking reduces the time spent in the rain. Therefore, the total amount of water accumulating on horizontal surfaces is smaller.

The calculation is described in more detail, among other things on this site.

Calculation offers two key insights, writes Treiver.

1. The faster you move, the less water hits the head and shoulder areas.

2. The amount of water hitting the vertical surfaces of the body remains the same regardless of the person’s speed – except if the person stays still. If you spend a shorter time in the rain, you can compensate for the fact that more raindrops hit your body per second.

So you should lean forward and move quickly when you get caught in the rain. And the speed must be increased enough.

Correction November 11 at 12: Added clarification to the statement that the amount of water hitting the vertical surfaces of the body remains the same, whatever the speed of the person. This is not the case if a person stays still.