I always run because I want to get out of the rain faster, but I wonder if I’m just running into more rain and actually ending up wetter…
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[https://www.youtube.com/watch?v=HtbJbi6Sswg](https://www.youtube.com/watch?v=HtbJbi6Sswg)
My boys Adam Savage and Jamie Hyneman have the answers for you. Spoilers: ~~walking is actually better.~~ >!They redid the experiment and running is better, but you get wet both ways. Learn something new every day.!<
In chemical reactor design, we teach the students that the time you spend in the reactor is the ‘residence time.’ The goal of the class is to show you how to have the reactants in the reactor long enough to react completely without wasting time.
Think of running in the rain as your time that you are in the reactor. You need X number of minutes in the rain to get fully saturated. If you run, your time in the reactor is less, and you will be less wet.
So, run fast, spend less time in the ‘get wet reactor’ and get less wet.
You will likely get wetter faster by running, but you will reduce your exposure time if running to and from a sheltered location.
You will get wetter at a slower rate by walking, but you will offset your lower wetness per unit of time with an increase in travel time between the same beginning and end points.
The relationship should technically be linear, then, but there are complicating factors: running will increase your surface area in the horizontal plane, meaning that per unit of time you are liable to be hit by a greater number of raindrops. You will also run through a higher number of intersection points between airborne water droplets and your horizontal movement. You will also be reducing the number of intersection points between water droplets and the same body part during your walking test because you are effectively running out from under a large percentage of those same water droplets that would have otherwise landed on that same given spot uf you were walking. These complicating factors may disrupt the linear relationship. Interesting things happen given complicating factors in statistical representation such as minimum and maximum points, inflection points, or logarithmic relationships.
I would suspect that you get wetter by walking given a requisite ratio of walking speed to running speed. You technically should get wetter faster by running given increased horizontal surface area and increased intersection points, but it is doubtful that you would get, say, 600% wetter per unit of time, versus the capability of people to run, say, 600% of their walking speed. I, for example, sprint at approximately 12 to mayyyybe 14mph, but I comfortably wall at 2 to 3mph. Given the same distance, then, I suspect you’d be better off running because although your rate of wetness increase per unit of time may be higher, you are cutting your exposure time by an even greater factor
The interesting question then becomes “At what walking versus running speed relationship does it become drier to walk? (Like if you can only run 30% faster than your walking speed) and how does, if at all, the density of rainfall effect the outcome?”
If you stand still you can dodge each drop of rain coming from up by slightly moving out of its way. But if you run, you have to do it for drops coming both from up and from front so it will be harder to dodge all of them.
For this kind of question, it’s often useful to look at extreme cases.
Imagine you walked very very slowly, so that it took you many hours to go from one shelter to another. Even if the shelters were inches apart – so close that you wouldn’t normally get wet at all – the slow movement from one to the next would leave you out in the rain for ages. So very slow speeds get you very wet.
Now, imagine you moved ultra-fast, so fast that the rain is basically stationary from your point of view. You’d punch a “hole” through the rain shaped like you, like an old cartoon character plowing through a wall. That means you’d only get as wet as the amount of water directly in front of you, which is not very much. (And a slightly more subtle calculus argument will show that this is the minimum you could ever hit.)
While this isn’t a mathematical guarantee, problems of this kind tend to have one of two behaviors: either they ramp up or down to a maximum or minimum and then taper off, or they smoothly increase/decrease. In this case, it’s the latter, and you get the least wet by moving quickly. You strike the same amount of “forward water” regardless (assuming the rain rate is constant), but are exposed to less of the falling water.
That said, things like splashing in puddles, non-constant rain rates, different parts of your body being differently sheltered by an umbrella, etc can make this idealized answer not apply in practice.
**The math is interesting**. This assumes no wind which can change the math.
* Walking (assume no “running into raindrops, all vertical fall connecting with horizontal surfaces)
There is less surface area (head, shoulders, swinging arms, tops of feet) but you are in the rain a longer time. **More water hits you because of the longer time**. This is perfect for an umbrella.
Wetness = surface area * rate of rainfall * time in rain
Lets say you are in the rain 5x longer than if you run.
* Running (assume you hit some raindrops with your front surface)
There is more surface area (front plus head, shoulders, swinging arms, tops of feet) but you are in the rain a shorter time. **less water hits you because of the shorter time**.
Wetness = surface area * rate of rainfall * time in rain
Lets say you are in the rain 1/5 as long as walking.
The surface area calculation and rate are now complex math as the front surface may get wet ad a different rate (based on running speed and rainfall speed). Lets guess you get approximately 2x more water but only for 1/5 as long, so you get less wet running.
Always run.
You can break the problem down into parts. One part is what falls on you during motion, mostly on your head. The other part is the stuff you walk into.
The stuff that falls on your head is a function of the rate of rain and how long you’re in it. This value will always be lower if you run,
The second part is a bit more difficult to explain. Think of what would happen if gravity turned off and all the raindrops suddenly stoped in mid-flight. If you walk through that, the number of drops that hit you is a function only of your size and the density of the rain. It’s like you’re punching a hole through it, that hole doesn’t change if you run or walk.
Now turn gravity back on. What changes? Nothing. Running will not increase the density of the drops.
So one of the two will not change and the other will go down if you run. So run!
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