PART 3: Final words (maybe) on 'Is it Worth Running in the Rain?'

May 2004

Four years ago this month the note was a summary of basic Physics considerations to whether it is worth running in rain. The next month was a JavaScript calculator in which you could plug in the various parameters about yourself and the rainfall conditions and see how wet you would get while going to a shelter. That had kept me amused for a couple of months and I expected that would be the end of it... I was wrong!

It turns out that many people are interested in thinking this through. A year and a half after writing these two pages it was coming up to the rainy season in Seattle. I was contacted by Eric Sorensen of the Seattle Times because he was interested in doing one of those humorous filler articles on this topic. His article was quite good, unfortunately it is no longer online as far as I know. (provided you register for a free subscription). I was immediately contacted by other media from coastal areas also entering the rainy season. This included radio stations in Newfoundland and Ireland. One interviewer laughed that the night before at the dinner table they had a conversation something like this:

  Him:  "Tomorrow morning I'm interviewing a scientist about whether
         it makes sense to run when it rains."
  Son #1: "Of course you do, the faster you get out of it the drier you are."
  Son #2: "Of course not, the faster you go the faster you get wet!"
  ... argument ensues.  The each leave the table considering the other to be an idiot.

Fortunately, Eric Sorensen's article included reference to people who had weighed clothes before and after walking and running in the rain - so I can now poin to experimental verification of what my calculations suggested.

The media attention has since dwindled, but not gone completely. The most recent article I'm aware of was last month in the Colorado Springs Gazette. This seems to be my enduring claim to fame as a Physicist. In fact, "everybody with a PhD in Physics" gets published in Phys Rev A/B/C/D/E... but how many have made the last page of the Old Farmer's Almanac? (see the 2003 edition)

So... what is left to say about running in the rain? Well, my original pages on the topic were intended to teach Physics. The running in the rain thing was an amusing excuse to talk about things like flux and relative velocity. The pages didn't make it easy for anybody to answer the question without learning some Physics. Maybe I should leave well enough alone, but after being asked for the answer so often I'm finally relenting and offering the following thoughts based on Physics and other considerations:

When a moderate rain is falling vertically, consider the following total amounts of rain striking a body.
light stroll: 19.2 mL
double that speed: 15.2
Olympic sprinter speed: 12.1 mL
... so simply picking up your walking speed cuts about 20% off the total water absorbed, but breaking into a sprint that would leave most of us gasping and writhing on the ground when we got to our destination only cuts off another 20%. I say, speed up by a comfortable amount, but don't kill yourself trying to stay dry.
If the wind is blowing from behind you, the optimal solution will be walking/running at approximately wind speed.
If the wind is blowing on your face or side, there is a greater benefit to running.
If you (trying to be as polite as I can about this) present a relatively large target as seen from above, there is a greater benefit to running. For example, your dog - opposite to the average human - presents a much larger top area than front area. Ergo, a dog should run to get out of the rain.
How much are you going to sweat if you run, and how should you add that contribution to the total wetness? Note that sweat is a lot less pleasant when it is rainy because with the high relative humidity it doesn't evaporate off very quickly.
On a related topic - "why do people hunch over in the rain". If you hunch over you probably create a larger vertical target and hence will get wetter.
Answering that last question - I think people hunch over because even if they get wetter, it is more pleasant to have really wet shoulders than a somewhat wet face. So, another consideration to this is not just how much water hits in total, but where it hits and how much tolerance you have to water in that area.

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