Does Running in the Rain Make You More or Less Wet?

[Shooting Star]

All of the mathematical models offered here and elsewhere are just that: models, and nothing more. Experiment is the arbiter in science. Experiment alone determines which models may be correct, and which cannot be correct. In this case, I'd listen to the MythBusters more than to anyone else, even if their show can be a bit sloppy at times.

- Warren

The whole of theoretical Physics are models and nothing but models. Some models has been proved to be wrong after a few hundred years. But in that time, the model has guided us towards stupendous achievements or advancements. Experiment was the arbiter in both rise and fall of the model. Without a model to guide us, how shall we understand the essence of a situation?

Your philosophy is commendable, but it’s mostly true for practical problems in engineering, where the number of parameters are high, or for a new theory in the frontiers of Physics, which is yet to be accepted as a law.

If I calculate how many electrons are emitted by a photoelectric material when EM waves of certain frequency are falling on it, nobody would think of questioning it if I mention the right formula. But the answer may depend on so many real factors pertaining to the apparatus and the situation. Then why such a fuss about an extremely well understood concept like the falling of rain?

If the Myth Busters do the experiment wearing yellow windcheaters, what does that say for red mackintoshes, or for somebody running without any clothes? You’ll probably reply that it makes no difference. How do you arrive at that model? Not experimentally, I’m sure.

After somebody constructs a good model, meaning many people are convinced by it, then of course we’ll call in the Myth Busters and verify it, but not before. Let me repeat what somebody I admire said once:

“It is theory which will dictate which experiment to perform.” – A.E.

Sorry for the rather long-ish post. But so much more to say...

 

[Shooting Star]

Hi all, I just thought that I'll remind you what everybody does when they want to cross the street in the rain, or go from the door maybe to the car parked a short distance away. They sprint.

 

[Huckleberry]

Sorry, I'm no mathematician. It would seem to me that in vertical rain the least wet one could get without shelter would be to stand still, rain/time. Any scenario with rain/distance and vertical rain, the faster you can move the better.

maybe (rain x time) / distance?
This would assume a constant rate of vertical rain.

edit - rain would have to be described as a certain volume/time. [liter/square meter/second]

edit - nevermind, that's wrong. If rain falls at some arbitrary rate, say 1Liter per Second, then one would need to know the speed that he could get to shelter. Speed is the distance crossed in a certain period of time D/T. So if your shelter is 20 meters away and you can move 5 meters/second then you will reach it in 4 seconds, enduring 4 liters of water plus a minimal amount for moving against the wind? [assuming a surface area that the rain impacts on of 1 square meter for the dude running.]

Eh, I'll leave this up to you. I passed college calculus 15 years ago and never looked back. It's a shame really.

 

[kanato]

But two years later in 1997, two meteorologists, Thomas C. Peterson and Trevor W. R. Wallace, from the National Climatic Data Center in North Carolina in the USA, published another paper called "Running In The Rain". They had read the "Raindrops Keep Falling On My Head" paper, and found a few mistakes, corrected them, and added in stuff like "rain driven by the wind".

Their equations showed that in a light rain with no wind at all, running will give you only a 16% reduction in wetness, as compared to walking. But if you're running rapidly and leaning forward in a heavy rain that is driven by the wind, you will end up 44% less wet than if you had walked. At this stage, Petersen and Wallace showed that they were fair-dinkum scientists and decided to do the experiment. Luckily they were roughly the same build, so they bought two identical sets of sweat shirts, pants and hats. They also bought two very large plastic bags to wear underneath these clothes, so that any rain which ended up on their clothes would not get soaked into their underclothes. They then measured out a 100 metre track behind their United States National Climatic Data Center office and waited for some rain. Soon, some heavy rain came along - falling at around 18 mm (or 3/4 of an inch) per hour. They made sure that they weighed the clothes both before and after the rain.

Dr. Wallace ran the hundred metres at around 14.4 kph, and his clothes picked up 130 grams of water. Dr. Petersen walked his hundred metres at a much more leisurely 5 kph, but his clothes soaked up 217 grams of water. Running, instead of walking meant that you got less wet by 40%, which was pretty darn close to their predicted 44%.

Their results can be summed up as:

"When caught in the rain without a mac,
walk as fast as the wind at your back,
but when the wind's in your face,
the optimal pace
is fast as your legs can make track".

That's an interesting study.. do you have a citation for their publication? I'd like to read that. Their conclusions sounds like the same I got from the simple model I posted above.

 

[cesiumfrog]

Their results can be summed up as: "[..] walk as fast as the wind at your back,

Where was it summed up thus? It seems slightly inexact: you should walk slightly faster because there is a trade-off between preventing rain striking your front (or back) and minimising the time spent with the top of your head exposed, without considering further factors. And I'd like to think their experimental results involved statistics of more than two data points (unlike the mythbusters "method").

 

[Gokul43201]

It seems like a read somewhere the answer to this was that you would get the same amount of wetness either way. Don't remember where I read it though.

Intuitively I would think this to be true. Since there is a certain amount of rain that the rather large cross-sectional area of your front side would be running into (as compared to that of your head and shoulders), any benefits of running faster to avoid the amount of time your head (plus any other horizontal areas) is exposed to the rate of rain fall would cancel out (if not cause you to get wetter).

I don't think the faster you run the less wet you would be. If you run too fast you will run into more falling rain. It would effectively change the angle at which the rain is acting against you, exposing a wider surface area for the drops to hit your body.

This is nicely covered in marcus' post, which I quote:

We assume that the density of water in the air is constant (so many grams per liter, say), given whatever velocity the drops have. The frontal surface sweeps out the same volume, hence same mass of water, from A to B regardless of how fast you walk or run, so the only difference is how wet you get on top. As stated above, the faster you go the less wet on top.

PS: mgb - do you have a citation/link for your quote?