# The Naked Scientists Forum

### Author Topic: Rain - Walk or Run to avoid getting wet  (Read 4171 times)

#### ejohnchat

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##### Rain - Walk or Run to avoid getting wet
« on: 28/10/2012 18:19:45 »
I was very intrigued by the question in late 2007 and early 2008 about how wet you can get running in the rain as compared to walking. I have always been a big proponent of walking as many of the summer rains here in the States are warm and quite enjoyable. So I began plugging some number into both the online calculator that you mentioned and another one I found. ( newbielink:http://www.dctech.com/physics/notes/0006.php [nonactive] , newbielink:http://www.amazingrust.com/Java/RainFall/RainFall.html [nonactive]) Both of them agree. You are better off traveling in the same direction of the horizontal component of the rain assuming of course that it has one and there a shelter nearby in that direction.  The calculations indicate that if your velocity is greater than the horizontal component of the rain's velocity, you get slightly wetter the faster you go. (My results were obtained with the rain's horizontal velocity at 2 m/s on the first website and the rain's angle at -22 degree to the front on the second website. All other inputs were left at the respective website's default values.) I found this very curious. So my advice is to just go with the wind.
Check it out and see if you don't get the same results.  Perhaps there is an assumption in the calculations that is causing them to give erroneous results.  Because even a very high travel speeds (>100 m/s) you still get wetter the faster you go!  So if you are going to run, don't run away from the rain unless it is faster than you are.
« Last Edit: 30/10/2012 00:34:04 by ejohnchat »

#### CliffordK

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##### Re: Rain - Walk or Run to avoid getting wet
« Reply #1 on: 28/10/2012 20:39:48 »
Interesting.

You may hit a minimum of overall body wetness only if you match the speed and direction of the rain.
If you are unable to match the speed and direction of the rain, then minimizing the time in the rain is beneficial.

One might also wish to consider the percent saturation of different body parts.

I.E.  Strolling in the rain, and your head might get completely drenched, but the front of your shirt may be generally dry.

Run in the rain, and your head gets struck with much fewer raindrops.  But, the front of the clothes gets wetter.  So...  say you get hit by 250 ml of rain.  Is it better for that whole 250ml to hit your head and shoulders, or is it better for it to be distributed over the whole body from head to toe?

#### ejohnchat

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##### Re: Rain - Walk or Run to avoid getting wet
« Reply #2 on: 30/10/2012 00:22:34 »
You only need to match the direction of the rain.  So long as you are moving faster than the rain, you are better off than running as fast as you can, assuming of course you can out run the rain and the equations closely model reality.

I agree that you only get marginally wetter if you run too fast.

Having all of the rain land on your shoulders and head is an advantage.  You have less surface area to dry off.

#### damocles

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##### Re: Rain - Walk or Run to avoid getting wet
« Reply #3 on: 30/10/2012 20:51:03 »
Assume an "ideal" person that has a flat front, a fixed height, and zero thickness front-to-back.

If such a person has to get from A to B in vertically falling rain, then the rain intercepted will always be whatever is contained in a parallelogram (parallelepiped) with base equal to the person's height and "altitude" -- actually a horizontal measurement in this case -- equal to the distance AB.

That means that in this ideal case it matters not whether you run or walk slowly.

However, most people have a finite front to back thickness. If we allow for rain that lands on the top of the person, as opposed to the front, then the amount of rain encountered will be proportional to the time spent in getting from A to B, so the faster the better.

As far as comfort is concerned, as opposed to actual water intercepted, it is really a choice between a brief discomfort and a more sustained lesser discomfort. But my practical advice would be to walk backwards as fast as you comfortably or safely can.

#### ejohnchat

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##### Re: Rain - Walk or Run to avoid getting wet
« Reply #4 on: 31/10/2012 03:12:36 »

Thanks damocles, you helped me figure out why it is that you are better off matching the horizontal speed of the rain.  It seems that the equations are not giving erroneous results.

Assume an "ideal" person that has a flat front, a fixed height, and zero thickness front-to-back.

If this same ideal person has to get from A to B and the wind is blowing the rain in the same direction as he is going and he travels very fast, he will intercept all of the rain directly in front of him, a rectangle equal in area to his height times AB.  Going slower, the rain he intercepts forms a parallelogram with his height at his starting position being the base of the parallelogram but the height of the parallelogram will be less than AB.  And the slower he goes, the shorter the parallelogram gets until he is going the horizontal speed of the rain.  Then the parallelogram height becomes zero. The area of a parallelogram is equal to its height times the base.  This explains graphically why it is best to match the rain's horizontal speed and direction and if you go faster than the rain, you will only get wetter.

#### imatfaal

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##### Re: Rain - Walk or Run to avoid getting wet
« Reply #5 on: 02/11/2012 12:00:37 »
Assume an "ideal" person that has a flat front, a fixed height, and zero thickness front-to-back.
.....

Sorry couldn't resist the obligatory xkcd

#### The Naked Scientists Forum

##### Re: Rain - Walk or Run to avoid getting wet
« Reply #5 on: 02/11/2012 12:00:37 »