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Math Science

Physicist Reveals Why You Should Run in The Rain (sciencealert.com) 107

Posted by BeauHD from the common-dilemmas dept.
Theoretical Physicist Jacques Treiner, from the University of Paris Cite, explains why you should run in the rain: ... Let p represent the number of drops per unit volume, and let a denote their vertical velocity. We'll denote Sh as the horizontal surface area of the individual (e.g., the head and shoulders) and Sv as the vertical surface area (e.g., the body). When you're standing still, the rain only falls on the horizontal surface, Sh. This is the amount of water you'll receive on these areas. Even if the rain falls vertically, from the perspective of a walker moving at speed v, it appears to fall obliquely, with the angle of the drops' trajectory depending on your speed. During a time period T, a raindrop travels a distance of aT. Therefore, all raindrops within a shorter distance will reach the surface: these are the drops inside a cylinder with a base of Sh and a height of aT, which gives:
p.Sh.a.T.

As we have seen, as we move forward, the drops appear to be animated by an oblique velocity that results from the composition of velocity a and velocity v. The number of drops reaching Sh remains unchanged, since velocity v is horizontal and therefore parallel to Sh. However, the number of drops reaching surface Sv -- which was previously zero when the walker was stationary -- has now increased. This is equal to the number of drops contained within a horizontal cylinder with a base area of Sv and a length of v.T. This length represents the horizontal distance the drops travel during this time interval. In total, the walker receives a number of drops given by the expression:
p.(Sh.a + Sv.v). T

Now we need to take into account the time interval during which the walker is exposed to the rain. If you're covering a distance d at constant speed v, the time you spend walking is d/v. Plugging this into the equation, the total amount of water you encounter is:
p.(Sh.a + Sv.v). d/v = p.(Sh.a/v + Sv). d This equation proves that the faster you move, the less water hits your head and shoulders, but the amount of water hitting the vertical part of your body remains constant. To stay drier, it's best to move quickly and lean forward. However, you'll have to increase your speed to offset the exposed surface area caused by leaning.

Physicist Reveals Why You Should Run in The Rain More | Reply

Physicist Reveals Why You Should Run in The Rain

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  • Umbrella (Score:5, Informative)

    by stooo ( 2202012 ) on Wednesday October 23, 2024 @06:03AM (#64886639) Homepage

    Use an umbrella. Duh.

    • Simpler explanation: In the limit, standing still forever is the worst and moving instantaneously is best. The optimum movement is somewhere in between.
      • I might characterize "somewhere in between" as "what will actually happen"... rather than "optimum", which'd be -- as you point out -- moving instantaneously. Well heck, as long as I'm being pedantic :) maybe the very worst would be walking exactly under the rain cloud as it moves forever. Or instantaneously running into the nearest ocean.
      • Yes, but it might not be a linear function. There might be local minimum and maximum only some of which are in the range of human speed. Although I agree that a linear approximation probably works just fine and agrees with the instinct to run as fast as you can.

    • Re: (Score:1)

      by Anonymous Coward

      But my umbrella always inverts when I run in the rain!

    • Re:Umbrella (Score:4, Interesting)

      by larryjoe ( 135075 ) on Wednesday October 23, 2024 @10:23AM (#64887193)

      Use an umbrella. Duh.

      The physicist is correct about running to decrease the amount of attracted water. He also mentioned that running faster is better. However, he left out the most important part, which is that running at the speed of light will minimize the water. Of course, that's silly. What's interesting is how fast one has to run to approach zero water.

      Then again, running in the rain maximizes splashing water on one's shoes and pants, so that's worse.

      We need to ask for Randall Munroe's thoughts on this.

      • Re: (Score:2)

        by higuita ( 129722 )

        if you run faster than light (FTL), you travel back in time and go back to the time where is no rain, so problem solved, run FTL

      • Re: (Score:2)

        by tlhIngan ( 30335 )

        Then again, running in the rain maximizes splashing water on one's shoes and pants, so that's worse.

        It also increases the chance of slipping and falling, and getting even more wet as you land on the wet surface you're walking on.

        I think just having an umbrella or wearing rain gear will just let you walk normally in the rain and not get wet. And you can take your time in case it's particularly slippery.

    • Re: (Score:3)

      by Sloppy ( 14984 )

      Or just live in the desert, so that you frolic in wetness whenever you're luck enough to get rain. (Best during the first monsoon of the summer.)

      • I remember one summer driving south along the I-25 through New Mexico and getting caught by a storm so heavy that it wasn't safe to drive faster than about 40 MPH. Not that that didn't stop some ID10Ts from zooming along at 65 or more, of course, and there were a fair number of cautious drivers pulled over to wait it out. Summer storms in the desert can get pretty nasty sometimes.
    • Using an umbrella complicates the problem significantly. Depending on the diameter of the umbrella, it's possible that you would stay completely dry while standing still but above a certain velocity would start to get wet as you moved horizontally into rain drops at lower height than the umbrella.

    • Re: (Score:2)

      by shanen ( 462549 )

      Completely different math with an umbrella. I guess you'd be looking for the speed limit where the umbrella is destroyed by the wind or water.

  • The quicker you reach shelter the drier you'll be. If you stay out in the rain it doesn't matter how you move you'll get just as wet, it'll just be on a different part of your body.

    • Clothing (Score:5, Funny)

      by Okian Warrior ( 537106 ) on Wednesday October 23, 2024 @09:36AM (#64887033) Homepage Journal

      The quicker you reach shelter the drier you'll be. If you stay out in the rain it doesn't matter how you move you'll get just as wet, it'll just be on a different part of your body.

      And to be pedantic, note that *you* are water proof, it's your clothes that sop up the water.

      Your body is well suited to being in the rain. Remove your clothes and it doesn't matter how wet you get, once you get out of the rain you'll dry very quickly.

      Then you can put your clothes back on.

      (This post is made in the spirit of all the other posts, such as "use an umbrella", currently in this thread :-)

      • Re: (Score:2)

        by piojo ( 995934 )

        And to be pedantic, note that *you* are water proof

        That is my favorite thing to remind people of when it's wet outside. Oh, don't worry about me. My skin is waterproof!

      • Re: (Score:2)

        by cstacy ( 534252 )

        The quicker you reach shelter the drier you'll be. If you stay out in the rain it doesn't matter how you move you'll get just as wet, it'll just be on a different part of your body.

        And to be pedantic, note that *you* are water proof, it's your clothes that sop up the water.

        Your body is well suited to being in the rain. Remove your clothes and it doesn't matter how wet you get, once you get out of the rain you'll dry very quickly.

        Then you can put your clothes back on.

        So I should strip off all my clothes, and run through the rain, while using my umbrella.

  • Did they factor in the chance of slipping? (Score:5, Insightful)

    by fleeped ( 1945926 ) on Wednesday October 23, 2024 @06:15AM (#64886653)
    If you run it's more likely you'll slip and get full-blown wet. That's not factored in, so, back to your equations, Dr Treiner...

    • If you run it's more likely you'll slip and get full-blown wet. That's not factored in, so, back to your equations, Dr Treiner...

      We've just been given a running recommendation, from an expert who understands everything, and yet nothing.

      Hell. I’ve never seen a finer example of an MBAs actual value.

      *$350/hr. golf clap*

      TL;DR - This.

    • Re: (Score:2)

      by mjwx ( 966435 )

      If you run it's more likely you'll slip and get full-blown wet. That's not factored in, so, back to your equations, Dr Treiner...

      If it's raining hard enough that you need to run, you're going to get full blown wet no matter what, you're running in order to spend less time in the rain. Where the fuck is my research grant money?

      • Im a runner: If its not raining, and its hot, you get wet no matter what from sweat. If its cold, not raining, and you wearing 2 layers, you get wet from sweat. Almost all conditions, you wet. Running in the rain in warm weather, assuming its light so you dont get blinded by the rain, is the same.

      • Re: (Score:2)

        by TWX ( 665546 )

        you're running in order to spend less time in the rain.

        This is really the only answer that anyone needs. Presuming that the rain is constant enough, the amount of water falling on the person is a function of exposure time. Running at around 6 miles an hour will result in being out in the rain half the time that walking at around 3 miles and hour will.

      • If you fall, there is an extra risk of being hurt. You don't want to be wounded when full-blown wet. The physicist also suggest leaning forward. So you get even more risk of slip and fall.

    • Re: (Score:1)

      by Tablizer ( 95088 )

      The Ministry of Falling Down approves this message.

  • You fly in a superman orientation infinitely fast. Then the only raindrops that hit you are those that were blocking your route.

    For light rain it's possible to dodge the raindrops completely.

    For heavy rain and a sufficient distance it's best to go vertically until you're above the clouds.

    • According to the article:

      The water hitting the vertical part of your body stays the same regardless of speed

      That intuitively seems false, but I can't figure out the math (the notation is weird, which doesn't help). I read the article several times, and I don't understand the logic of this physicist. The organization of the article is horrific. For example, this sentence:

      Let p represent the number of drops per unit volume

      What is "unit volume?" Volume of what? The only other use of the word "unit" in the entire article is unit of time!

      • Re: (Score:3)

        by Ambvai ( 1106941 )

        'Per unit volume' is used all the time in cases where the exact unit doesn't matter, other than it being volumetric. 15,000 raindrops per cubic meter, 120 raindrops per cup, etc. Outside of my field, I think the most I've ever seen that term used is to define density: "Mass per unit volume".

        As far as the first point goes: Try envisioning a horizontal rectangle with a constant amount of dots in it, where the dots move downward from the top at a constant rate at a random point along the x-axis, with a bar swe

        • Re: (Score:2)

          by piojo ( 995934 )

          Try envisioning a horizontal rectangle with a constant amount of dots in it, where the dots move downward from the top at a constant rate at a random point along the x-axis, with a bar sweeping across the rectangle. Over repeated trials, the bar will touch the same amount of dots regardless of how fast it sweeps across the rectangle, but the odds of a dot spawning right on the bar (rain landing on your head) is decreased at higher speeds.

          That doesn't sound right. Regardless of speed, the bar's top always occupies the same proportion of the horizontal space.

      • Re: (Score:3)

        by unrtst ( 777550 )

        According to the article:

        The water hitting the vertical part of your body stays the same regardless of speed

        That intuitively seems false, but I can't figure out the math (the notation is weird, which doesn't help). I read the article several times, and I don't understand the logic of this physicist. The organization of the article is horrific. For example, this sentence:

        Agreed. I had started a reply cause I thought it was making sense, but I was reading that as "the water hitting the _HORIZONTAL_ part of your body", and I thought it just needed a timespan qualification, like:
        "The (amount of) water hitting the (horizontal) part of your body stays the same (over a given period of time) regardless of speed"

        But that's not what they're saying. And it makes zero sense to me that the water hitting the vertical part of your body would remain the same regardless of speed. This can

    • Or fly around the planet in the opposite direction of earth's rotation at the speed of light, until you turn back time just enough to before you left your house. This time don't forget your umbrella.

  • Mytbusters did this one (Score:5, Informative)

    by GrumpySteen ( 1250194 ) on Wednesday October 23, 2024 @06:42AM (#64886689)

    They came to the opposite conclusion but they were test IRL [youtu.be] and not just theorizing.

    • It was mostly due to splashing, which wet the lower half of their bodies. The devil is in the details when it comes to models.

    • Theorizing has the benefit of being backed by some specific formulas that describe a system. If Mythbusters ever used a scientific method they'd probably bust that too. There are many "myths" they "busted" which were none the less actually facts not myths. You need to remember they are an entertainment show, not very scientific.

      My favourite one was the experiment that assumed that a boat had the same forces applied to it by water as by air and called the myth busted despite there being photographic evidence

    • Paper (Score:2)

      by JBMcB ( 73720 )

      My uncle and his friends wrote a sorta-joke paper about this in college, but it was a serious analysis with a few experiments thrown in.

      His findings were similar to the MythBusters, it didn't really make a difference if you were running or walking. He didn't take into account leaning, though.

      As for his bona fides, he has a doctorate in physics with a specialty in thermodynamics.

  • MythBusters would like a word (Score:3)

    by quonset ( 4839537 ) on Wednesday October 23, 2024 @06:45AM (#64886697)

    MythBusters did this very experiment [youtube.com] a while back. According to their experiment, running increases the amount of rain falling on you compared to walking.

  • Next up (Score:3, Funny)

    by cascadingstylesheet ( 140919 ) on Wednesday October 23, 2024 @06:51AM (#64886711) Journal

    Some research on running with scissors.

    Of course, the form you have to sign to participate is a bit longer ...

  • once while riding a motorcycle (Score:3)

    by FudRucker ( 866063 ) on Wednesday October 23, 2024 @06:51AM (#64886715)
    it started raining, you know what rain feels like at 65 mph? it feels like being poked with lots & lots of needles, when i got home i was chilled to the bone, pulled into the garage and pulled the door shut, stripped naked water poured out of my boots, my clothes were dripping and made a puddle i had to sweep out the door, then i went straight to a hot shower and hot bowl of soup afterwards,

    • Re: (Score:2)

      by Sique ( 173459 )
      But that's exactly one of the points: The amount of water hitting you in the front stays the same for the distance. If you travel faster, you will encounter more front splash per time. If you are not moving at all, you encounter infinitely little front splash, but it takes infinitely long to get out of the rain.

      So, all that matters is the rain dropping on head and shoulders, and this one you can minimize by reducing the amount of time out in the rain.

      • I didn't realize that motorcycles could operate in the rain. I thought that you had to park under an overpass and wait until the weather cleared up.

        • Re: (Score:2)

          by Sique ( 173459 )
          Depends on your tires and your risk management. Friend of mine commutes to work using the motorcycle, and only under really bad conditions, he comes in late.
    • A good fairing is one of the most underrated features of a motorcycle. Almost no reviews even talk about their efficacy....

    • Rules (Score:3)

      by JBMcB ( 73720 )

      This is veering into Rule 34 territory, especially the part about the soup :)

      • I had to look it up.

        Rule 34, Federal Rules of Procedure: Producing Documents, Electronically Stored Information, and Tangible Things, or ... for Inspection and Other Purposes.

        I'm not a lawyer, but it sounds to me like it says pictures are required.

  • ... one should not run in the rain. Years of scientific research have proven so many times already that one should DANCE IN THE RAIN! :-D

  • What about the drops of water splashing back up off the ground which will increase when you start walking/running?

  • Physicist Reveals HOW You Should Run in The Rain

    HOW you should run in the rain. Not WHY. What sort of raving incompontent produces this drivel and... .oh, wait. This is one of those AI blurbs isn't it.

    Journalism, and communication, is dead. This is our dystopian future.

    • Actually, it is "why" as in why you should run instead of walk. In addition to the why, he also explained how you should run, i.e., leaning.

  • no use (Score:3)

    by znrt ( 2424692 ) on Wednesday October 23, 2024 @07:08AM (#64886745)

    no use to complain,
    when you're caught out in the rain,
    your mother's quite insane.
    cat food, cat food, cat food, again?

    • Re: (Score:2)

      by msauve ( 701917 )
      If the rain comes, they run and hide their heads
      They might as well be dead
      If the rain comes
      If the rain comes
  • I run in the rain like an idiot and wind up breaking my ass. Then I'm really wet. I prefer umbrellas.

  • Seems incorrect (Score:4, Insightful)

    by Paul Carver ( 4555 ) on Wednesday October 23, 2024 @08:24AM (#64886881)

    This doesn't seem to account for the horizontal velocity of the rain and it includes the false statement "When you're standing still, the rain only falls on the horizontal surface".

    Generally speaking, rain and wind are highly correlated because the atmospheric conditions that lead to rain are also involved in the creation of wind. Although it is possible for rain to fall straight down, it's not a valid assumption to say that it will fall straight down.

  • ah man, now I need to go and apologise to my brother, he told me this when were like 8 or 7, I laughed at him. Damn science!

  • Was this peer reviewed?

    Until then I will continue to use an umbrella to stay drier in the rain.

  • Here I thought I was reading about rain somehow creating less wind resistance or better for your body, etc. only to find out it's an article about staying dry.

  • I'd have thought that the volume you pass through remains constant. Assuming constant rainfall, the amount of water in this volume is also constant and you collect all of it. Thus the Sv component shouldn't change. The faster you move the less rain from above so faster is better for the Sh part.
  • This is bullshit math.

    But I wasn't really paying attention because I was unfolding an umbrella.

  • To stay drier, it's best to move quickly and lean forward. However, you'll have to increase your speed to offset the exposed surface area caused by leaning.

    Skipping that this contradicts the statement preceding it, where forward movement just shifts the raindrop hits to the front of your body from the top, while the total number of hits remains constant, when I read the headline, I thought this was going to be a physiological argument where the rain kept you cooler, so you could exercise harder and longer, like drinking a slushie into your gut during a marathon.

  • In the documentary featuring Gene Kelly, and what he recommended

  • This was a homework question in my first-year physics course in 1976.

    This ain't "news".

  • So, you might get less wet but then you bust your tail ;P

  • There are 2 things that the computations are neglecting: 1) wind and 2) that one may prefer to have 1 side of the body relatively dry.
    1) Wind will cause 1 side of the vertical surface to be impacted by more water droplets than the other.
    Depending on which way one is facing and the velocity of the wind, it is possible that 1 side remains dry at the expense of the opposite side.
    2) I generally don't mind if my back gets wet, because it will often be placed next to the back of a chair/seat minimizing the effect

  • They weighed a jumpsuit after walking and running through simulated rain. Came to the opposite conclusion.

    • I remember watching that. I also remember that being one of the least scientific-method-y tests that they ever did. I was immediately skeptical then, and remain now.

      • I was in the Navy at the time it dropped and multiple people around me stopped running to get out of the rain after watching it.
        I'm not sure what the episode said but those people got really fucking wet and despite being relatively drenched, they still felt the need to explain that mythbusters has assured them that my drier clothes were not the result of running.

        Trust me man it's on TV.

  • I suspect that both the speed and the direction of any wind that's blowing render the analysis in TFA invalid.

  • a spherical cow

  • I didn't even get wet. Honnist.

  • I had a physics class where we had to do this analysis.

  • The excellent newsletter of the Danish engineer's union "IngeniÃren" worked on this problem a while back, though for bikers. Turns out when you move fast enough, you also get an "air envelope" that pushes done of the drops that would otherwise have hit you out of the way.

  • ... until the rain stops.

  • Del Shannon thinks you should be walking in the rain [youtube.com].

  • So you can fall on your ass running on wet pavement and said physicist can get a laugh.

  • The surface area exposed to rain increases with velocity, Id rather be shoulder wet the front wet. Plus a fall hazard exists when running

  • But please - don't leave the cake out in it.

  • I think putting on a raincoat and holding a bag over your head would be better than running without protection. Also, maybe you'd like to NOT risk a slip-and-fall or a collision when you can't maneuver as well as you are used to. There are more things to optimize for than "number of raindrops hitting you". Bu of course this is a physics problem featuring spherical physicists. Fritz Zwicky was right.

  • Will you stay dryer if you dance in the rain?

  • I imagine a rectangle with a narrow top and a large front . As you move through the rain you rotate it forward so only the top catches the rain.

    Only, you can transform this into another problem. Simply a car driving over a large plain . It starts on (x,0) and drives towards the Y axis while keeping a constant velocity in the Y direction.
    How should it drive then to minimize the driving distance. Well, the higher the speed in X direction the better.
    The amount of rain you catch is now equivalent to the driving

  • Of someone hunched forward running in the rain, as always is the case?

    Yay! We figured out what physics now tells us a long time ago.

  • As a physics grad student at UCLA, I worked out this identical problem in 1982 for undergraduate students in a homework session for an introductory physics course, and my solution yielded the exact same results. The problem had been assigned by the professor for those students, so it doesn't take a "theoretical physicist" to figure out.

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