walking or running in the rain

[brikis98]

here's a random thing i've always wondered: say you need to get from your car to a building across a big parking lot. would you actually stay more dry by running or walking?

it seems obvious at first, since running would reduce the amount of time you're being hit by water. but here's where it gets confusing to me: when you run, you not only have the rain falling ON you, you also have (a lot more) rain that you run IN to. that is, if you picture yourself in a big cube of falling rain, when you standstill, the only rain that hits you is the stuff right above you. as you move forward, you get hit by the stuff right above you AND the stuff that had already fallen in front of you. i've noticed when i drive in the rain, for example, that the faster i go the more water seems to hit my windshield. this may be the effect of the water hitting harder and splashing more, but it seems like going faster makes things worse.

so i guess the question is what's the ideal speed to move at to stay as dry as possible?

edit: i'm guessing the answer will most likely depend on the rain - how dense it is, how much is falling, if it's at an angle, etc...

 

[f95toli]

If you are a built roughly like a rectangular box (a cuboid) you should run because that will allow you to get into the warm building quicker; but you will get just as wet as if you walk.
I think you have already figured it out: If you run you "hit" more rain from the side and it turns out that this exacly compensates from the fact that you spend less time in the rain. You can actually calculate this; you should be able to find the calculation on the web. I saw it in a book a few years ago.

This is obviously an approximation (it also assumes that you are moving, if you let the speed go to zero you would get infinitly wet before you reach the building) but it seems it is not too far from the truth. However, in real experiments it turns out that it is somewhat better to run; possibly because most people are not cuboids.

 

[Matthias99]

Mythbusters did a piece on this... their conclusion was that walking was better than running. However, they also talked to some researchers who had done some (relatively informal) experiments and got the opposite conclusion. I don't think they really answered the question definitively.

The answer probably depends on how hard the rain is, how the wind is blowing, etc.

as you move forward, you get hit by the stuff right above you AND the stuff that had already fallen in front of you.

Just to note, you're also *not* getting hit by some of the stuff above you that would have hit you if you were standing still. The amount of rain falling right on you should be the same (or very close to the same) if you are standing still, moving slowly, or moving fast; what differs is the amount of rain that you 'run into', so to speak, per second. The faster you move, the more rain you 'run into' per second -- but the less time you're in the rain overall.

 

[f95toli]

I should point out that the calculation I refered to above also assumes that there is no
wind.

 

[brikis98]

hm, i wish i had seen the myth buster's episode. theoretical calculations - where we might make unrealistic assumptions, such as people being cubes - are one thing, but actual experiments seem much more practical in this instance.

oh, and yeah, i think assuming the rain falls vertically down (no wind) is a reasonable assumption. it would be interesting to see something definitive though.

 

[CycloWizard]

I was wondering about this a while ago... Let me see if I can whip up my own model real quick and see what you guys think.

 

[CycloWizard]

OK, it looks like the relationship for weight gain with time is the following:

dW/dt = D*c*(v_d*a+v_r*b)

Where dW/dt is the rate of weight gain (the rate of getting soaked )
D is the rain 'density' (drops/m^3 or some such)
a is the person's 'thickness' (front to back)
b is the person's height
c is the person's width (shoulder-to-shoulder)
v_d is the rain's falling velocity
v_r is the velocity that the person is moving forward

Integrating gives the right-hand-side multiplied by t, where t is time in the rain and the left-hand-side becomes the total weight change.

W = D*c*(v_d*a+v_r*b)*t

I treated the person as a rectangular prism moving at a constant velocity, along with other standard assumptions you all mentioned above (rain falls straight down, constant density, constant velocities of rain and the person). Just looking at the equation tells me that the rate at which you get wet will always be higher for the person running (since v_r is higher). However, you need to determine the time required to get from place to place to know which will have a larger net wetting effect. I'll throw this in Excel and see how it comes out. It may even be that the answer differs for the fat guy over the tall guy, but if you're tall and fat, you're going to get soaked no matter what.

Edit: It looks like the net weight change is always lower for running. However, the walking skinny guys get as wet as the really fast, running fat guys. There is an asymptote at high velocities for each body type.

 

[f95toli]

The problem with actual experiments is that the result will depend on the shape of the body, the type of rain etc. I doubt you would find a clear answer.
Also, even simple models are sometimes useful and in this case I don't think the model is too unrealistic to give a usefull answer; people are not cuboids (not cubes, THAT would be an unrealistic assumption) but it is still pretty close to the real shape (if you stand still with your arms straight down they only thing that is "sticking out" fromt he cuboid shape is your head), there is a reason why many older FPS games had cuboid-shaped hitboxes (modern games tend to divide to body into more boxes, e.g. a separate cuboid for the head).
Hence, you would expect the answer to be approximately correct; i.e the difference between running and walking won't be huge regardless of the shape of your body.

 

[silverpig]

Walk.







With an umbrella.

 

[brikis98]

Originally posted by: CycloWizard
OK, it looks like the relationship for weight gain with time is the following:

dW/dt = D*c*(v_d*a+v_r*b)

Where dW/dt is the rate of weight gain (the rate of getting soaked )
D is the rain 'density' (drops/m^3 or some such)
a is the person's 'thickness' (front to back)
b is the person's height
c is the person's width (shoulder-to-shoulder)
v_d is the rain's falling velocity
v_r is the velocity that the person is moving forward

Integrating gives the right-hand-side multiplied by t, where t is time in the rain and the left-hand-side becomes the total weight change.

W = D*c*(v_d*a+v_r*b)*t

I treated the person as a rectangular prism moving at a constant velocity, along with other standard assumptions you all mentioned above (rain falls straight down, constant density, constant velocities of rain and the person). Just looking at the equation tells me that the rate at which you get wet will always be higher for the person running (since v_r is higher). However, you need to determine the time required to get from place to place to know which will have a larger net wetting effect. I'll throw this in Excel and see how it comes out. It may even be that the answer differs for the fat guy over the tall guy, but if you're tall and fat, you're going to get soaked no matter what.

Edit: It looks like the net weight change is always lower for running. However, the walking skinny guys get as wet as the really fast, running fat guys. There is an asymptote at high velocities for each body type.

interesting, wanna post the graph?

 

[smack Down]

Running is better.

The amount of rain that you run into depends on distance and not speed. Think about it as if all the rain stopped in mid-air.

 

[PowerEngineer]

Here's my take on this...

If you start with the assumption of a uniform rain (in which the density of drops within any volume of space is identical and constant, and as many drops are entering as are leaving), then it seems to me that your body will "run into" the same amount of water at any speed (because you essentially clear the same volume of space in moving from the parking lot to the building). Perhaps this is easier to visualize if you freeze all the rain drops in place and then think about moving through them.

Besides the water your body "runs into", we have to account for the rain that runs into you. For my "uniform rain", this should just be a function of the time you are out in it (and your rain shadow); it's the same regardless of your speed.

So, your total wetness should be some constant (tied to the volume of space you clear on the way to the building) plus another constant multipled by the time it takes you to reach the building. The faster you run, the drier you should stay.

I did see the Mythbuster episode, and I couldn't believe they didn't subject their results to more rigorous examination. I think their experiment must have been flawed.

 

[CycloWizard]

Originally posted by: brikis98
interesting, wanna post the graph?

I planned on it, but my department's webserver is down for maintenance. If someone can host, PM me and I'll send it to you.

 

[lyssword]

I noticed when I walk I get less wet, since rain falls mainly on top of my head. If I were to run the front of my legs + chest+face would get wet.

 

[brikis98]

ok, i've done some thinking and here is my simplistic view of the problem. say we represent the person with a rectangular prism with height h, width (shoulder to shoulder) w, and depth (chest to back) d...

(1) whether you run or walk, the top of the person is always exposed to the water - that's a surface area of dw... the amount of water collected by this part of the body is directly proportional to the time spent outside.

(2) as you move forward, however, you also allow for the front of the body - a much bigger surface area of hw - to collide with the rain that's below height h (and above the ground). the amount of water collected from this surface is directly proportional to the distance you cover.

if you stand still, only (1) gets you wet. you minimize wetness by getting inside as quickly as possible. however, as you start to move forward, provided that the vertical distance between rain drops is less than h, some part of (2) will begin to collide with the rain. however, since (2) is independent of movement speed... we again conclude that moving as fast as possible is the key.

however, the flaw is that humans are NOT rectangular prisms and although it's a nice approximation, it's inaccurate. the issue is that walking/running can increase the surface area exposed in (1) and (2) by a non-trivial and speed dependent amount.

for example, any walk/run causes you to swing your legs and arms forward and back. this obviously increases the surface area in (1). but, perhaps just as importantly, swinging the limbs back and forth means that you are making more contact with droplets ahead - and even behind you - at a height between 0 and h as in (2). as speed increases, a person swings their arms and legs more rapidly and even tilts forward, further increasing both (1) and (2).

therefore, there is a trade off between moving fast to minimize (1) but not so fast as to increase (1) and (2) from the flailing of your body... however, i have no clue how to formalize this "flailing" into a formula, so i again argue for a nice scientific experiment

 

[CycloWizard]

Originally posted by: brikis98
therefore, there is a trade off between moving fast to minimize (1) but not so fast as to increase (1) and (2) from the flailing of your body... however, i have no clue how to formalize this "flailing" into a formula, so i again argue for a nice scientific experiment

Of course that's all true. There are additional factors as well, such as how much rain bounces off versus how much sticks (which I considered adding to my model, but neglected for simplicity, as any numbers I used would be pretty arbitrary). You could add it in as a simple probabiltiy sticking factor (i.e. a multiplier from 0-1 for each impact direction). Adding in the more complex motions and geometries is a real task though... I might have time in a few weeks to come up with a more sophisticated model, but it would be a lot more work - not something I can do in ten minutes.

 

[Kelnoen]

too hard to answer accurately, it depends on so many factors it's ridiculous.

Walking/Running speed
Body shape
height
wind
drop density
drop size
distance
probability of water running off/soaking in

too complex
deep blue couldn't work that one out

 

[brikis98]

Originally posted by: Kelnoen
too hard to answer accurately, it depends on so many factors it's ridiculous.

Walking/Running speed
Body shape
height
wind
drop density
drop size
distance
probability of water running off/soaking in

too complex
deep blue couldn't work that one out

actually... i bet it wouldn't be that hard to simulate in a 3d modelling/animation program that can do hit detection.

all you'd need is to create (er, download) a decent 3d model of a human and animate them walking/running at various speeds... this kind of thing shouldn't be hard to find/make as it's been done a million times.

you then represent raindrops with simple dots and have them fall at some uniform rate while your model moves (at various speeds) through the rain. you then simply count the number of collisions and the speed which minimizes them is the winner.

 

[f95toli]

Yes, you are right. However, we already know from the approximate theoretical result that the difference won't be very large, and while adding more "situation-specific" parameters to the model will give a clear result it will only be valid for a that situation.
Some set of parameters are likely to give you the answer that it slightly better to walk, other that it is better to run.

This is the fundamental problem with numerical modelling: It is very difficult to draw GENERAL conclusions and when the parameter space is as large as in this problem it becomes almost impossible to test all possible cases.
That is the reason why physicists prefer to complement even very sophistacated computers models with simpler ones that can be handled analyticaly, the latter can often be used to predict qualitative properties.

 

[brikis98]

Originally posted by: f95toli
Yes, you are right. However, we already know from the approximate theoretical result that the difference won't be very large, and while adding more "situation-specific" parameters to the model will give a clear result it will only be valid for a that situation.
Some set of parameters are likely to give you the answer that it slightly better to walk, other that it is better to run.

This is the fundamental problem with numerical modelling: It is very difficult to draw GENERAL conclusions and when the parameter space is as large as in this problem it becomes almost impossible to test all possible cases.
That is the reason why physicists prefer to complement even very sophistacated computers models with simpler ones that can be handled analyticaly, the latter can often be used to predict qualitative properties.

yeah, i agree, it would be tough to get a general answer... as a compromise though, you can just get an answer for the average situation - that is, for the average rain storm, the average person, moving at the average speed, in an average parking lot... and then you can probably just use that as a "rule of thumb"...

 

[KeithP]

would you actually stay more dry by running or walking?

It depends it you are running or walking on a tread mill that matched your pace exactly.

-KeithP

 

[Madwand1]

Originally posted by: PowerEngineer
I did see the Mythbuster episode, and I couldn't believe they didn't subject their results to more rigorous examination. I think their experiment must have been flawed.

FWIW, they "revisited" this, and their second test overturned their original result, so the status is "confirmed" -- runners were drier.

But, if they can flip-flop in this way, perhaps their second experiment wasn't good either, and the truth is closer to that the way in which you do the collection and measurement makes a bigger difference in this experiment than the conditions supposedly being tested?

http://en.wikipedia.org/wiki/MythBusters

To date, two previously "Busted" myths have actually been overturned. The "Chicken Gun" myth became "Plausible" and the "Who Gets Wetter?" myth, which asserted that running in the rain gets one less wet than walking in it, became "Confirmed" after their revisits

 

[DrPizza]

This was one of many that mythbusters screwed up. (They're good entertainment, not good science.) But, to give them credit, they revisited it and realized that you get less wet if you run. Now, this is dependent upon having somewhere dry to run to... If you're going to be outside for an hour, running around until it stops raining, then you'd certainly get wetter (simply because you're sweeping out a larger volume).

However, if you have a destination to head to, say an entrance to a building 100 yards away, then common sense should dictate that you'll be less wet if you run than if you walk. To realize this, figure out what happens to someone who walks reallllllllly slowly for that 100 yards, as in their out there for an hour before they make it to the door. i.e. the more time you are out there, the wetter you are going to get, with the only difference being that (well, I like the analogy above about freezing water droplets in place) you sweep out a volume of space in less time (but, the same volume of space.) A couple of years ago, I found a source (online, perhaps?) that went into great depth on the issue, invoking the gods of differential equations before proving the solution.

 

[Jeff7]

Probably depends on the wind too. Here in Erie, when it's storming, you're going to get wet. I don't care if you run or walk, with or without an umbrella. You'll get wet. The wind will laugh at your umbrella, and the rain will continue unimpeded.

Best bet: full-length trenchcoat or raincoat.

 

[SRoode]

I'm a big believer in instincts. Since the human instinct is to run in the rain, it's probably the way to go. Millions of years of pre-programming there...