Re: walk to enjoy the rain

[Doug Craigen <dcc@CYBERSPC.MB.CA>]

> > > One question - does one get wetter walking in the rain or running. We are
> > > thinking of doing an expt with two students, two lab coats, and measure
> > > mass of coat before and after. Saving it for a rainy day.
> > By running, theoretically, as may be inferred from the limiting case as
> > v--> infinity.
>
> But how about the other extreme at v--> zero where one leisurely strolls
> through a rainstorm of infinite duration?  Fair is fair: If you can have
> infinite v, I can have infinite t :-)

There was a paper on this topic in the European Journal of Physics about
a decade ago.  If you stop and think about it, it does depend on the
question you ask, but usually one is expecting to get from point A to
point B.  In this case divide the rain velocity up into components
relative to your body and you have a certain flux hitting top areas
(bald spot, shoulders...) and another flux hitting the larger vertical
areas (chest etc).  In the latter case what you are looking at - for
vertically falling rain - is this: there is a certain amount of water at
any instant between your body and its destination... you will get
exactly this amount of water hitting the vertical parts of your body
regardless of what speed you run.  That leaves the parts hitting the
bald spot and shoulders.  Obviously this amount is propotional to the
time spent in the rain, and thus decreases with increasing speed.

The cases of non-vertical rain or "hunching over" in various fashions
while running can now be tackled as well by analogous approaches, but it
gets very messy. One case is to match your speed to the horizontal
component of the rainfall, hence reducing the rain to hitting the much
smaller head and shoulders targets.  One could also ask about the
dependence of the evaporation with increased running speed.

The general prediction from the EJP paper was that if you can run like
Donavan Bailey for your entire trip (not just 100 m), you might get some
significant benefit, but the rest of us shouldn't bother.

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Doug Craigen
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