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Staying dry in rain: run or walk?

Published: 
Monday, July 23, 2012
If you run, you are out in the rain for less time, yet you run into more drops—so what is the optimal speed?

A physicist has put forth new ideas in the long-running question of how best to keep dry when moving in the rain. If you run, you are out in the rain for less time, yet you run into more drops­—so what is the optimal speed? Franco Bocci, reporting in the European Journal of Physics, now asserts that both wind direction and a person’s stature figure into the answer. In most cases, the general answer is to run as fast as possible; but the answer changes in a tailwind, or for the thin. Prof Bocci is by no means the first to address the problem, which is far more mathematically complex than it seems on the surface. In the 1970s, a number of papers came out in mathematics magazines debating the question, each more fully exploring the issues at hand. The battleground for this bit of hobby mathematics now seems to be the UK’s Institute of Physics publication the European Journal of Physics. In 1987, another Italian researcher asserted in the journal that changing strategies did not make a substantial difference. In 2011, a textile expert and a physicist used the same publication to suggest that an optimal speed existed, depending on the wind direction. “For the most part in the previous work, there was a simple answer, but I found that the problem is much more complicated,” Prof Bocci told BBC News. What complicates the question is the human shape; for simplicity, previous attempts to crack the thorny problem assumed people to be thin sheets or upright, rectangular boxes.

When Prof Bocci considered a more general case—likely to be the case you would face in the rain—he found that the answer may depend on an individual’s height-to-breadth ratio as well as wind direction and raindrop size. Luckily there are a few generalisations in the analysis, to spare you having to calculate the cosine of the angle between your path and the wind direction. “Let’s say that in general, the best thing is to run, as fast as you can—not always, but in general,” said Prof Bocci. “If you’re really thin, it’s more probable that there will be an optimal speed. Otherwise, it’s better to run fast.” As for wind direction—and again, in general—you should run as fast as you can unless the wind is behind you, in which case the optimal speed will be exactly the speed of the wind. Prof Bocci said the problem promises to get even more complicated as more factors are taken into account, but that for now he is drying his hands of the question. (BBC) 

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