Friday, July 20, 2012

How to Stay Dry in the Rain Even If You Don't Have an Umbrella

Well, it's more like how to get less wet, although a WaPo headline says "How to Stay Dry", but there is some physics that can be applied. It's very complicated physics that even includes the shape of your body.

Brad Plumer at WaPo explains:

In the latest issue of the European Journal of Physics, Franco Bocci ponders one of life’s enduring mysteries. If you’re caught in the rain without an umbrella, is it better to run or walk in order to stay as dry as possible?

Running seems entirely sensible—you can get to dry shelter quicker. But you also hit more raindrops by running. So there’s a genuine trade-off. Bocci used math to explore the problem more thoroughly and found that running is usually better, although it really depends on the situation:
The problem of choosing an optimal strategy for moving in the rain has attracted considerable attention among physicists and other scientists. Taking a novel approach, this paper shows, by studying simple shaped bodies, that the answer depends on the shape and orientation of the moving body and on wind direction and intensity. For different body shapes, the best strategy may be different: in some cases, it is best to run as fast as possible, while in some others there is an optimal speed.
Basically, the best strategy for staying dry (or at least somewhat dry) is to run as fast as possible. Unless you’re really thin, in which case there may be a more optimal speed. And if you’ve got a tailwind behind you, then you should run exactly as fast as the wind at your back.
This sounds like a weird, random topic, but the BBC’s Jason Palmer reports that the raindrop problem has famously vexed physicists and mathematicians for decades:
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….
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.


  1. I move at whatever speed minimizes the amount of rain that hits my bottom.

    I don't mind getting wet hair, wet shoulders, wet pants, and so on, but a wet bottom, while sitting in a chair, is the worst.

  2. Dude, don't you watch discovery? Myth busters solved this years ago!

  3. How about this? I suppose that you will hit the same number of rain drops whether you run or walk for a certain distance (relative velocity etc.,), if the rain falls at the same rate. If you run, you hit them faster, if you walk, you hit them slier, but basically the same number. Even in that case, the outcome is not entirely clear as at that point in time that you decide to either run or walk, you do not know how the rate of rainfall will change in the future. What if the rain fall rate increases? You will be better off running and getting to shelter sooner. What if the rainfall rate will slow down? Then you will be better off walking. What if it stops soon? Are you not missing the opportunity to walk in rain free climate if you run faster while it is raining? It seems as soon as you incorporate the actor into the picture, the problem of imperfect information and uncertainty become issues. It would be interesting what the econometricians (who are neither physicists nor economists) have to say about this.