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Flying circus of physics

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Run or walk in the rain?

Should you run or walk when crossing a street in the rain without an umbrella (Fig. 1-1)? Running certainly means that you spend less time in the rain, but it also means that you may be intercepting more of the raindrops. Does the answer change if a wind blows the drops either toward or away from you?

Answer  
If the rain falls straight down or if the wind blows it toward you, you should run as fast as possible. Although you run into raindrops, the decreased time in the rain leaves you less wet than if you move slower. To decrease the number of drops you run into, you should minimize your vertical cross-sectional area by leaning forward as you run. To move rapidly while also bending over, you might, as one researcher suggested, ride a skateboard through the rain, but that is certain to attract attention and, besides, a skateboard is more trouble to tote than an umbrella.

If the wind is at your back, the best strategy is to run at a speed that matches the horizontal speed of the falling drops. In that way, you still get wet on the top of your head and shoulders, but you do not run into drops along your front surface, nor do they run into you along your back surface. However, this strategy does not work for an object being moved through the rain if the object has a much larger horizontal cross-sectional area than you do. Such an object will collect an appreciable amount of water on its top surface even if its speed matches the horizontal speed of the raindrops. To minimize the wetting, such an object should be moved as quickly as possible.

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