Running through the rain will keep you a lot drier (not just 10% drier) than walking.
First the theory. We divide the raindrops hitting you into two categories: (1) head drops, which fall from above and would hit you even if you were standing still; and (2) chest drops, which you run/walk into and which wouldn't hit you if you were standing still. We can all agree that the number of head drops is strictly a function of how long you're out in the rain; if you run, fewer head drops. The question is whether the allegedly larger number of chest drops you get when running outweighs the definitely larger number of head drops you get while walking.
If we ignore aerodynamic effects, we can show mathematically (but won't) that while you'll collect many fewer head drops running rather than walking, you'll get exactly the same number of chest drops, regardless of the speed at which you travel. Bottom line: you'll be a lot wetter if you walk.
http://www.straightdope.com/columns/read/827/which-will-keep-you-drier-running-through-the-rain-or-walking
