This is one of the questions that dog many an undergraduate physicist. The question of whether to run or walk in the rain. Now it is not as easy as you might think. Both Brainaic and Mythbusters have done this and come up with the conclusion that it is better to walk. Alas they are wrong. So first off what is the thinking that leads people to believe running is worse.

Basically the idea is that as you are running you are covering a larger area of rain. If you think of the rain falling down. imagine the rain that is falling on your head. When that rain falls on your head at a point directly in front there is the rain that is going to hit your feet  Say you are 2 meters high moving at 1 meter a second and the rain is falling at 2 ms second. That means that the rain that is one meter ahead of the rain falling on your head will hit the ground 1 second later or the time taken for you to put your foot there. Increasing your speed means that the distance from the drop that hits your head and your foot increases meaning you are hitting more rain in between those to points.

Ie. if you increase the adjacent side (4) the Hypotenuse (5) gets larger.  Therefore says the Walking fans you get wetter running.

The Runners however will point to the fact that the faster you go the less time you spend in the rain the less rain you hit.

Both of these Theory’s are true. However the question is which factor outweighs the other. The answer to which was given in 1987 by Alessandro De Angelis of the Istitto di Fisica Dell’Universita di Udine Italy. In his paper (which you can read if you have subs with the IOP). He modeled this assuming in the best physics tradition that a human is a rectangular box (physicists love assumptions particularly a particle in a infinite square well) of dimensions Dx, Dy, Dz. As time t is equal to distance over speed. u=sv/(dxdydz) where small d is the distance between drops of rain. ie as the distance between drops increase ie there is less rain the less wet you are going to get. The longer you spend in the rain the more drops you hit. v is your speed. Vop is the speed of the rain. So he came up with this equation.

Where N is the Number of drops. From this it was seen that as your speed increases your N decreases to a limit.
However before you all go sprinting in the rain. The result of the plot needs to be told. In his paper De Angelis compared a brisk walk to a world record run and found a difference of 10% in how wet you are going to get. So unless you are aiming for gold at the Olympics you are going to get less then 10% wetter walking. Which probably does not merit the effort or indeed the sweat( that portion of moisture not calculated) required to run.

So in conclusion. Buy an umbrella your going to get wet anyway.

Nice Java Applet to play with here