The short answer:
It's better to run.
The long, and slightly more entertaining answer:
Thomas Peterson and Trevor Wallis, meteorologists from the National Climatic Data Center in Asheville, NC set out to disprove the 1995 research of Dr. Stephen Belcher, of the University of Reading in Britain: the doctor had stated in the meteorological journal Weather that there was little to no difference between walking and running, and that the same number of raindrops hit you regardless of your speed (Don't dismiss that thesis out of hand just yet--it'll come up later).
The clever lads bought two identical sets of sweats and caps and measured off a 100-meter test track. Then, during a downpour, one of them walked (1.5 m/s) and one of them ran (4 m/s) the distance. They wore garbage bags under their clothes to eliminate the contribution of sweat to the clothing's wetness, and weighed the two sets of apparel afterward. The walker absorbed .22 kilograms of water while the runner absorbed 40 per cent less, about .13 kilograms. Their 1997 report, also published in Weather, was celebrated as the authoritative word on the ages-old question. Wallis stated that the research "confirms something that much of us learn as children. Loitering in the rain gets you wetter."
But there's a catch!
This holds true for a rainstorm of finite distance, but over long distances or at automobile speeds, other effects come into play that change the game. Yes, it's child's play to note that the less time you spend in the rain (by moving through its space boundary, the door, sooner) the dryer you will be. But what about rainstorms that don't have a boundary in space? Let's take a quick look:
Imagine the rain as an even distribution of water particles with a known vertical velocity vy and a known density ρ. Furthermore, this rainstorm is infinite in space: it goes on evenly in all directions, with the same velocity characteristics throughout. We'll simplify your clothing as a rectangle with area A, and assume that the rain has no velocity perpendicular to gravity and your motion: that is, no crosswind. Since the rain is uniform no matter how far you go, it's easy to imagine that you are stationary and that the rain is moving horizontally towards you at a variable speed vx, resulting in each raindrop having an identical velocity vector V. One last thing: since the area A is approximately your cross section perpendicular to the flow of raindrops, A is going to be a function of vx, varying from your footprint (roughly your shoulder width times your bum-to-belly or bum-to-bust depth) to your frontal area (your height times your shoulder width). A(v) will become a maximum when the ratio of your speed to the vertical speed of the rain is equal to the ratio of your depth (that's bum-to-belly or bum-to-bust) to your height. A(v) will be minimized at one of the two extremes--your footprint or your frontal area, whichever is smaller.
How wet you get over a finite time t can then be calculated as
Wetness = t * q * A(v)
where q = V * ρ
Since we hypothesize a rainstorm of finite time and infinite space, t is not under your control. Likewise, the rain's downward velocity and density are not yours to change. You have already been given the opportunity to change your shape--once stuck in the rain, we hypothesize that a crash diet won't help you. The only parameter under your control is your horizontal speed! Luckily, that influences both A(v) and V. To minimize Wetness, we come to the following conclusions:
- If your height is larger than your depth, stand still. A(v) and V are minimized, and while you will get wet, you are better off than if you ran.
- If your depth is larger than your height (imagine Superman flying horizontally), you need to make your horizontal speed equal to the vertical speed of the rain times the ratio of your depth to your height; the quantity A(v) * V is darn near minimized. Otherwise, just lay still. If you're capable of orienting yourself so that your smallest cross section is horizontal, do so, and stand still.
- If you want to get as wet as possible, run at a horizontal speed which maximizes A, or make your horizontal velocity as high as possible.
A special case
If you are in a
convertible with the top stuck down or other
very fast vehicle which has its own surface area (some of which you do not want to get wet), there are certain
aerodynamic effects I have ignored. If you
tailgate a tractor trailer (or an
articulated lorry if none are available), you will rest inside a tunnel of relatively
stagnant air insulated from this ideal but infinite storm by a
boundary layer of fast-moving air. If your car goes fast enough, you may even be able to get most of the convertible's front seat inside the
boundary layer created by the car's own windshield. The downside is that you're trading a wet frontal area for a dry top surface--your
windshield wipers will need to work harder to keep the windshield clear, and you risk reducing your visibility to near-zero as you increase speed.
Disclaimer: As with all of my pseudo-science nodes, feel free to try this at home, but I'm not going to be responsible if you catch a cold or blow your fingers off.
Follow-up:
Cecil Adams, of
The Straight Dope fame, has also answered this question, though not in as much gory detail as I. His column (which also mentions Peterson & Wallis,
supra) can be found at
http://www.straightdope.com/classics/a3_395.html