Why does wet skin sunburn faster?
I don't know of any research to find out if skin sunburns faster when wet, though someone did a comparable experiment to find out if plants can be burnt by sunlight focussed through drops of water after the plants have been watered.
You need to be clear what is being measured here. The total amount of sunlight hitting you, and a plant, is unaffected by whether you're wet or not. The question is whether water droplets can focus the sunlight onto intense patches causing small local burns.
The answer is that under most circumstances water droplets do not cause burning because unless the contact angle is very high they do not focus the sunlight onto the skin. Burning (of the plants) could happen if the droplets were held above the leaf surface by hairs, or when the water droplets were replaced by glass spheres (with an effective contact angle of 180º).
My observation of water droplets on my own skin is that the contact angles are less than 90º, so from the plant experiments these droplets would not cause local burning. The answer to your question is (probably) that wet skin does not burn faster. I would agree with Will that the cooling effect of water on the skin may make you unaware that you're being burnt, and this may lead to the common belief that wet skin accelerates burning.
I'm no expert in biology, and biology may describe this phenomena? I am applying what I know about physics.
Sunburn is caused by excessive exposure to skin-damaging UV light.
I tend to believe that wet skin by itself does not cause you to burn faster, one merely feels cool in water, so one is prepared to stay out in the sun longer. I can't see any reason for why wet skin would amplify the effect of burning (barring localized droplets, possibly causing what I will call "spotted sunburn"). In fact, some frequencies of UV are absorbed by water, which would seem to reduce the effect (by how much, I'm not sure). In fact, having a layer of water on your skin would seem to help reduce sunburn (however slightly) because some of the UV will be reflected by the water.
That being said, consider a large body of water. Here are a couple of situations you might find yourself in:
If you have your body submerged in water it is possible that more UV could strike your skin due to the water surface not being flat (a lensing effect).
If you are above the water (for example, on a boat, or just your head and shoulders poking out of the water, etc.) there will definitely be more UV striking your skin, due to there being not only direct light from the sun, but reflected light from the surface of the water.
Consider these facts: [cdc.gov]
- Grass reflects from 2.5 to 3 percent of UV rays hitting its surface.
- Sand reflects 20 to 30 percent of UV rays.
- Snow and ice can reflect 80 to 90 percent of UV rays.
- Depending on the angle of reflection, water can reflect up to a full 100 percent of UV rays striking the surface.
As such, one explanation might be:
Most UV rays from the sun (which cause tans and sunburns) can reflect off the surface of water. This is actually why so many people get sunburnt while swimming or wading in water -- UV radiation will reflect off the surface of the water onto whatever part of a person is above the water, giving a swimmer or wader a double dose of UV radiation, versus someone far enough inland to not be affected by reflection off the water. The UV reflection is strongest when sunlight's angle to the water is at its lowest, within a few hours of sunrise or a few hours before sunset. By contrast, more UV rays penetrate water and can cause sunburns on completely submerged divers during mid-day, when the sun is high overhead.