19) Are the risks of ultraviolet (UV) exposure at the beach less on a cloudy day?
Not necessarily. The effect of clouds on UV radiation is as varied as the clouds themselves. Fully overcast skies lead to reductions in surface UV irradiance. On average, scattered or broken clouds also cause reductions, but short-term or localised UV levels can be larger than for cloud-free skies if direct sunlight is also present. Clouds tend to randomise the directions of the incoming radiation (because of scattering) so that a hat may provide less protection on a cloudy day relative to a clear day.
Furthermore, people often change their behaviour on cloudy days. If they spend more time out in the open, or forego the use of sunscreen, they may end up with a very bad sunburn. In general, less UV radiation is received per hour under an overcast sky than under a clear sky, but extending one�s stay at the beach may easily compensate for this effect. A completely cloud-covered sky may still transmit substantial amounts of UV-B radiation. In principle, any amount of UV-B radiation exposure contributes to the skin cancer risk.
Sunbathing
20) Will sunscreens protect one from harmful effects of increased UV-B radiation?
Not always. Sunscreens applied to human skin limit the penetration of UV radiation into the skin, and thus sunburn can be prevented. Sunscreens were primarily developed for this purpose. The effectiveness of sunscreens in protecting against skin cancer and immune suppressions is under debate. Any effectiveness in these respects may well be lost if the sunscreen is used to stay out in the sunlight longer than would be done without the sunscreen. It should also be kept in mind that there are other ways to protect the skin. These include staying out of the sunlight during the hours when the UV-B is maximal around solar noon, seeking the shade, wearing clothes, and especially hats.
21) Will getting a suntan help prevent skin cancer?
No. There is no evidence that getting a suntan will help prevent skin cancer. The UV exposure needed to acquire the tan adds to the skin cancer risk. The fact that one is able to tan well does, however, signify that the personal risk is lower (by a factor of 2 to 3) than for people who do not tan. Naturally dark-skinned people have a built-in protection of their skin against sunlight.
http://www.gcrio.org/UNEP1998/UNEP98p73.html