Clouds block only as much as 20 percent of UV radiation. UV radiation also can pass through water, so don't assume you're safe from UV radiation if you're in the water and feeling cool. Also, be especially careful on the beach and in the snow because sand and snow reflect sunlight and increase the amount of UV radiation you receive. People with darker skin will resist the sun's rays by tanning, which is actually an indication that the skin has been injured. Tanning occurs when ultraviolet radiation is absorbed by the skin, causing an increase in the activity and number of melanocytes, the cells that produce the pigment melanin.
Melanin helps to block out damaging rays up to a point.
Those with lighter skin are more likely to burn. Too much sun exposure in a short period causes blood vessels to dilate, resulting in sunburn. A sunburn causes skin redness, tenderness, pain, swelling, and blistering. Sunburn becomes a more serious problem with fever, chills, upset stomach, and confusion. If these symptoms develop, see a doctor.