Sunburn and Sun Exposure

sun exposure at 45 degrees north latitude

Figure 1. Sun exposure at 45 degrees north latitude near the summer solstice according to time of day and exposure angle. Zero degrees means the skin faces directly upward, 90 degrees means the skin is vertical (e.g. legs when walking).

Exposure of skin to sunlight can cause sunburn, skin cancer, and premature aging. When is sun exposure greatest? Sunlight intensity depends upon the time of day, time of year, and angle between the sunlight and the exposed skin. Figure 1 give the direct beam sunlight exposure according to angle and time of day. The graph is drawn for 45 degrees latitude near the summer solstice. All exposure angles (N, S, E, W) are weighted evenly to obtain an average value. The worst situation is for 0 degrees (horizontal skin) at solar noon. This occurs when laying out on the beach to get a tan. The best case is for vertical (90 degree) skin (e.g., the legs when standing).

Prudent hikers to not expose their skin to the sun. A hat with a wide brim will stop noontime rays. The shoulders and any other horizontal portions of skin should be covered. The interesting observation from the graph is that the worst time of day for exposure depends upon skin angle. Vertical portions of the skin such as legs when walking, the parts one cannot easily protect with a hat, have greater exposures in morning and afternoon than they do around solar noon. This effect, in general, becomes more pronounced when moving north or south of the equator.

Many other factors are of greater concern for outdoor sun exposure. Clouds, snow, and water cause reflection of sunlight. This turns the direct beam shown in Figure 1 into a diffuse radiation coming more equally from all directions. Diffuse radiation situations often cause sunburns as common protections such as hats are less effective, we get less warning when the sun does not feel as hot, and vertical or even downward pointing portions of skin receive abnormally high radiation loads. Diffuse loads are generally higher at solar noon.