It's not that uncomun for people to be allergic to the sun, but there is special type of sun allergy, called solar urticaria, that is very rare. It is said that very few people in the world suffer from it. Solar urticaria (SU) is a rare condition in which exposure to ultraviolet or UV radiation, or sometimes even visible light, induces a case of urticaria or hives that can appear in both covered and uncovered areas of the skin. The most common locations include the "V" of the neck, the back of the hands, the outside surface of the arms and the lower legs. (Link)