The intensity of ultraviolet radiation (UVR) from the sun is directly related to the angle of the sun or altitude above the horizon. Solar altitude at any given clock time varies with season and latitude-longitude. The shadow rule indirectly determines the sun's altitude by observing the length of a person's shadow during the time course of a day. When the shadow on a horizontal surface in the sun is equal in length to height of the person casting the shadow, then the altitude of the sun above the horizon is 45 degrees. The tangent of 45 degrees altitude (tan alt) equals 1 or the ratio of the height of the person casting a shadow (y) to the length of the person's shadow (x). In middle latitudes, the intensity of UVR is approximately inversely proportional to the length of the person's shadow. Thus, when a person's shadow is shorter than the person is tall, the intensity of UVR from the sun is more likely to cause sunburn.