THE SHADOW RULE - A SIMPLE METHOD FOR SUN PROTECTION

The intensity of ultraviolet radiation (UVR) from the sun is directly related to the angle of the sun or altitude above the horizon. Solar a ltitude at any given clock time varies with season and latitude-longit ude. The shadow rule indirectly determines the sun's altitude by obser ving 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 s un above the horizon is 45 degrees. The tangent of 45 degrees altitude (tan alt) equals I or the ratio of the height of the person casting a shadow (y) to the length of the person's shadow (x). In middle latitu des, the intensity of UVR is approximately inversely proportional to t he length of the person's shadow Thus, when a person's shadow is short er than the person is tall, the intensity of UVR from the sun is more likely to cause sunburn.