SPF stands for Sun Protection Factor. SPF indicates how much solar energy (ultraviolet radiation) is required to cause sunburn on an individual's skin when using sunscreen compared to the amount of solar energy required to cause sunburn on unprotected skin. Factors influencing the total amount of solar energy include, the amount of time in the sun, the time of day of sun exposure (an hour of sun exposure at 9:00 am may be equal to 15 minutes of solar energy obtained at noon), location, with greater intensity occurring at lower altitudes, and weather conditions, as clouds can absorb solar energy thereby making solar intensity greater on clear days. SPF is a relative measure of sunburn protection provided by sunscreens. As the SPF increases, so does the sunburn protection. An SPF of 30 prevents 97% of UVB rays from reaching the skin’s surface. An SPF of 50 prevents 98% of UVB rays from reaching the skin's surface. No sunscreen provides 100% protection.