SPF stands for sun protection factor and is a measure of the protection against UV-B rays offered by a given sunscreen. The higher the SPF number, the greater the degree of protection. The number is calculated as the amount of UV radiation required to cause a sunburn with a user wearing the sunscreen as opposed to not wearing the sunscreen. However, the efficacy of any sunscreen depends on several variables, including the time at which a person is outside (more intense radiation occurs between 10am-3pm); the type of skin a person has (lighter-skinned individuals require much less UV radiation to burn than those with darker skin); and the amount of sunscreen applied (in general, the average adult in a bathing suit needs 1-2 oz of sunscreen to cover the body appropriately which means that the standard 12 oz bottle should only last for 6-8 uses!).
You are absolutely correct that as a person with pale skin you are at higher risk for sun damage. In general, to protect the health of your skin and minimize your risk of skin cancer, it is recommended to stay out of the sun between the hours of 10am-3pm when UV radiation is at its most intense. If you do have to be in the sun, wearing a wide-brimmed hat and a cover-up is a good idea. A sunscreen with a high SPF and protection against both UV-A and UV-B rays--and applied liberally--is also strongly recommended.
Questions about skin health or about worrisome skin findings should always be discussed with you primary care physician
. He or she can discuss strategies to protect your skin and be sure to conduct regular skin exams.