The more intense the sun, the greater your exposure to UV rays. The amount of UV that will reach you depends on the following:
Time of Day—UV is greatest when the sun is at its highest in the sky (between 10 am and 4 pm) and less in the early morning and late afternoon.
Season—While UV exposure is the greatest in the summer (May—August) in the United States, it is important to remember that UV rays reach Earth every day and you should be sun safe year-round—including wintertime! Snow can reflect 85% to 90% of the sun's UV rays!
Altitude—The air is cleaner and thinner at higher altitudes, so UV exposure is greater in the mountains than in the valleys. (For example, you can still get sunburned while skiing in the winter!)
Location—UV is strongest at the equator and gets weaker as you move towards the poles. Going tropical? Be prepared and take your sunscreen with you on family vacations.
Exposure Time—The longer you are out in the sun, the more UV rays you receive. Remember, you are exposed whenever you're out: picnics, Saturday yard chores, long drives, spectator/sports events, and more!