Our understanding of precisely what damage Ultraviolet-A (UV-A) and Ultraviolet-B (UV-B) rays cause to the skin has evolved over time as new research has come out.
UV-B rays are Burning rays and are the predominant cancer-causing rays. They are most intense between 10 AM and 2 PM, in the summer, closer to the equator, at high altitudes, and on reflective surfaces such as water, snow or ice, which mirror back up to 80% of the rays.
UV-A rays are Aging rays; they cause wrinkles, brown spots, enlarged pores, and visible capillaries. UV-A rays are present from dawn until dusk every day of the year, and they penetrate ozone, clouds, and glass. Incandescent and LED lights emit small amounts of UV-A radiation. Compact fluorescents have a coating designed to prevent emission of UV-A rays; however, a study at Stony Brook University found that every bulb investigated had cracks in the coating, and thus released dangerous levels of UV-A rays. UV-A is the predominant tanning ray, causing cumulative skin damage and premature aging over time. UV-A rays are not as carcinogenic as UV-B rays, but they do contribute to and may even initiate the development of some skin cancers.
What we learn from this is that carcinogenic ultraviolet rays from the sun get to our skin even on cloudy days and even through glass; so we need to wear sunscreen against both UV-A and UV-B rays every day of the year. An added bonus of wearing sunscreen this way is that it will also prevent premature aging of the skin due to sun damage.