On those super frigid days it might seem like the sun’s not pulling its weight. But don’t be deceived: It’s working as hard as ever. While UVB radiation does weaken in winter, UVA rays stay at a constant level all year round—and they cut through clouds. These rays play a big role in skin aging and wrinkling, and may also up your risk of cancer. So before you head into the cold, dark day, put on your hat, your scarf—and your sunscreen.