UV rays are present all year round. Even when the sky is cloudy, up to 80% of UV (A & B) rays hit the ground. It doesn't matter if it’s summer or winter or if you're spending the majority of your time indoors. UVA rays penetrate through windows and they're responsible for photoaging. That means that they cause everything that you don't like about your skin when it gets older (fine lines, wrinkles, loss of firmness, pigmentation, etc.). That's why the best thing that you can do for your skin is wearing sunscreen every. single. day.