Long-haired cats have two coats of fur they switch between when the seasons change. It's one part genetics and one part reaction to the weather, sunlight, and other stimuli. When winter starts to creep in and the days get shorter and colder, your cat will start to grow a thicker, fluffier undercoat. This coat is highly insulating and helps keep them warm in the coldest months, even if they spend time outside or on a cold windowsill.