Dark snow often contains black carbon—a particle emitted by burning wood, diesel and coal—it heats up faster, just like when you go outside in the sun wearing black. Usually, clean snow reflects about 90 percent of the sun's light and heat. But if it becomes impure, this lessens its reflectance, or albedo. The less reflective the snow is, the more it heats up and melts. The more the snow melts, the more it exposes dirt, which reflects about 70 percent less sunlight, thus heating up the Earth even more. It's called a "positive" feedback loop, which is scientist lingo for "vicious cycle."