The most obvious difference between these shoes and previous UltraBOOST X releases? These have a knit, sock-like upper (the part of the shoe that covers your foot) that extends to just above the ankle, to keep you warm and prevent water from getting in your shoe, according to Alexa Andersen, category director for women’s running at Adidas North America. "Just think about those times when you go for a run in the cold weather, and your ankles get cold due to the gap between your tights and shoes," Andersen said. When we wore the shoes, it was pretty warm out, but they definitely have just enough height to fill that shoe-legging gap for when it gets colder (and the fabric is breathable enough to wear when it's 80 degrees out). They're also designed with "more aggressive outsole lugs," so you have better traction and grip in wet conditions, Andersen said.