Why do fair people's skin turn blotchy and red when they go outside in the cold?

Vascular changes. Changes in temperature can cause changes in superficial blood flow to the skin. This is what often happens when one goes quickly from a warm environment to a cold one. Because your skin is fair this makes the change more obvious.
Circulation . The skin turns blotchy and red because the blood vessels go into spasm affecting blood supply. This can be a normal response or sign of blood vessel problems. Fair skinned people show the blood vessels under the skin more than people with dark skin. It happens to all people.