I used to think that my itchy legs were caused by the cool Fall air when I ran outside; turns out, it has nothing to with the weather and everything to do with my running schedule. Itchy legs are caused by capillaries and arteries in your muscles that expand rapidly as your heart beats faster. If you haven't run for a while, your blood vessels begin to collapse and don't allow as much blood through. The message is sent by nerves to your brain, which registers the sensation as itchy.
The best solution? Don't slack off! The more often you run, the less chance you have of experiencing the uncomfortable itchiness.