Final answer:
The optimal campsite in a cold winter backcountry setting would be an area shielded from the wind and capable of retaining heat, such as a spot nestled between evergreen trees and a cliff.
Step-by-step explanation:
The best place to set up a campsite on a backcountry trip with cold, clear skies and light winds would be an area that provides natural shelter and can retain warmth during the night. Drawing from references to camping in literature, one suitable location would be between spruces or other evergreen trees and a cliff face, as described in the provided passages. These areas can offer shelter from the wind, trap heat that has been absorbed by the rocks throughout the day, and minimize the exposure of the campsite to the open sky which can lead to temperature loss through radiation. Moreover, selecting a site with dry ground and utilizing natural rock formations or caves for additional protection can create a comfortable and secure camping environment.