That warmest part of a slope above a valley floor lying between the layer of cold air that forms over the valley floor on calm, clear nights and the cold hilltops or plateaus. The air flowing down the slopes is warmed by mixing with the air above ground level and to some extent also by adiabatic compression. The frostless zone is not a fixed belt but varies in level from night to night and season to season according to the initial temperature, the length of the night, and the clearness of the sky. Its lower limit is sometimes clearly marked by the upper limit of frost damage to crops, following the hillsides at a small angle to the horizontal. See thermal belt.