timberline, elevation above which trees cannot grow. Its location is influenced by the various factors that determine temperature, including latitude, prevailing wind directions, and exposure to sunlight. In general, the timberline is highest in the tropics and descends in elevation toward the polar regions; in the north it intersects the land surface approximately at the Arctic Circle. For example, the timberline is at about 2,500 ft (750 m) on Mt. McKinley, Alaska; 6,500 ft (1,950 m) on Mt. Shasta, Calif.; and 11,500 ft (3,450 m) in the San Francisco Mts., Ariz. These figures represent elevations on the sunny side of the mountains; the timberline is lower on the shaded sides. The timberline is roughly marked by the location of the 50°F (10°C) isotherm (see isopleth) during the warmest month.