The troposphere is heated from below by latent heat, longwave radiation, and sensible heat. Surplus heating and vertical expansion of the troposphere occurs in the tropics. At middle latitudes, tropospheric temperatures decrease from an average of 15°C (59°F) at sea level to about −55°C (−67°F) at the tropopause. At the poles, the tropospheric temperature only decreases from an average of 0°C (32°F) at sea level to about −45°C (−49°F) at the tropopause. At the equator, tropospheric temperatures decrease from an average of 20°C (68°F) at sea level to about −70°C to −75°C (−94 to −103°F) at the tropopause. The troposphere is thinner at the poles and thicker at the equator. The average thickness of the tropical troposphere is roughly 7 kilometers greater than the average tropospheric thickness at the poles.