A curvature of 1.0 appears as a circle of an angular radius of 57.3° corresponding to an altitude of approximately 2,640 km (1,640 mi) above Earth's surface. At an altitude of 10 km (6.2 mi; 33,000 ft), the cruising altitude of a typical airliner, the mathematical curvature of the horizon is about 0.056, the same curvature of the rim of circle with a radius of 10 m that is viewed from 56 cm directly above the center of the circle. However, the apparent curvature is less than that due to refraction of light by the atmosphere and the obscuration of the horizon by high cloud layers that reduce the altitude above the visual surface.