Zones, the name given to belts of climate on the surface of the earth marked off by the tropical and polar circles, of which the former are 23½° from the equator and the latter 23½° from the poles, the zone between the tropical circles, subject to extremes of heat, being called the Torrid Zone, the zones between the polar circles and the poles, subject to extremes of cold, being called respectively the North Frigid Zone and the South Frigid Zone, and the zones north and south of the Torrid, subject to moderate temperature, being called respectively the North Temperate, and the South Temperate Zone.