A front that forms as a cyclone moves deeper into colder air. This front will separate air behind the cold front from air ahead of the warm front. This is a common process in the late stages of wave-cyclone development, but is not limited to occurrence within a wave cyclone. There are three basic types of occluded front, determined by the relative coldness of the air behind the original cold front to the air ahead of the warm (or stationary) front. 1) A cold occlusion results when the coldest air is behind the cold front. The cold front undercuts the warm front and, at the earth's surface, coldest air replaces less cold air. 2) When the coldest air lies ahead of the warm front, a warm occlusion is formed, in which case the original cold front is forced aloft at the warm front surface. At the earth's surface, coldest air is replaced by less cold air. 3) A third and frequent type, a neutral occlusion, results when there is no appreciable temperature difference between the cold air masses of the cold and warm fronts. In this case frontal characteristics at the earth's surface consist mainly of a pressure trough, a wind-shift line, and a band of cloudiness and precipitation. See bent-back occlusion.