- the boundary or transition zone between the mesosphere and the ionosphere.Compare mesosphere(def 1).
- the top of the mesosphere, determined by the appearance of a temperature minimum near an altitude of 50 miles (80 km).Compare mesosphere(def 2).
- meteorol the zone of minimum temperature between the mesosphere and the thermosphere
- The boundary between the upper mesosphere and the lower thermosphere, approximately 80 km (50 mi) above the Earth’s surface. It is the site of the coldest temperatures in the Earth’s atmosphere, with temperatures of -100°C (-148°F).