part of adjacent to stratosphere troposphere atmospheric layer middle atmospheric level high atmospheric level A part of the troposphere which extends from 3 km to 8 km (10 000 – 25 000 ft) height in the polar region, from 5 km to 13 km (16 500 – 45 000 ft) height in the temperate region, and from 6 km to 18 km (20 000 – 60 000 ft) height in the tropical region. high étage