part_of part of 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. To be revised when we have information about the real definition. The definitions given by WMO cloud atlas are entered as comment, as these are not the defining features of the levels but the result of whatever it is that defines the levels (I assume temperature, density, etc. but have not yet found clear information about this). high étage