50 Miles High, an Upper Atmospheric Boundary
Puzzle 2 of Cold Places Easy Pack
Question is: 50 Miles High, an Upper Atmospheric Boundary
and the answer is: MESOPAUSE
Word Meaning:
MESOPAUSE — (meteorology) In the atmosphere, the boundary between the mesosphere and the thermosphere or ionosphere. It is the atmospheric boundary where the temperature reaches its minimum value.
Definitions from Wiktionary.