And the troposphere in the atmosphere. Very high up.
I can't remember why it is important.

troposphere
Noun 1. troposphere - the lowest atmospheric layer; from 4 to 11 miles high (depending on latitude)
layer - a relatively thin sheetlike expanse or region lying over or under another
atmosphere - the envelope of gases surrounding any celestial body
tropopause - the region of discontinuity between the troposphere and the stratosphere