mes·o·sphere 
(mĕzə-sfîr′, mĕs-)
(mĕzə-sfîr′, mĕs-)n.
The portion of the atmosphere from about 50 to 80 kilometers (31 to 50 miles) above the surface of the earth, characterized by temperatures that decrease from 10°C to -90°C (50°F to -130°F) with increasing altitude.
mes′o·spheric (-sfîrĭk, -sfĕr-) adj.