stratosphere

The atmospheric shell lying just above the troposphere which is characterized by an increasing temperature with altitude. The stratosphere begins at the tropopause (about 10 - 15 km height) and extends to a height of about 50 km, where the lapse rate changes sign at the stratopause and the beginning of the mesosphere.
Source:
PAC, 1990, 62, 2167 (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2217