Tropopause

The tropopause is the atmospheric boundary between the troposphere and the stratosphere. Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry. More formally, it is the region of the atmosphere where the lapse rate changes from positive (in the troposphere) to negative (in the stratosphere). This occurs at the equilibrium level (EL), a value important in atmospheric thermodynamics. The exact definition used by the World Meteorological Organization is:

the lowest level at which the lapse rate decreases to 2 °C/km or less, provided that the average lapse rate between this level and all higher levels within 2 km does not exceed 2 °C/km.