the boundary layer between the troposphere and stratosphere, where an abrupt change in temperature lapse rate usually occurs; it is defined as the lowest level at which the lapse rate decreases to 2 degrees Celsius per kilometer or less, provided that the average lapse rate between this level and all higher levels within 2 kilometers does not exceed 2 degrees Celsius per kilometer; occasionally, a second tropopause may be found if the lapse rate above the first tropopause exceeds 3 degrees Celsius per kilometer.