Why doesnt air mix between the troposphere and stratosphere?

Air does not mix between the troposphere and stratosphere primarily due to the presence of a temperature inversion at the tropopause, which is the boundary between these two atmospheric layers. In the troposphere, temperature decreases with altitude, while in the stratosphere, it increases. This temperature difference creates a stable layer that inhibits vertical mixing, preventing the two layers from intermingling significantly. Additionally, the stability of the stratosphere prevents turbulence that could facilitate mixing.