The turbopause marks the altitude in the Earth's atmosphere below which turbulent mixing dominates. The region below the turbopause is known as the homosphere, where the chemical constituents are well mixed and display identical height distributions; in other words, the chemical composition of the atmosphere remains constant in this region for chemical species which have long mean residence times. Highly reactive chemicals tend to exhibit great concentration variability throughout the atmosphere, whereas unreactive species will exhibit more homogeneous concentrations. The region above the turbopause is the heterosphere, where molecular diffusion dominates and the chemical composition of the atmosphere varies according to chemical species.
The turbopause lies near the mesopause, at the intersection of the mesosphere and the thermosphere, at an altitude of roughly 100 km.