The Rayleigh lidar technique was applied to study the thermal structure of the middle atmosphere. Observations were carried out on a routine basis for one year (130 clear nights) at the main campus of York University near Toronto (44 ¿N, 80 ¿W). Mesospheric temperature inversions were generally found to occur below a height of 70 km during winter and above during summer. The most interesting aspect of our observations was that the inversions were often associated with an overlying nearly adiabatic lapse rate which extended for several kilometers. We interpret this as being an indication (or signature) of a well-mixed turbulent layer. A one-dimensional numerical model was applied to demonstrate that a well-defined turbulent layer within the mesosphere can bring about a thermal structure quite similar to that which was commonly observed - an inversion with overlying nearly adiabatic lapse. ¿ American Geophysical Union 1995