The above figure is a temperature profile of Venus’ polar region (at 71° north), calculated with different upper-boundary conditions.
Regardless of the conditions, the models converge at the same temperature distribution below 90 km. The different atmospheric layers are indicated in the figure. The tropopause, at 62 km marks the transition between the troposphere and the mesosphere.
An isothermal layer, or a layer of approximately uniform temperature can be seen marked by a black box. It is the upper cloud layer in Venus’ atmosphere.