  The interface between a pure liquid and its vapor is usually close to
saturation temperature, hence strongly hindering any thermocapillary flow. In
contrast, when the gas phase contains an inert gas such as air,
surface-tension-driven convection is easily observed. We here reconcile these
two facts by studying the corresponding crossover experimentally, as a function
of a new dimensionless number quantifying the degree of damping of interfacial
temperature fluctuations. Critical conditions are in convincing agreement with
a simple nonlocal one-sided model, in quite a range of evaporation rates.
