  Evaporation of a liquid layer on a substrate is examined without the
often-used isothermality assumption -- i.e., temperature variations are
accounted for. Qualitative estimates show that nonisothermality makes the
evaporation rate depend on the conditions the substrate is maintained at. If it
is thermally insulated, evaporative cooling dramatically slows evaporation
down; the evaporation rate tends to zero with time and cannot be determined by
measuring the external parameters only. If, however, the substrate is
maintained at a fixed temperature, the heat flux coming from below sustains
evaporation at a finite rate -- deducible from the fluid's characteristics,
relative humidity, and the layer's depth (whose importance has not been
recognized before). The qualitative predictions are quantified using the
diffuse-interface model applied to a liquid evaporating into its own vapor.
