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1.20 We have discussed the restrictions on cloud~base height but not on the depth of the convective layer. This is because for a given snow-surface temperature, tJ. rv varies with the height of cloud base (Fig. 1.10) but is almost independent of the depth of the convective layer up through the cloud to cloud top. AS long as the cloud base lies within range, the cloud depth can be of any value which matches the 0w criterion with the overlying cloud-free air (Telford and Chai, 1984). 3 . Concl asioDs . It is shown on the basis of the moist convective plume model that vapor flux from the evaporating snow surface can drive convection, and maintain a super-cooled water cloud layer, without the assistance of heat flux from the surface, or entrainment or radiative cooling, at cloud top. Since the saturation vapor pressure over water is higher than that over ice, the base of the super-cooled water cloud has a lower limiting height. When the cloud base is lowered to this height, the air at the bottom of the convective layer is just saturated with respect to ice and the evaporation of snow stops and with it the vapor-driven convec~ion. This limiting cloud base height depends on the snow-surface temperature. The lower the snow temperature, the higher the cloud base height limit for continued convective transfer from the surface. Since there are many factors controlling the convective field over snow, carefully designed observational experiments are indicated. Sensi- tive remote sensing instrumentations may have an advantage over other means of observations. References: Chai, S.R. and J.W. Telford (1983): Convection model for stratus cloud over a warm water surface, Boundary-Layer Meteorol., li, 25-49. Telford, J.W. and S.R. Chai (1984): Inversions, and fog, stratus and cumu- lus formation in warm air over cooler water, Boundary-Layer Meteorol., li, 109-137. I I I I ,I I I I I I I I I I I I' I I I