A note on the adiabatic lapse rate for saturated air at temperatures below 0°C

Abstract

AbstractI t is generally acknowledged that super‐cooled water drops can exist at temperatures far below freezing and are of common occurrence in cloud. In many cases, therefore, it is to be expected that a s saturated air rises, as for instance in a large cumulus cloud, its surplus water vapour, over and above its saturation water vapour content, will condense into super‐cooled water drops and not into ice particles.But a mass of air, at temperatures below freezing, ascending and condensing its surplus water vapour into super‐cooled water drops, will cool a t a rate different from that when the surplus water vapour condenses into ice. This is because the latent heat of vaporisation of water is less that the latent heat of sublimation of ice, and also because the pressure of saturated water vapour is less over ice than over water.Thus, since less heat is released on condensation into water than on condensation into ice, the adiabatic lapse fate of saturated air condensing its surplus water vapour into super‐cooled water drops, would be expected to be greater than that for air releasing its surplus water vapour to ice. This appears to be so, at any rate to temperatures as low as about 245 to 250°A.; below 245°A., the different behaviour of the saturated vapour pressures over water and ice respectively probably reverses the magnitudes, though the differences in the two lapse rates are small.Tables are given below for the computation of the two saturated adiabatic lapse rates, assuming that the heat content of the solid or liquid water is negligible compared with that of the air. It is also shown how to make the necessary (small) corrections to these saturated lapse rates to allow for the heat content of the condensed water vapour. The cumulative effect of the difference between the two lapse rates is shown to result in a temperature difference of about I°C. between the tops of super‐cooled water and ice clouds respectively, after convecting through 10,000 ft. above freezing level.