A Laboratory Experiment of Thermals with Generation of Heat

Abstract

A model experiment of the developing stage of a cumulus cloud was performed by the use of NH3 gas and HCl gas which release heat of reaction by mixing. The white smoke of NH4Cl (thermal) grew upwards because of the buoyancy created by heat of reaction.The thermal had a strong tendency to grow upwards at the vigorous reacting stage and the half-broadening angle was inversely proportional to the height of the top of the thermal. Such properties were also observed in a large natural cumulus cloud. The entrainment coefficient of this thermal was 0.24. This value is in fairly good agreement with that of McCarthy's observation (0.3). The Froude number of thermal in the experiment was estimated as 4.0. This value was larger than that of a thermal with a constant total buoyancy.