The top-hat jet/plume model has recently been employed to obtain simple closed-form expressions for the mass of fuel in the flammable region of a vapor "cloud" produced by an axisymmetric (round) continuous-turbulent jet having positive or negative buoyancy [1]. The fuel release may be a gas or a volatile liquid. In this paper, the top-hat analysis is extended to obtain closed-form approximate expressions for the total mass (fuel + entrained air) and volume of the flammable region of a release cloud produced by either a round or a plane (two-dimensional) buoyant jet. These expressions lead to predicted average fuel concentrations in the flammable regions of the release clouds which, when compared with the stoichiometric concentration, serve as indicators of the potential severity of release cloud explosions. For a fixed release mass, the combustion overpressure following ignition of a hydrogen/air cloud is anticipated to be significantly lower than that due to ignition of a hydrocarbon/air cloud. The predicted average hydrogen concentration within the flammable region of the release cloud is below the lower detonability limit. The facility with which the expressions can be used for predictions of combustion overpressures is illustrated for propane releases and deflagrations in a closed compartment. (C) 2007 Elsevier B.V. All rights reserved.