Venting is a common method to mitigate damage caused by explosions in low strength building and industrial enclosures. External explosion caused by combustion of external cloud (formed by vented gases) plays a crucial role in internal pressure rise. However there appears to be no modelling effort for external cloud formation in the existing literature. This paper presents a phenomenological model for the formation of the external cloud in vented explosions. This model employs experimental observation of flame propagation to formulate a correlation for flame velocity and prediction of the time for the internal flame to reach the formation. It utilizes the theory of vortex bubble formation, based on previous theoretical and experimental investigations. Detailed description of the modelling process and major assumptions involved are presented. The model is then validated with the cloud formation data obtained from experimental work reported in literature for hydrogen and methane, in enclosure volumes ranging from 1 m3 to 100 m3, for cylindrical and cuboidal geometries. Further predictions are made for various experimental configurations used for vented hydrogen explosions. variation of cloud dimensions with enclosure geometry and mixture reactivity is studied in detail. Inferences on dependence of cloud shape on various parameters are discussed and the observed trends are explained.