Dynamics of the dispersion of a fixed mass of cryogenic natural gas (NG) vapour produced from LNG (liquefied natural gas) boil off is examined. The vapour is considered to have been suddenly exposed to an overlaying atmosphere within open vertical cylindrical enclosures with a negligible pressure difference. A 2-D axis-symmetric CFD model was used to resolve the resulting complex temporal changes of the concentration and temperature fields. Focus was made on the features of the flammable regions, from their formation to dissipation, both inside and immediate outside of the enclosure. It was observed that with adiabatic walls, the structure of the flow changes from diffusion driven to buoyancy driven as methane warms up on mixing with the air to the atmospheric temperature. The effect of heat transfer from the walls on the evolution of the flammable cloud is discussed. The associated potential fire hazard is assessed based on the propagation rate of the lower flammability limit of the NG vapour cloud. (C) 2010 Elsevier Ltd. All rights reserved.