Releases and Dispersions: Dispersion: (1) Dispersion: The process of dilution of a hazardous substance by the surrounding fluid. (2) Gas: Adequately understood and used in common parlance. “Vapour” is sometimes used instead, particularly for the evaporation of a spill of liquid. (3) Gas Cloud: The mass of gas/air mixture within a particular envelope of concentration mix. (4) Peak Concentration: The highest concentration predicted at a point by a dispersion model. (5) Isopleth A surface joining points of equal concentration, i.e. A three-dimensional “contour”, in a gas cloud. (6) Dense Gas Cloud: A gas cloud which is heavier than the surrounding air immediately after the release process, because either the gas is a dense gas, or the mixture has a temperature sufficiently below ambient. (7) Neutral Density Gas Cloud: A gas cloud, which has a density equal to that of the surrounding air. (8) Buoyant Gas Cloud: A gas cloud, which is lighter than the surrounding air. (9) Dense Gas: A gas whose density exceeds that of air at the same temperature. (10) Passive Dispersion: A dispersion process depending only on atmospheric conditions in which the properties of the dispersing material do not affect the local turbulence. (11) Wind Speed: The mean speed of the air past a stationary point at a specified height, e.g. 10m. (12) Surface Roughness: A measure related to that component of the turbulence of the atmosphere which is aided by the departure of the ground profile from perfect smoothness. (13) Weather Category: A measure related to that component of the intrinsic turbulence of the atmosphere, which is specifically determined by thermal stability. (14) Puff: The gas cloud resulting from an instantaneous release. (15) Plume: The gas cloud resulting from a continuous release.