Releases and Dsperions: Behaviour of Releases: The type of release depends on the manner in which the containment system fails, the physical properties of the material involved and the storage conditions. Some initiating events lead to such a rapid release of inventory that they are termed Instantaneous Releases. Other produce discharge over a prolonged period and are termed Continuous Releases. Quantification of the rate of release is achieved using fluid mechanics principles applied to single or two phase flow. Gas held under pressure as liquids form an important category of hazardous substances. On depressurisation a proportion of such materials vaporise with a resulting decrease in temperature. This can result in two phases flow in the leak path, for example, if a liquid off-take pipe is severed at a point some distance away from a vessel. This is known as Flashing Flow. In many cases the driving pressure is sufficiently high for this flow to become choked and so the substance is still under pressure on release to the atmosphere, where further flashing occurs. Once depressurisation to atmospheric pressure is complete the temperature of the material will have fallen to its normal boiling point. The proportion which would be vaporized if the entire depressurisation were carried out adiabatically, is known as the Flash Fraction. It provides an estimate of the maximum proportion of a superheated liquid emission which promptly vaporises on release to the atmosphere. Rapid depressurisation is a violent process and such a pressure burst may be hazardous in its own right and much of the remaining liquid fraction may be atomised. Momentum Turbulence will entrain the surrounding fluid and a proportion of these droplets may rain-out and subsequently vaporise. Heat transfer between the surrounding medium usually the atmosphere  and the suspended liquid droplets may also lead to further vaporisation. These factors tend to increase the vapour fraction beyond the theoretical flash fraction. Pools, formed by spills of materials, which are normally liquids at atmospheric temperature and pressure, evaporate by atmospheric convention and solar heating. Pools, formed by gases, which are liquefied by low temperature, vaporise, also taking heat from their surroundings, in this case mainly from the sub-medium. The rate of these processes is characterised by the Regression Rate of the liquid pool. This rate is enhanced significantly if the vapour is flammable and is ignited, in which case the term linear burning rate is usually used. The quantity of released material made airborne, its form, composition, and temperature, can therefore depend on many factors. The description of the release required as input to consequence models, particularly dispersion calculations, is known as the Source Term.