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Flashing ratio is the portion of liquid that turns into vapor when a liquid is released into ambient conditions of lower pressure. As the high-pressure liquid escapes, the sudden pressure drop drives part of it to flash into vapor, creating a two-phase mixture of vapor and liquid. This ratio, defined as the mass (or volume) of liquid that flashes divided by the initial liquid amount, determines how much vapor is produced in the release.

Why this matters is that the amount of vapor in the cloud controls its size, buoyancy, dispersion, and the hazard it poses. A larger flashing ratio means more vapor, which can spread wider and faster and increase risks from flammable or toxic vapors and potential ignition scenarios. A smaller flashing ratio implies a jet with relatively more liquid and different hazard characteristics.

This concept is distinct from the ratio of vapor to liquid inside a closed tank or from condensation processes, or from the fraction that remains liquid, which do not describe the immediate flashing behavior during release.