The term used to identify the minimum temperature at which a liquid gives off vapor in sufficient concentration to form an air-vapor mixture at the surface of the liquid that can be ignited by a small ignition source.
Class I flammable liquids have flash points below 100ºF. The flash point of motor and aviation fuels ranges from -36ºF down to -50ºF. Such fuels are easily ignited at almost all ambient conditions likely to be encountered. The minimum flash point for diesel fuel ranges from 100ºF to 130ºF, although some states require diesel fuels to have higher flash points. Liquids with a flash point above 100ºF are classified as combustible liquids.
Flash point is established through use of a precise laboratory procedure by passing a flame over a cup of the liquid as the liquid is slowly heated.
See also Flammable liquid.