Fourth edition of tables for calculating the six standardcomponents of the Canadian Forest Fire Weather Index System. Thefirst three components are fuel moisture codes that follow dailychanges in the moisture contents of three classes of forestsfuel; the final three are fire behaviour indexes that representrate of spread, amount of available fuel, and fire intensity. The system provides a uniform method of rating fire danger acrossCanada.

Forest fire managers must be able to predict the normal variation in fine fuel moisture around the clock, since it is a critical factor that affects the behaviour and control of wildfires and prescribed burns. The Canadian Forest Fire Weather Index System contains a Fine Fuel Moisture Code (FFMC) that produces a standard daily index of expected fine fuel moisture at the mid-afternoon peak of fire danger, but does not output normal diurnal variation in FFMC. This paper documents the development and updating of an earlier model of diurnal variation in FFMC, presents the underlying equations and a lookup table, and announces the availability of a computer program version of the model for various user applications.