The Canadian Forest Fire Weather Index System takes one daily noon observation — temperature, relative humidity, wind speed, and rainfall over the previous day — and computes six values in a fixed chain. Three fuel moisture codes come first, each with its own memory: the Fine Fuel Moisture Code for litter, responding within a day; the Duff Moisture Code for loosely compacted organic layers, over roughly a fortnight; and the Drought Code for deep compact organic matter, over roughly two months. Two behaviour indices follow: the Initial Spread Index combines the fine fuel code with wind to rate expected spread rate, and the Buildup Index combines the two slower codes to rate available fuel. The Fire Weather Index itself combines those two into a single intensity rating. The confusion this resolves is treating the six numbers as alternatives; they are a pipeline, each stage consuming the last. After this Concept you can say which stage of the Canadian system answers a given question about a day's fire potential.
The Fire Weather Index (FWI) System models forest fuel flammability as a set of interacting moisture "codes" that function as daily-updated memory variables: each code accumulates the effect of wetti…