Fire danger indices are not computed from general forecasts; they are computed from a specific, standardised observation set. A remote automatic weather station in a fire weather network reports hourly air temperature, relative humidity, wind speed and direction at the standard height, peak gust, precipitation, solar radiation and, at many sites, fuel temperature and fuel stick moisture. Two conventions matter more than the hardware. First, siting: a station must stand in fuels and exposure representative of the area it rates, or every index derived from it is representative of nothing. Second, timing: several systems key their daily rating to an observation taken at the hottest, driest part of the afternoon — around \( 13{:}00 \) local standard time in the United States and around noon in the Canadian system — because the day's peak is what the rating is meant to describe. The confusion this resolves is imagining an index summarises a whole day. After this Concept you can trace a published fire danger number back to the observation that produced it.
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…