Fire Weather Index (Canadian Fire Weather Index System)

The Fire Weather Index is the headline number of the Canadian Forest Fire Weather Index System. It rates how intense a fire would be if one started, by bringing together two inputs: the initial spread index $ISI$, a rating of how fast a fire would spread, and the buildup index $BUI$, a rating of how much fuel is available to burn. It is a rating of weather and fuel moisture in a standard mature pine forest. It is not a prediction for any one fire, and it is not a physical intensity in kilowatts.

The calculation runs in stages. The buildup index, written $U$ in the formula, is first turned into a factor $f(U)$, with one form up to 80 and a second form above it. That factor is multiplied by $ISI$ and by 0.1 to give $B$. Up to $B = 1$ the index is just $B$; above that it is stretched logarithmically, so that the scale does not run away at the high end. The index has no upper limit, and agencies set their own classes for it by region, so a value is read against the classes that apply where the station is. Higher means a more intense fire.

Worked examples. The station values below are from the Canadian Wildland Fire Information System of 22 Oct 2026. Manchester Airport, with $ISI$ 19.8 and $BUI$ 35.5, has an index of 30.2. Albany has $ISI$ 5.0 and $BUI$ 126.6, which is above 80, so the second form of $f(U)$ applies, and the index is 21.6. Argentia, with $ISI$ 1.9 and $BUI$ 3.8, falls where $B$ is 1 or less, so there is no stretching and the index is 0.7, a published value rounded to a tenth. The page also runs backward: an index of 30.2 at a buildup of 35.5 returns an $ISI$ of 19.8.

Sources and limits. Van Wagner (1987) describes the development and structure of the system (Forestry Technical Report 35), and Wang, Anderson and Suddaby (2015) give updated source code for calculating its indices (Information Report NOR-X-424). Because the index rates conditions in a standard stand, not a particular fire, it tells you about the weather and the fuel moisture, not about what one fire will do.