Initial Spread Index (Canadian Fire Weather Index System)

The Initial Spread Index answers one question: how fast would a fire spread? It is the first of the three indices of the Canadian Forest Fire Weather Index System, and it is built from only two things, the wind and how dry the finest fuels are: needles, grass and the top of the litter. It says nothing about how much fuel is there; the buildup index does that. Natural Resources Canada publishes it every day for each weather station in the fire season. The index is a plain number with no unit, open-ended, and higher means faster.

The two inputs enter as a product. The wind factor is $f(W) = e^{0.05039\,W}$, with $W$ the wind at 10 m in the open at the noon observation. Because it is exponential, each extra kilometre per hour raises the index by about 5 per cent, and the page does not cap it at high wind. The fuel factor starts from the fine fuel moisture code $F$, which runs from 0 for soaked litter to 101 for bone-dry (90 is about 11 per cent moisture). The formula turns the code back into a moisture content, $m = 147.2\,(101 - F)/(59.5 + F)$, and then $f(F) = 91.9\,e^{-0.1386\,m}\left(1 + m^{5.31}/4.93\times10^{7}\right)$. The negative exponent makes the factor fall as the moisture rises. The constant 0.208 sets the scale of the product.

The worked example is a station of the Canadian Wildland Fire Information System (CWFIS) on 22 Oct 2026: Manchester Airport, with a code of 90.9 and wind of 27.8 km/h, rated 19.8. Bridgeport, with a drier code of 93.3 but a lighter wind of 20.4 km/h, rated 19.2, almost the same. Duck Lake, with damper fuel at 80.2 and a strong wind of 48.3 km/h, rated 13.2, which shows how much work the wind factor does when it is not capped; an index with the wind factor flattened above 40 km/h gives 11.3 there. The page also runs backwards: the wind CWFIS used at Duck Lake, 48.3 km/h, comes back from the index of 13.2 and the code of 80.2.

The sources are Van Wagner's 1987 report on the development and structure of the system and the 2015 updated source code of Wang, Anderson and Suddaby. The published index is rounded to a tenth, and the code and wind it was computed from are rounded as well, so the checks against the station table allow a tolerance of 0.3 to 1 per cent.