Daily Severity Rating (Canadian Fire Weather Index System)

The daily severity rating turns a day's fire weather index into a number for how hard that day is on fire control. The formula is short: raise the fire weather index $FWI$ to the power 1.77 and multiply by 0.0272. Both quantities are plain numbers with no unit, open-ended, and higher is more severe. The index being rated is itself made from the spread index and the buildup index, so this page adds no new weather; it only changes the scale.

The change of scale is the point. Because the power is greater than one, it weights the high end more heavily than the fire weather index does. A day at 40 is rated about 40 times as severe as a day at 5, though the index itself is only eight times as large. The constant 0.0272 sets the scale of the result. Read the answer as a ranking of how demanding days are, with the worst days standing out far more than they do on the index alone.

That is what the rating is for. Daily severity ratings can be added over days or averaged over a month to give a seasonal severity rating that tracks the effort a fire season demands, which the fire weather index itself cannot be. The worked example uses the Canadian Wildland Fire Information System table of 22 Oct 2026: at Manchester Airport an index of 30.2 gives a rating of 11.3, and at Albany an index of 21.6 gives 6.3, with the rating rounded to a tenth. Run backwards, a rating of 11.3 returns the index of 30.2, since the inverse is $FWI = (DSR/0.0272)^{1/1.77}$.

The rating is only as good as the index put in, and the exponent of 1.77 means that a percentage error in the index shows up as a larger percentage error in the rating. The sources are Van Wagner's 1987 report on the development and structure of the Canadian Forest Fire Weather Index System and the station values published by Natural Resources Canada.