A fire danger rating combines weather, fuel moisture and fuel type into a number describing the general severity of conditions over an area for a day. Three properties define what it can and cannot do. It is relative: values are interpreted against the local climatology of the same index, so a value that is alarming in one region is ordinary in another, which is why agencies place the raw index against its local history (often as a percentile) before binning it into the published categories such as Moderate, High or Extreme. It is areal and daily: it describes a rating area for a day, not a point at an hour. And it presumes nothing about ignition: a maximum rating with no ignition produces no fire at all. Danger ratings are therefore planning instruments — for staffing, restrictions and public messaging — while a fire spread model answers what a particular burning fire will do next. The confusion this resolves is reading a high rating as a forecast that something will burn. After this Concept you can state exactly what claim a fire danger rating makes and what it leaves out.
The National Fire Danger Rating System (NFDRS) is a science-based method for measuring environmental conditions and correlating them to predicted fire occurrence, spread, and impact, producing standa…