Fire Danger Rating, 2009

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Abstract

Fire danger rating is the process of systematically evaluating and integrating the factors that determine the ease of a fire starting and spreading, the difficulty of control, and the resulting impacts based on an assessment of ignition risk, the fire environment (fuels, weather, and topography) and values at risk.

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The familiar colour-coded fire danger signs seen in forested areas across Canada provide the public with an easily understood indicator of the risk of fire, from blue for low through to red for extreme hazard.

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Figure 1. Fire Danger Indicator Road Sign Used in Whitehorse, Yukon

The Canadian Forest Fire Danger Rating System, developed by Natural Resources Canada’s Canadian Forest Service (Table 1), is used across Canada for assessing fire danger. This system consists of several subsystems, two of which have been used by fire management agencies for a number of years. They are the Fire Weather Index System and the Fire Behavior Prediction System. Two other subsystems are still under development: the Fire Occurrence Prediction System, which relates to the risks of fire, and the Accessory Fuel Moisture System, which relates to weather.

Table 1. Fire Danger Classifications

Fire Weather Index System

The Fire Weather Index System provides estimates of fire danger based on a continuous record of weather observations taken daily at noon. The continuity of the record is required to track the moisture content of fuels (combustible material). Moisture content is estimated for fine fuels (twigs, leaves and needle litter), duff (loosely compacted organic layers of moderate depth) and drought (deep, compact organic layers). Temperature, relative humidity, wind speed and rainfall are inputs to the system. Wind speed and temperature directly influence a fire the most; however, rainfall is most important in determining fuel moisture. Frequently, the current weather conditions are not a good indicator of the fire hazard. For example, the hazard would be much higher on a hot, windy day after a week of dry weather than it would be on a similar day after a week of rain.

Fire Behavior Prediction System

The Fire Behavior Prediction System uses fuel type and terrain (elevation, slope and aspect) data, along with outputs from the weather-based Fire Weather Index System, to make predictions of fire spread rate, fuel consumption and fire intensity. Unlike the codes and indices of the Fire Weather Index System, these predictions are quantitative (that is, they are measurable quantities). For example, the spread rate is in metres per minute. The system uses 16 different fuel types, which are distinguished by characteristics of the vegetation and forest floor. Fire behavior is strongly influenced by the size and arrangement of fuels. For example, fine fuels, such as needles and twigs, ignite more readily than heavy fuels, such as tree trunks.