j.g. pausas' blog

Recently, a colleague asked me about the difference between fire hazard, fire risk and fire danger. I’m not an expert in these concepts, but here is my understanding of these and other related terms. In short, fire hazard is related to fuel (forestry), fire risk is often used for mapping the probability of ignitions (geography), and fire danger is typically associated with weather conditions (meteorology); below is a longer answer (see Fig. 1). Feel free to improve or qualify these definitions by leaving a comment (see top right).

Fire weather: Weather conditions that influence fire ignition, behaviour, and suppression. E.g., extreme (or severe) fire weather refers to very low moisture, high temperatures (heat waves) and strong winds. Fire weather indices (FWI) can provide information for estimating the fire danger (see below).

Fire hazard: the degree of ease to fire ignition and propagation, and the resistance to control (given an ignition source). It depends on the quantity and continuity of the vegetation (fuel) and it is independent of the weather (in contrast to fire danger). It reflects the potential fire behaviour associated with static properties of fuel (fire hazard doesn’t change from one day to the next, in contrast to fire danger). Fire hazard reduction treatments refer to fuel treatments.

Fire risk: typically it refers to the probability of ignition, i.e., the chance that a fire might start (risk of ignition). It can be split into lightning fire risk and human fire risk; the latter typically decreases with distance to roads and increases with population density in rural or semi-rural areas. Other authors define fire risk as potential damage (or degradation risk), and thus they include fire hazard and fire vulnerability in the concept of fire risk. Fire risk is relatively static (in contrast to fire danger; e.g., a zone with high fire risk), and often used to produce maps (fire risk mapping).

Fire danger: sum of the factors affecting the initiation, spread, and resistance to control in a given area; it is typically expressed as a semi-quantitative index (e.g., from very high to very low). Very often it largely depends on weather (i.e., moisture; sometimes also lightning activity) and is reported by meteorological agencies (weather-based fire danger). Because it considers the weather, fire danger is very dynamic (e.g. fire danger today; daily fire danger forecast). Note that if fire danger is based on weather only: (1) the fire danger may be very high in areas where the likelihood of having a fire is very low due to their low fuel (i.e., overestimation in arid ecosystems; Fig. 2 below); (2) weather-based fire danger may fail to capture short-term increases in dead fuel due to strong droughts (underestimation); and consequently, (3) predictions of fire danger for the future under climatic warning may be questionable. A good prediction of fire danger should consider fire risk, fire weather and fire hazard (including fuel dynamics).

Fire damage: detrimental changes in value after a fire (e.g., ecological fire damage, social fire damage); i.e., it refers to negative fire effects. Note that fire may damage some species and favour others; also it depends on the temporal scale, as some short-term effects may be different from mid- or long-term effects.

Fire vulnerability: probability of fire damage; potential effects of fire on values. It is often presented as fire vulnerability maps. Ecological fire vulnerability is typically computed from the type of vegetation, soil and topography, to estimate postfire erosion risk and regeneration capacity. Societal vulnerability is typically associated with exposure, sensitivity, and adaptive capacity; these characteristics depend on socio-economic level and cultural experience.