Wildfire danger depends on factors affecting the initiation, spread, and resistance to fire control in a given area. It is typically expressed as an index describing very low to very high danger. It largely depends on the weather conditions 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).
Weather-based fire danger
The weather conditions affecting the fire danger are described by the Fire Weather Index: the higher the index value, the more favorable the meteorological conditions to trigger a wildfires are. Specific conditions can be aggravating factors that raise the hazard. There are 4 major weather parameters directly affecting the hazard and formative to other conditions that favor fire.
PRECIPITATIONS: dry seasons are aggravating factor as they may facilitate the ignition and spread of fire
TEMPERATURE is aggravating factor if higher than 38/40°C
WIND: gust and change in directions are aggravating factors
HUMIDITY can act as aggravating factor, increasing the likelihood of fire, or as blocking factor, helping fire extinguishing
Humidity below 20%
Humidity below 45%
Humidity larger than 50%
Other weather conditions:
CLOUDS: no clouds and of clear sky increase ground temperature up to 10 or 20 °C
FOEHN WIND: it can heavily increase fire hazard as it affects three of the weather aggravating factors: it causes increase in temperature and wind and a decrease in humidity
HEAT WAVES: they can heavily increase fire hazard and have an impact on the burning temperature of vegetation making difficult restoration
The hazard of a wildfire strictly depends on the quantity and continuity of vegetation that makes up for the fuel component of the fire triangle. In the fire hazard the weather is not considered. Fire hazard reduction treatments refer to fuel treatments.
The key- role of fuel load – The amount and type of vegetation, known as fuel load, play a critical role in wildfires hazard. Areas with dense forests and an accumulation of dry, dead vegetation are more prone to blazes. Highly flammable vegetation, such as coniferous trees and grasslands, can increase the hazard.
Vegetation dryness is an important indicator of how large the fire could become and how much smoke it will emit.
Fire climbs up slopes – The shape and slope of the terrain does not cause wildfires but it influences fire behavior. Slopes favor fire to climb up. Steep slopes can accelerate the spread of fires, while valleys and canyons can channel flames and increase their intensity. Topographic features can also affect wind patterns, further influencing fire behavior. Slopes steepness and orientation to sun affects the insolation and, in turn, humidity content of vegetation.
Some soil types can favor fire – The type of soil affects the humidity content of vegetation and in turn the danger of wildfires.
Anthropogenic factors can increase the hazard – Human activities can directly affect the ignition and spread of fire. Pyromaniac pressure is one of the major causes of wildfires. However, campfires, discarded cigarette butts, fireworks, and equipment sparks, can unintentionally ignite wildfires. Additionally, activities like logging, agriculture, and urban development in or near forested areas can increase the risk of fire damage.
Forest management: Forest management largely affects the likelihood of wildfires. In forests where no management activities are performed, there is an increased amount of fuel, and thus an increased fire hazard.