Weather variables commonly used in wildfire risk assessment indicate those atmospheric variables that affect ignition or propagation (temperature, relative humidity, precipitation, wind speed and direction, etc.). They can refer to current (instantaneous), aggregated (last days-weeks) atmospheric conditions, or even historical conditions (past scenarios). These variables are obtained from meteorological observations or short-term weather forecasts based on climate models. Additionally, adaptation to future wildfire conditions requires understanding future weather patterns, which is done through Climate and Dynamic Global Vegetation Models (DGVM) [1] to represent future values of meteorological danger indices [2], but also to estimate future fire occurrence by linking DGVM with fire models [3].
Weather variables are a critical element and widely used in existing wildfire risk assessment systems, which generally address only the fire danger component. Weather plays a key role in the ignition and propagation of wildfires. Temperature, atmospheric humidity (relative humidity and vapour pressure deficit), wind speed and direction are the common meteorological inputs to danger rating systems [4].
Several authors distinguish between conditioning and determinant factors. The former are those related to ignition by conditioning the load and moisture content of fine fuels. This group includes the following factors: precipitation, air temperature, and relative humidity. Precipitation, expressed in millimetres (litres of rainfall that fall on the ground per square metre), affects the water content of the soil and also the moisture content of forest fuels (live and dead). The persistence of precipitation at the beginning of the hydrological year (October in the Northern Hemisphere) favours the growth of fine vegetation, which may be available to support the fire spread in the summer. However, the absence of precipitation during the months leading up to summer, with values much lower than the climate normal (average precipitation values that characterise a region over a period of 30 years), can lead to a situation of drought or water deficit that favours the general dryness of forest fuels, which are available to support the fire spread and contribute to larger burned areas by summer fires [5]. The air temperature changes in a diurnal cycle due to the effect of solar radiation, but it can also change due to the entry of warmer or colder air masses. Typically, the temperature increases during the day and decreases at the end of the day. This diurnal cycle is of great importance in wildfires as it directly affects the flammability of forest fuels since the amount of heat required to raise the temperature of the fuels to the ignition point depends on their initial temperature and that of the surrounding air and on the fuel moisture content that is greatly dependent on the air temperature [6]. Relative humidity of the air (RH) is the percentage of the mass of water vapour in the air in relation to the mass of vapour necessary to saturate the environment. A RH value equal to 100% corresponds to a saturated atmosphere, in which there is condensation, while a value of 30% or less corresponds to very dry air, which also favours the drying of fuels. RH also has a diurnal cycle but tends to decrease throughout the day and increase at the end of the day – basically, it changes inversely to the temperature cycle.
Determining weather factors are those parameters that affect directly the fire spread. This group includes wind and atmospheric stability. Wind is generally defined as the horizontal movement of atmospheric air (atmospheric wind) characterised by the following components: windspeed (or intensity) in m/s, and direction (from where the wind comes) referred to the North in degrees. Windspeed and direction change from one point to another, and at a given point it changes continuously over time (slow variation). There is another variation due to the turbulence of the wind flow, which is manifested by sudden variations in windspeed in a short period of time (gusts). These variations have a direct influence on fire propagation conditions. The windspeed profile increases with altitude (it is zero near the ground and increases until it reaches a maximum value at the top of this layer). However, in a wildfire, this profile can be changed due to the wind induced by the fire, which can lead to fast fire development even if the atmospheric wind is low. Therefore, in a wildfire, the wind can be defined as the sum of the atmospheric wind and the wind induced by the fire.
Atmospheric stability is characterised by the air temperature variation in the vertical direction, measured by its gradient of temperature or rate of variation, dT/dz (ºC/m), which determines whether the atmosphere is stable, neutral, or unstable. Atmospheric stability may either encourage or suppress vertical air movement. The heat of fire generates vertical movement, at least near the surface, but the convective circulation thus established is affected directly by the stability of the air [6:1]. In turn, the indraft into the fire at low levels is affected, and this has a marked effect on fire intensity. Also, in many indirect ways, atmospheric stability will affect fire behaviour, for example winds tend to be turbulent and gusty when the atmosphere is unstable, and this type of airflow causes fires to behave erratically [6:2]. Thunderstorms with strong updrafts and downdrafts develop when the atmosphere is unstable and contains sufficient moisture [6:3]. Their lightning may set wildfires, and their distinctive winds can have adverse effects on fire behaviour [6:4]; for more information see FirEUrisk deliverable D2.6.
In addition to weather or short-term atmospheric conditions, climate patterns are relevant to fire occurrence, particularly for the build-up of vegetation fuels. Climate plays a role in determining the fuel available to burn, the length of the fire season as well as the presence of lightning, which is the most common natural source of ignition. Climate including long-term averages has been used to explain the spatial variation in fire regimes. Additionally, climate variability helps explain the global interannual variability in areas burned, especially forested areas [7][8]. Many studies describe climate-fire relationships along a continuum, ranging from fuel limited to fire weather limited systems. Fire weather limited systems are largely driven by extreme fire weather and fuel moisture [9] [10], whereas fuel limited regions are positively related to precipitation prior to the fire season that promotes vegetation growth and subsequent biomass for combustion during the fire season. Lastly, and perhaps most importantly, human-caused climate change is causing profound changes in global fire regimes through changes in fire season length, fuel moisture, fire intensity and fire severity [11].
Weather patterns also affect natural ignitions, both directly to the convergence of lightning and drought, as well as the characteristics of strikes and thunderstorms. Lightning strikes are one of the most common natural causes of wildfires, accounting for around 10–15% of all wildfires worldwide. Lightning-induced wildfires are particularly prevalent in regions with a high incidence of thunderstorms and dry vegetation [12] [13] [14]. While lightning strikes can occur at any time, they are most common during the summer months when hot and dry conditions make it easier for fires to start and spread. One of the significant challenges in identifying lightning-induced wildfires is the prolonged latent phase, known as the holdover time, between ignition and fire detection. Typically, lightning-caused fires start as smouldering in the organic matter surrounding the base of the tree struck by lightning. This smouldering phase can last from several minutes to one or three days or even several weeks in rare cases, making it difficult to accurately identify the lightning that caused the fire. This phenomenon poses a great challenge to the development of predictive methods that can establish a cause-effect relationship, which is currently limited to specific regions [15]. A robust association between lightning and wildfires can aid in understanding the natural fire regime, such as estimating holdover duration, identifying igniting lightning characteristics, and modelling lightning fire occurrence. The principal possibility of building such a model was demonstrated by Wotton and Martell (2005)[16] from Ontario, Canada. Unfortunately, there are currently no global datasets that unambiguously link igniting lightning to its corresponding wildfires. However, improvements in lightning location systems (LLSs) and fire databases [17] have facilitated the identification of possible igniting lightning, and numerous methodologies have been developed to match wildfires and related lightning [15:1]. Consequently, identifying the igniting lightning of a wildfire by searching the lightning dataset remains challenging, making predicting single lightning-ignition events difficult. However, utilising a statistical approach that incorporates lightning data from Numerical Weather Prediction (NWP) models and other environmental factors occurring when a fire is observed may offer a potential solution [18]. The target variables for prediction could be widely available satellite products, such as Fire Radiative Power (FRP) or Burned Area (BA).
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