Written by Henrietta Appleton, GWCT Policy Officer
For any fire to start and spread it needs fuel, oxygen and an ignition source. But when it comes to wildfires at a landscape scale there are additional factors such as the fire environment (weather, topography and fuel) and the fire regime (landform, climate and vegetation).
Understanding how these interact over spatial and temporal scales is fundamental to predicting the behaviour of a wildfire; in terms of its movement across the landscape (speed) and its intensity (heat) and therefore its severity (capacity to cause damage).
Many consider that wildfire threats are greatest in the Summer. But there are two fire seasons in the UK with the main one being the Spring and the secondary one mid-late Summer. This is because fire activity is related to the amount of dry vegetation and Spring is the time when upland vegetation has yet to come to life after the winter and so is dry.
The current spell of Spring-like weather with overnight frosts has highlighted the threat of wildfire at this time of year by creating what is termed “fire weather”. Low rainfall and humidity as well as high temperatures and wind dry out the dormant fuel and increase its flammability. These factors have been drawn together in a recently launched UK fire behaviour prediction system called FireInSite. FireInSite is a free to use web-based app which forecasts fire behaviour using fuel models designed specifically to describe key fire prone UK vegetation types, the time of day, temperature, relative humidity, rainfall and wind.
An example of the output from the FireInSite app for Thursday 6th March for a site in the Peak District is shown below. The outputs show estimates of fire spread, heat per unit area, fire intensity, fuel moisture, probability of ignition and crown fire transmission ratio but I have chosen flame length. This is because there are two useful rules of thumb which demonstrate the increased risk of a severe wildfire presented by the large expanses of mature vegetation that can dominate our uplands. Firstly, a flame length with no wind is approximately twice the height of the vegetation; this is increased a further 2-3x by wind and aspect. A flame length of 3.5m is beyond the direct control of the Fire & Rescue Services. You will note that at 4pm on 6th March the model predicted that this limit would be reached for 5 vegetation types and in particular grasses and bracken.
(The codes on the left represent the vegetation types e.g. CLh is calluna high load, FN is largely bracken and GRh is grass high load. For more information, please see FireInSite fire behaviour prediction system ).
Unfortunately the last few days have seen the Fire & Rescue Services called out to a number of wildfires in the Peak District. Whilst the causes are unknown, the potential for another severe wildfire event (or events such as witnessed in 2022) is heightened by the current trend to rewild or reduce management interventions in our uplands. Add to this expected climate change trends increasing the likelihood of hotter drier summers after wetter, warmer winters, and you have all the ‘ingredients’ for a wildfire disaster.
Whilst we cannot change some of these ‘ingredients’, we can manage our habitats to protect them from the threat of wildfire and the risk to wildlife and biodiversity, carbon stores, water quality and our cherished cultural landscapes.