A conceptual model based on the dynamic interaction between fire, the fuel bed and the surrounding flow to explain the non-monotonic or intermittent behaviour of fires is proposed. According to the model, even in nominally permanent and uniform boundary conditions, the fire-induced flow modifies the geometry of the flame and its rate of spread. After an initial acceleration, there is a reduction in the rate of spread followed by one or more cycles of growth. Carefully controlled experiments of fires in slopes and canyons show that the evolution of fire properties, namely flame angle and rate of spread, have high-frequency oscillations superimposed on the low-frequency fire growth cycle described above.

Based on analysis of the interaction between a spreading fire and its surrounding environment, in nominally constant and uniform boundary conditions, it is observed that the evolution of the fire front is characterised by fluctuations of its properties, including (in particular) its rate of spread (ROS). Using a database with a wide range of fires with different time-space scales, it is shown that the amplitude of the fluctuation in ROS is proportional to the average value of the ROS and that the frequency of oscillation varies with the type of fire, and for a given fuel, increases with the average ROS. In fast-spreading fires, the large amplitude of ROS increase and sudden decrease promote the intermittent behaviour of the fire. In general, the amplitude and period required for the ROS increase are larger than for its decrease. However, the acceleration and deceleration phases in junction fires do not follow this rule, suggesting the existence of different convective processes of interaction between the flow and fire. This oscillation explains the variability in many fires at all scales and challenges the current interpretation based on the three factors affecting fire spread and the classification of wind or topography-driven fires.

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The Fire ROS Calculator is a software tool built using MATLAB to assist researchers from the wildfire research area in analysing wildland fire behaviours. In particular, it measures the rate of spread (ROS) of a fire propagating over a surface in a laboratory setting and constructs its propagation contour map. An algorithm is used to calibrate the camera used for filming the fire spread. Various algorithms and image processing procedures are applied to the images to obtain the ROS. The software has a graphical user interface (GUI) and documentation stored in a GitHub repository alongside the source code.