Rate of Spread (ROS) is the speed at which a wildfire advances across the landscape, typically measured at the leading edge (head) of the fire. ROS is expressed in chains per hour (ch/h) in the US system, meters per minute (m/min) in the Canadian system, and miles per hour (mph) in some operational contexts.
ROS is one of the two primary inputs to the Fire Behavior Index (FBI), alongside Flame Length (FL). Together, they determine the overall severity and spread potential predicted by Technosylva's fire behavior models.
ROS is highly dynamic. Small changes in wind speed, slope, or fuel moisture can produce large changes in spread rate. Values can shift significantly within a single forecast period as weather conditions evolve, making ROS one of the most volatile outputs in a fire behavior simulation.
ROS depends on:
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Wind speed and direction: the primary driver; wind both dries fuels and physically pushes the fire forward.
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Slope: uphill spread accelerates ROS; steep terrain can produce rates comparable to extreme wind events.
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Fuel type and load: fine, dry, continuous fuels (grass, light brush) support faster spread than heavy or patchy fuels.
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Dead fuel moisture: particularly 1-hour (1H) moisture, which responds rapidly to ambient conditions.
ROS directly affects how quickly a fire reaches structures, roads, or other values at risk, and how much time responders have to act. A fire spreading at low ROS may be manageable with ground resources. At moderate to high rates, aerial resources and evacuation coordination become critical. At extreme rates, containment during the run is generally not feasible.