A wildfire is an unplanned fire burning in vegetation, whether it was started by lightning, by an accidental or deliberate human ignition, or by a prescribed fire that escaped. Federal land managers use wildland fire as the umbrella term for any non-structure fire in vegetation, with wildfires as the unplanned kind and prescribed fires as the planned kind.
Wildfire versus prescribed fire
The National Park Service sorts wildland fire into two kinds: planned fire, which is prescribed fire, and unplanned fire, which is wildfire. A prescribed fire is ignited on purpose under an approved plan to meet specific objectives.
A prescribed fire that escapes and is declared a wildfire becomes a wildfire under these definitions. The label describes how the fire started and how it is managed, not how large or intense it is.
What drives fire behavior
Fire managers describe fire behavior with a triangle of fuels, weather and topography. Fuel moisture matters most among fuel traits, because heat from the fire must drive off moisture before the fuel will burn.
Wind brings fresh oxygen and pushes the fire toward new fuel. At low humidity, fuels dry out, catch fire more easily and burn faster, and warmer fuels ignite more readily than cold ones.
Where forecasters come in
The National Weather Service (NWS) issues Fire Weather Watches and Red Flag Warnings when the combination of fuels and weather supports extreme fire danger or fire behavior. These alert land managers to the potential for widespread new ignitions or control problems.
Once a fire is burning, local forecast offices may issue spot forecasts for the site on request. Any federal, state, tribal or local public safety official supporting wildland fire operations can request one.
How fires are tracked, and what the numbers miss
Fires are counted by incident and sized in acres burned. National totals from the National Interagency Fire Center come from the Incident Management Situation Report published by the National Interagency Coordination Center.
Mapped outlines, called fire perimeters, are shared publicly for many fires but not every incident. Progress toward stopping spread is reported as percent containment.
An acreage is a running total that changes as the fire grows and as it is mapped again. It also says nothing about how severely each part of the fire burned, which is mapped separately after the fire and matters for flooding and erosion in the burn scar.