Wildfire Glossary

Definitions for terms used in wildfire reporting, fire management, and satellite fire detection.

FIRMS (Fire Information for Resource Management System): NASA's system for aggregating and distributing near-real-time satellite fire detection data. FIRMS collects active fire detections from the MODIS instrument (Terra and Aqua satellites) and VIIRS instrument (Suomi NPP and NOAA-20 satellites) and makes them available as map layers, alerts, and data downloads. Data is typically available within 3 minutes of satellite overpass for the NRT (Near Real-Time) product. NearbyFires uses FIRMS as its source for active fire detections.
NIFC (National Interagency Fire Center): The coordination center for federal wildland fire management in the United States, located in Boise, Idaho. NIFC coordinates resources from multiple federal agencies (Forest Service, BLM, NPS, BIA, FWS) and serves as the primary source for official named fire incident statistics. The NIFC publishes annual fire statistics, the Wildland Fire Potential Outlook, and maintains the IRWIN system for incident tracking.
FRP (Fire Radiative Power): A measure of the rate at which a fire releases energy through radiation, expressed in megawatts (MW). FRP is measured by satellite thermal sensors and is directly related to the rate at which biomass is being consumed. Higher FRP indicates more intense fire behavior. FRP values are used as inputs to smoke and air quality models. Typical values range from <10 MW for small agricultural burns to 1,000+ MW for extreme wildfire behavior.
MODIS (Moderate Resolution Imaging Spectroradiometer): A satellite instrument aboard NASA's Terra and Aqua satellites that detects fires at approximately 1km pixel resolution. MODIS uses thermal infrared bands (particularly band 21/22 at ~3.9 microns) to identify anomalous heat signatures consistent with active fire. Terra crosses the equator in the morning; Aqua crosses in the afternoon. Together they provide approximately 4 overpasses per day for most US locations.
VIIRS (Visible Infrared Imaging Radiometer Suite): A satellite instrument aboard NOAA-20 and Suomi NPP that detects fires at 375-meter pixel resolution — significantly higher resolution than MODIS. VIIRS uses I-band channels (I4 at 3.74 microns and I5 at 11.45 microns) for fire detection. VIIRS detects smaller fires that MODIS may miss and produces less spatial blurring at swath edges.
Containment: A fire is "contained" when a completed control line (fireline, road, river, or other fuel break) encircles the entire fire perimeter. Containment percentage refers to the fraction of the perimeter that has a completed control line. A fire at 100% containment may still have active burning inside the perimeter — it is simply not expected to spread beyond its current boundaries. Containment does not equal extinguishment.
Prescribed Burn (Prescribed Fire): An intentional, planned burning of accumulated fuels under specific weather conditions to reduce wildfire risk, improve wildlife habitat, or maintain ecosystem health. Prescribed fires are conducted by trained crews under a written prescription specifying acceptable weather, fuel moisture, and wind conditions. Prescribed burning is the most scalable tool for reducing fuel loads in fire-adapted ecosystems. Also called "controlled burn."
Fuel Moisture Content: The amount of water in vegetation or dead organic material, expressed as a percentage of the material's dry weight. Live fuel moisture below 80% and dead fuel moisture below 10% are considered critical fire weather thresholds. Fuel moisture is the most important controllable variable in fire behavior — moist fuels resist ignition and burn with lower intensity. Fuel moisture stations (Remote Automated Weather Stations, or RAWS) provide real-time fuel moisture data to fire behavior analysts.
Red Flag Warning: A National Weather Service warning issued when a combination of dry conditions, low humidity, and high winds creates an extremely dangerous fire weather environment. Criteria vary by region but typically include relative humidity below 15–25%, winds sustained above 25 mph or gusting above 35 mph, and critically dry fuels. A Red Flag Warning indicates conditions under which any ignition could lead to rapid, potentially uncontrollable fire spread.
Fireline (Control Line): A linear fuel break constructed to stop or redirect fire spread. Direct firelines are built adjacent to the fire edge; indirect firelines are built ahead of the fire. Hand crews use tools to scrape soil down to mineral earth; bulldozers can construct wider lines in accessible terrain. The width required depends on fire intensity — ground fires may be contained with a 4-foot hand line; crown fires may require 100-foot dozer lines or aerial retardant drops.
WUI (Wildland-Urban Interface): The zone where human development (homes, businesses, infrastructure) meets or intermingles with undeveloped wildland vegetation. The WUI is the highest-risk zone for structure loss in wildfires — structures are close enough to wildland vegetation for embers to reach them, but fire suppression in this zone is complicated by the presence of civilians and structures. An estimated 44 million US homes are in the WUI.
Spot Fire: A new fire ignited by burning embers (firebrands) that have been transported ahead of the main fire by wind. Spot fires can be carried miles in extreme wind events, creating ignitions that outflank control lines and cut off escape routes. The 2018 Camp Fire's extraordinary speed was partly due to spotting — new ignitions well ahead of the main fire front prevented systematic evacuation of the areas between spots.