Reference
Fire & Water Glossary
Plain-language definitions of the terms used across this site. Free to use and adapt with attribution.
- Snow Water EquivalentSWE
- The depth of water that would result if a snowpack melted completely. It is the standard measure of how much water is stored as snow, tracked at automated SNOTEL stations. Low April 1 SWE means a drier, earlier fire season. Snowpack & water supply →
- Vapor Pressure DeficitVPD
- A measure of how strongly the atmosphere pulls moisture out of soil and plants. VPD rises steeply with temperature; higher VPD dries fuels faster and correlates strongly with area burned. Fire Climate 101 →
- Energy Release ComponentERC
- A US National Fire Danger Rating System index representing the potential heat energy at the head of a fire. Higher ERC indicates drier fuels and more intense potential fire behavior.
- Fire Radiative PowerFRP
- The rate of radiant heat energy released by a fire, measured in megawatts by satellites such as VIIRS. Higher FRP indicates a more intense detection. FRP helps distinguish an active, growing fire from a small or cooling one.
- Air Quality IndexAQI
- The EPA's 0-500+ scale for reporting daily air quality. For wildfire smoke the driving pollutant is fine particulate matter (PM2.5). Values above 150 are Unhealthy for everyone. Air quality & smoke →
- Fine Particulate MatterPM2.5
- Airborne particles 2.5 micrometers or smaller. They penetrate deep into the lungs and enter the bloodstream. Wildfire-smoke PM2.5 can be substantially more harmful per microgram than typical urban PM2.5.
- El Nino-Southern OscillationENSO
- The warm (El Nino) and cool (La Nina) phases of the tropical Pacific that shift the winter storm track. ENSO does not ignite fires, but it loads the dice a season ahead by shaping snowpack and spring fuel growth. El Nino & the fire corridor →
- Defensible Space
- The managed area around a structure, divided into zones, kept clear of combustible material to reduce the chance embers or flames ignite the building. Most homes ignite from embers, not the flame front. Preparedness handbook →
- Fire Perimeter
- The mapped outer boundary of the area a fire has burned. Historical perimeters (e.g. from NIFC) show where fire has occurred; they are not a forecast of where it will burn next.
- Fuel Load
- The amount of burnable vegetation, living and dead, available in an area. A century of fire suppression let fuel loads accumulate in dry western forests, so escaped fires now burn hotter and larger.
- Prescribed Fire
- A planned, controlled burn set under specified conditions to reduce fuel loads and restore fire-adapted ecosystems. Thinning followed by prescribed fire can cut later fire severity by more than 60%.
- Corsi-Rosenthal Box
- A do-it-yourself air cleaner made from four MERV-13 furnace filters and a box fan (about $60-100) that rivals commercial purifiers at filtering wildfire-smoke PM2.5 from a room. How to build one →
Start with Fire Climate 101 for how these terms fit together, or the Methods page for the datasets behind them. Corrections welcome: contact@pnw-firemap.org.