Wildfire maps: where official fire data comes from

Open three different wildfire maps during a fire and you can get three different pictures. One shows a neat outline, another a scatter of red dots well outside it, and a third a pin with an acreage that hasn't changed since yesterday. None of them is necessarily wrong. They are showing different kinds of data, collected in different ways and updated on different schedules.

This guide explains where official US wildfire data comes from, what perimeters and hotspots actually are, and how fire weather alerts and smoke information fit in.

Incident reports: the fire as an event

When a wildfire is reported, the agency responsible for that land records it as an incident with a name, a location, a start date, and later the acreage and containment. Who is responsible depends on the land. It could be a federal agency such as the US Forest Service or the Interior Department's US Wildland Fire Service, a state agency such as CAL FIRE, or local and tribal fire departments.

Several official places publish this information:

• The National Interagency Fire Center (NIFC) in Boise coordinates resources between the federal agencies and states. It publishes the national Preparedness Level and a daily Incident Management Situation Report summarising significant fires across the country.
• InciWeb is the interagency incident information site. Larger or longer-running fires get a page with updates, maps, closures and announcements from the team managing the fire. Not every fire is on InciWeb. Small and fast-moving fires often aren't.
• Incident location data is published as open data through NIFC's Wildland Fire Interagency Geospatial Services (WFIGS), on the NIFC Open Data site. Many apps and news maps draw from it.
• State agencies run their own incident lists. In California, CAL FIRE's incidents page covers fires it responds to.

Acreage and containment come from the fire managers' reports, so they update when a new report is filed, often once or twice a day. Containment describes how much of the fire's edge has a control line around it. It doesn't mean the fire is out or that the area is no longer dangerous.

Perimeters: the mapped outline

A perimeter is the mapped edge of the burned area. Perimeters are drawn by fire mapping specialists from several sources, including infrared flights over large fires. The Forest Service's national infrared program flies at night, so a large fire's outline is often refreshed about once a day. WFIGS collects perimeters from these sources and keeps the most current one, and not every fire has a perimeter at all (NIFC Open Data). A perimeter is a snapshot. On a fire spreading quickly, the real edge can be well beyond the last mapped line by the next afternoon.

When you look at a perimeter, check its date and time. Most official maps show when it was last updated.

Hotspots: what satellites see

The red or orange dots on many fire maps are satellite hotspots, also called thermal anomalies. NASA's Fire Information for Resource Management System (FIRMS) distributes detections from the MODIS and VIIRS instruments on polar-orbiting satellites. NOAA's Hazard Mapping System combines satellite fire detections with analyst-drawn smoke plumes.

A hotspot is a pixel where the satellite detected more heat than expected. That makes it useful, and also easy to over-read:

• Each detection stands for a whole pixel, roughly 375 metres across for VIIRS and about a kilometre for MODIS. The dot marks the pixel's centre, not the exact flames.
• Polar-orbiting satellites pass over a given spot only a few times a day, so there are gaps between detections. Geostationary GOES satellites look more often but with much larger pixels.
• Thick smoke or cloud can hide a fire, and some hotspots aren't wildfires at all. Industrial heat sources, gas flares and prescribed burns can all show up.

So a hotspot outside a perimeter may mean fire has spread there, or it may be a pixel edge, an older detection or something else. NASA advises against using hotspots to estimate burned area, and NOAA describes its fire and smoke analysis as general guidance, not a basis for decisions such as evacuations. Treat hotspots as a sign of where heat was seen, and rely on incident information and local officials for the actual situation.

Fire weather alerts: before a fire starts

The National Weather Service issues fire weather alerts when conditions favour fast-spreading fires. A Fire Weather Watch means weather that could lead to widespread fires or extreme fire behaviour is possible, typically one to four days ahead. A Red Flag Warning means critical fire weather, usually some mix of strong wind, very low humidity, dry fuels or dry lightning, is happening or expected within about 48 hours (NWS). Each NWS office sets the criteria with local fire managers, so what triggers a warning differs by region. These alerts are about the risk of fire behaviour, not about an existing fire. During a Red Flag Warning, avoid anything that could start a fire outdoors, and check local burn restrictions.

Smoke: a separate map

Smoke travels much further than fire. Communities hundreds of miles away can get unhealthy air from a fire they will never see. The official place to check is the AirNow Fire and Smoke Map, run by the EPA and the US Forest Service. It shows air quality readings from official monitors alongside readings from low-cost sensors that have had a correction applied, plus satellite smoke plumes and fire locations. Its fire icons for large incidents come from the national situation report and can be a few days old (map FAQ).

Smoke plumes on a satellite image aren't the same as smoke at ground level. A thick plume can pass high overhead while the air you breathe stays fairly clean, and the reverse can happen too. Ground-level readings on the AQI scale are what tell you about the air you're breathing. For what the numbers mean, see What the AQI numbers and colors mean.

Evacuations: always local

Evacuation orders and warnings come from local authorities, usually the county sheriff or fire department, not from the agencies that publish fire maps. A fire map can't tell you whether your street has been told to leave. Your county's alert system, Wireless Emergency Alerts and local officials can. Sign up for your county's alerts before fire season, and know your evacuation zone if your area uses them.

Putting it together

When a fire is near, a sensible order is:

1. Local official alerts and evacuation information for your area.
2. The incident page (InciWeb or the state agency) for the fire's status, acreage and containment.
3. The latest perimeter, with its timestamp.
4. Hotspots, as a rough sign of recent heat, read with their limits in mind.
5. The AirNow Fire and Smoke Map for air quality where you are.

In Herebeat, US wildfires come from that same interagency WFIGS data. Each fire shows its name, size and containment as last reported, on the same map as official alerts, with a note that these figures can lag conditions on the ground.