How Satellites Spot Wildfires: Reading a Fire Hotspot Map

How a satellite sees fire

Fire gives off a lot of heat in the infrared, far more than the ground around it. Instruments on satellites measure infrared light in every small square of the Earth below them, called a pixel, and flag any pixel that is much hotter than its neighbours. That flag is a hotspot, or thermal anomaly.

NASA's Fire Information for Resource Management System (FIRMS) collects these detections and publishes them for the whole world, usually within three hours of the satellite passing over.

Which satellites

• VIIRS, on the Suomi NPP, NOAA-20 and NOAA-21 satellites, sees pixels about 375 metres across. It can pick up fires much smaller than its pixel when they are hot enough.
• MODIS, on NASA's older Terra and Aqua satellites, sees pixels about 1 kilometre across, and has the longest record, back to 2000.
• Geostationary satellites such as GOES, Meteosat and Himawari watch one side of the Earth every few minutes, but with much coarser pixels, so they spot large fires quickly but miss small ones.

The VIIRS and MODIS satellites orbit from pole to pole, and each passes over most places about twice a day, once in daylight and once at night. So a fire that starts and burns out between passes may never be seen.

What a hotspot does and doesn't mean

• A hotspot means something in that pixel was burning or very hot when the satellite passed. The dot marks the centre of the pixel, not the exact spot of the fire.
• It doesn't tell you the size of the fire. A small, hot fire and a large one can each fill a single pixel. A big wildfire shows up as many hotspots in a cluster or a line along its front.
• It is not a map of burned area or of a fire's edge. Official fire perimeters come from fire services and aerial surveys.
• Each detection has a confidence level. Low-confidence hotspots are more likely to be something else.

Why there are hotspots with no wildfire

Not every hotspot is a wildfire. Many are farmers burning fields, controlled burns to reduce fuel, gas flares at oil fields, steel works and other hot industrial sites, and active volcanoes. Sunlight glinting off water or metal roofs can occasionally cause a false detection too. Fire maps usually mark long-standing hot sites, such as gas flares, so you can tell them apart.

What the satellites can miss

• Thick cloud hides fires completely.
• Dense smoke and tree canopy can hide parts of a fire.
• Small or brief fires may burn between satellite passes.
• The data is hours old, and a fire driven by strong wind can move many kilometres in that time.

That is why a satellite map is a powerful overview but never a substitute for local information. If there is a fire near you, follow your fire service and local authorities, and leave early if you are told to.

Smoke and air quality

Wildfire smoke can travel thousands of kilometres and harm air quality far from the flames. The particles that matter most for health are the finest, called PM2.5. When smoke is heavy, stay indoors with windows closed where you can, and follow your local air-quality advice. The Fire app shows the air-quality forecast for your place next to the fires around it.