Forest fire: understand, detect, follow

What a forest fire is

A forest fire, or wildfire, is a fire that reaches natural land and spreads without control. The fuel is the vegetation itself: trees, undergrowth, dry grass, conifers rich in flammable resins. Three conditions turn an ignition into a fire: dry vegetation, wind that brings oxygen and pushes the flames, and terrain or continuous cover that lets the fire run.

A fire does not burn an area all at once. It advances along a front whose speed depends on wind and slope, leaving behind blackened ground that cools within hours. Satellites see that hot front, not the area already burnt.

What to do in a fire

GISFire is not a warning tool. A detection describes a situation already a few hours old, and no detection proves nothing. In a fire, call the emergency services: 112 in the European Union, 911 in North America, 000 in Australia. Move away from the fire with the wind at your back, do not take roads through the area, and follow the instructions of local authorities, who alone decide on an evacuation.

To prevent fires, respect burning bans and access restrictions during high-risk periods, clear vegetation around buildings where the law requires it, and never park a hot vehicle on dry grass.

What causes forest fires

In Europe, more than nine ignitions out of ten are human in origin according to the European Commission Joint Research Centre (EFFIS annual reports), most often accidental: field work, a cigarette, a barbecue, a power line, a vehicle parked on dry grass. Lightning is the first natural cause, and it matters more in the boreal forests of Canada and Siberia, where a dry thunderstorm can start dozens of fires in one night.

How a satellite detects a fire

A satellite does not see flames, it measures temperature. The VIIRS sensors (375 m resolution, on Suomi-NPP, NOAA-20 and NOAA-21) and MODIS (1 km, on Terra and Aqua) observe the mid-infrared, around 4 micrometres, where even a small fire radiates far more than the ground around it. Each thermal anomaly becomes a point, with its radiative power in megawatts, the FRP.

These polar-orbiting satellites pass over the same spot two to four times a day, and NASA publishes their detections about three hours after the pass. A fire starting at 2 pm may only appear on the map in the late afternoon. The geostationary satellites Meteosat, GOES and Himawari fill that gap: they watch the same disc of the Earth continuously, every ten minutes, at a coarser resolution of several kilometres.

Clouds hide everything. Under thick cover or a dense smoke plume, a fire can burn without any detection coming through. Conversely, an industrial flare, an agricultural burn or a metal roof in the sun can trigger a detection without any wildfire.

Reading the active fire map

On GISFire, each point is a detection, coloured by its power. Detections close in space and time are grouped into fires, with a 2,500 m radius and a minimum of three points over 48 hours. A fire has a first and last detection date, a cumulative power and a footprint in hectares.

The footprint is the union of the ground footprints of the detected pixels. It overstates the area actually burnt: a 375 m VIIRS pixel is fourteen hectares and counts in full even if only its edge is burning. The burnt area is measured afterwards, from Sentinel-2 burn scars or by fire services, and it is always smaller.

Around the detections, the map overlays smoke plumes estimated from the wind, forecast wind, air quality, recent lightning and the live position of water bombers tracked by ADS-B. A timeline replays the last twelve hours, twenty-four hours or seven days, and a zone selector reframes the map on a country or region.

Where and when the world burns

Every region has its season. The Mediterranean burns from June to September, with peaks under strong wind: mistral and tramontane in France, meltemi in Greece, southerly winds in Spain and Portugal. The boreal forests of Canada, Alaska and Siberia ignite from May to September, often from lightning, and produce the largest smoke plumes on the planet.

The Brazilian Amazon and Cerrado, Bolivia and Paraguay have their fire season from August to October, at the end of the dry season, largely tied to land clearing. The savannas of southern and central Africa burn from June to October, over huge areas but with fast, low-intensity grass fires. South-eastern Australia has its big fires from December to February, South-East Asia from February to April.

These rhythms show on the map: at the height of the European winter, the world still burns, in southern Africa or Australia. The country pages below give the situation of the last thirty days for each.

Frequently asked questions

What is the difference between a forest fire and a wildfire?

They are largely the same thing: a wildfire is any uncontrolled fire in natural land, and a forest fire is a wildfire that burns forest rather than grass or scrub. GISFire's map shows every thermal anomaly detected, vegetation fires but also agricultural burns or industrial flares, which it cannot tell apart with certainty.

Is a point on the map always a fire?

No. It is a thermal anomaly seen by a satellite. Most are vegetation fires, but some come from refinery flares, controlled burns, volcanoes or sun glint. An isolated, low-power point seen only once deserves doubt. A cluster of points that persists from one pass to the next is almost always a fire.

Why did yesterday's fire disappear from the map?

A satellite detects heat, not a burnt area. Once the front has passed, the ground cools within hours and there is nothing left to detect, even though the land stays black for months. The fire list keeps every fire for thirty days, with its dates and footprint.

How long before a fire appears?

About three hours after a polar satellite pass, the time NASA needs to process and publish the data. Geostationary satellites are faster, often under an hour, but only see fires powerful enough. GISFire queries both and refreshes every thirty minutes.

Is the footprint in hectares the burnt area?

No, it overstates it. The footprint is the union of the ground footprints of detected pixels, each counting in full even if only its edge is burning. The burnt area is measured afterwards, from imagery or on the ground, and it is always smaller than the footprint.

Sources

• NASA FIRMS, VIIRS and MODIS active fire detections
• EFFIS, European Forest Fire Information System (Copernicus)
• EUMETSAT, Meteosat satellites
• Copernicus EMS, burnt area mapping

Go further

• Open the live fire map
• The list of fires of the last thirty days
• Frequently asked questions about the map
• About GISFire
• GISFire Pro: zone monitoring and alerts