Haze cover. Copernicus CAMS PM2.5 is read on a grid of 325 points from Sumatra to Kalimantan, then filled in every 0.1° (about 11 km). Anywhere above 55 counts as haze, because that is where NEA's 1-hour reading turns Elevated. The area is the sum of those grid cells, each sized by its latitude. CAMS often reads peat smoke too low, so near Singapore it is scaled to match the live NEA sensors, and that correction fades out over about 300 km.

Fresh fire smoke. Every fire cluster releases small parcels of smoke each hour, sized by how hard it is burning (fire radiative power). Each parcel drifts with the real wind about 800 m up, where peat smoke travels, at the spot where it actually is. It also spreads with turbulence, loses some to rain (more rain means more is washed out) and slowly settles out over about 4 days. The run starts 48 hours ago, so smoke that is already in the air is counted too.

The score. Half comes from how many hours Singapore sits inside the line that holds 90% of the fire smoke, and half from how thick that smoke is over Singapore compared with the core of the plume. A direct hit all day scores close to 100, and smoke heading elsewhere scores 0.