Methodology

Laterorlah on laterorlah.com answers one question: for this person and this outing, is it GO, CAUTION, or NO right now? Only official NEA readings decide that. The Plan chart and the early-warning line never change it.

What each source calculates

• NEA via data.gov.sg real-time API

  Air is 24-hr PSI and 1-hr PM2.5. Heat is WBGT. Lightning is the strike map. Wind and the 2-hour rain text are saved and do not set the verdict.

• NEA air-quality bands

  The Good to Hazardous names on PSI, and the guidance those bands quote.

• Haze site

  Nothing in the app. It is the public page for the same NEA readings.

• Open-Meteo Air Quality API

  The Plan chart. One CAMS global PM2.5 series for Singapore, scaled so the current hour matches NEA.

• CAMS, Copernicus Atmosphere Monitoring Service

  The model behind that Open-Meteo series. About a 45 km grid, so it is not a separate forecast per region.

• NASA FIRMS

  Hotspot counts for the early-warning line. Not used for GO, CAUTION, or NO.

The verdict

24-hr PSI: 0–50 Good, 51–100 Moderate, 101–200 Unhealthy, 201–300 Very Unhealthy, above 300 Hazardous. 1-hr PM2.5: 0–55 Normal, 56–150 Elevated, 151–250 High, above 250 Very High. The air level is the worse of the two. WBGT under 31 is Low heat, 31–32.9 is Moderate, and 33 or above is High. The region’s heat is the middle station in that region, so one hot station does not set the whole region. High heat makes a very hard session NO for a healthy adult. An easier session stays a warning unless the air is already stricter. The card shows three words: air, heat, and lightning. Lightning is NO when a strike is in the region, and GO when it is not. The large word is the strictest of the three. If any one is NO, the large word is NO.

PSI is a trailing 24-hour average, so it lags a sharp change. The 1-hour PM2.5 is the “now” number. Both are published for five regions: north, south, east, west, and central.

NEA says reduce, minimise, or avoid prolonged or strenuous outdoor activity. It does not define effort tiers. Splitting light, moderate, hard, and very hard effort is this product’s interpretation. Healthy adults are treated less strictly than children, older adults, pregnant people, and people with heart or lung conditions. An hour or more counts as prolonged. The verdict word is the strictest of air, heat, and lightning.

Plan times are an estimate

The chart is the Open-Meteo CAMS series, scaled so the latest hour matches the official 1-hour PM2.5. The scale is held between 0.5 and 2. Dragging chooses a clock time from 4 am to noon, or from 5 pm to 9 pm, when the app suggests one. Other hours still move, and the number stays the model number. A future hour uses that hour’s PM2.5 band and the same GO, CAUTION, and NO rules as Now. It does not copy today’s PSI, heat, or lightning. A window that includes the current hour uses the full Now verdict, including lightning. A chip that says “Go 6–8 am” means that window is GO. “Hold off” is CAUTION. “Stay in” is NO. “Cleanest” is the lowest GO stretch in the series, including an odd hour, if you still want to choose it. If no hour is GO, the day card stays on the usual hour and says Hold off or Stay in.

Region rows on Plan are that same island series, shifted by today’s gap with East. They are marked ESTIMATE. There is no separate hourly forecast for each region.

The early-warning line

The grey SIGNAL line is not a news feed and it does not decide the verdict. It turns on only when two saved fields are both present: a hotspot count above 1.3 times its 7-day average, and wind from the south (S, SSE, SE, SSW, or SW). The wind comes from the NEA stations on data.gov.sg. The hotspot count is the input NASA FIRMS is for. The words are then “Likely worse in 12–24 h”.

The pages read the saved reading. Rebuilding that reading from the local copies refreshes PSI, PM2.5, heat, lightning, rain, wind, and the CAMS chart. Those copies do not include a FIRMS hotspot file, so a rebuild leaves the warning off. It comes back when a saved reading contains both the hotspot counts and the wind.

This is not medical advice. If you feel unwell, stop and go indoors.