At 12:15 Singapore time on 14 October 2026, the 30 Meteorological Service Singapore (MSS) stations reporting a wet-bulb globe temperature (WBGT) read between 29.2 and 31.8 °C: 24 in the Low band, 6 in Moderate and none in High. We estimated the weather near each sensor from the nearest of 18 weather stations' air temperature and humidity. A shortcut from Australia's Bureau of Meteorology (BoM) put all 30 in High, between 2.3 and 5.2 °C above the measured value. The standard indoor WBGT formula with a calculated wet bulb, which assumes no sun and is meant to read low, came in under all 30, by a median of 2.5 °C. A sunlit WBGT cannot be recovered from temperature and humidity alone, and only the measured MSS figure is official; MSS says WBGT and air temperature "are not interconvertible". That is one moment, with weather readings taken 18 minutes later and up to 5.9 km away, and cloud and wind unrecorded.
The 30 sensors drew on only 14 distinct weather stations. The lowest BoM figure, 33.1 °C, cleared the High cut-off by 0.1 °C, and a re-run with the air temperatures on MSS's own 12:15 map, keeping each pair's moisture content, still put all 30 in High. Nor does the snapshot show that the shortcut always runs high: there was no High reading anywhere that day, and our model below has the same shortcut reading low in strong sun and still air.
Official readings blend three thermometers in fixed proportions
MSS's heat stress explainer gives the outdoor formula as WBGT = 0.7 Tnwb + 0.2 Tg + 0.1 Td. Tnwb is the natural wet bulb, a thermometer under a wetted wick left open to sun and breeze. Tg is the black globe, a thermometer inside a black metal sphere that soaks up radiant heat. Td is ordinary air temperature. MSS reports the result "as a 15-minute average value, updated every 15-minutes".
So WBGT is not the single wet-bulb reading of climate reporting. A heat index is a different quantity again, a shade figure on a scale of its own, and our guide to why relative humidity needs a temperature covers that side.
Wet-wick readings combine heat, humidity and wind
Seventy per cent of the weight sits on the wick, which is easy to mistake for a humidity term. MSS says the wick reading "represents the integrated effect of air temperature, humidity, and wind speed", and that the wick is "exposed to the natural prevailing air movement as well as radiation". Sunshine reaches WBGT through the wick as well as the globe.
A formula fed temperature and humidity can only produce the shaded, psychrometric wet bulb. Stull's 2011 approximation carries errors between −1 and +0.65 °C. Kong and Huber's 2024 GeoHealth paper found that in daytime, using it in place of the natural wick pulls WBGT down by around 0.8 °C at the commonest error, and using air temperature in place of the globe pulls it down by around 1.5 °C.
The Bureau of Meteorology shortcut stirs in a fixed dose of sunshine
BoM's thermal stress page credits its shortcut to a 1984 American College of Sports Medicine position stand: WBGT = 0.567 × Ta + 0.393 × e + 3.94, where e is water vapour pressure from temperature and humidity. BoM says it "assumes a moderately high radiation level in light wind conditions", and expects it to overestimate in cloud or wind, or with the sun low, and to underestimate slightly in clear full sun with low humidity.
Those assumptions did not match the weather near Singapore's sensors at 12:15 on 14 October. Kong and Huber's 2022 study found that the shortcut "greatly overestimates heat stress in hot-humid areas", and recommended discontinuing it.
Shade formulas dilute the globe down to air temperature
With no sun the globe settles near the air, and the outdoor formula collapses to 0.7 × wet bulb + 0.3 × air temperature. Our WBGT heat stress calculator uses this with Stull's wet bulb when it has only temperature and humidity; in daytime sun it reports the result as a lower bound and declines to print a sunlit figure. The snapshot supports that refusal; in the model below the shade figure sat under the sunlit reference at all 448 sunlit points we modelled.
At night the floor gives way, because the globe reads below the air: around 1 °C and up to 3 °C in Kong and Huber's figures, 0.3 to 0.8 °C in our model. At our night-time grid points the shade figure ran up to 0.44 °C above the modelled WBGT.
Until 14 October 2026 the calculator's page described a WBGT of 30 in stronger terms than Singapore's bands, which place 30 in Low. It also attributed to Kong and Huber's 2022 paper a percentage the abstract does not contain and called the shade figure a floor after dark. The revision committed that day removes all three, and the page now warns that the figure may run a few tenths high at night.
Sun and wind churn the gap between the two shortcuts
We also re-implemented the 2008 WBGT model of Liljegren and colleagues. Our grid spans the Changi climate normals that our density altitude guide also uses, light air to a 3 m/s breeze, and sunshine on a surface from none to 900 W/m². The model is ours and unchecked against Liljegren's code, and grid points are not a share of Singapore days.
Across the grid, the two shortcuts differed by 4.6 to 9.0 °C on identical temperature and humidity, more than twice the width of the 2 °C Moderate band. Under the strongest sun the modelled WBGT runs higher in still air than in a breeze, and the BoM error changes sign with it: on a typical afternoon of 31 to 33 °C and 60 to 70% humidity, that shortcut read 0.8 to 2.4 °C low at 0.5 m/s and 1.7 to 3.1 °C high at 3 m/s. With no sun it ran 5.7 to 6.9 °C high, as BoM expects once the sun is down.
Recorded high-heat days mix the weather with a growing sensor network
MSS's The Year in Numbers 2025 counts 29 days of high heat stress in 2025, up from 21 in 2024, counting a day when "any hourly-average WBGT at a station is equal to or greater than 33°C". It says the second-half increase "was due to the deployment of more Wet-Bulb Globe Temperature stations". When the advisory was announced on 24 July 2023, the Ministry of Sustainability and the Environment described "an islandwide network of nine sensors"; the data.gov.sg WBGT feed carried values from 30 stations at 12:15 on 14 October 2026.
The NEA/MOH heat stress advisory sets Low below 31 °C, Moderate from 31 to below 33 °C and High from 33 °C. MSS's heat stress page says it "applies to prolonged outdoor activities (under direct sun exposure) for the general population". The advisory sends outdoor workers to the Ministry of Manpower's guidelines and students to their schools, and does not apply to the SAF and the Home Team. The official readings are the MSS station figures on that page and in NEA's myENV app.
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