避暑助手 HeatSafe HK — a Hong Kong heat index app for where you are standing
Hong Kong's official heat index gives direct sunlight a weight of 5%. The international WBGT standard gives it 20%. In the sun, that gap is the whole story — so the app shows both.
This app is still in development and is not yet listed on the App Store or Google Play. This page describes what has been built.
One number, several definitions
Hong Kong's work heat warning is based on the Hong Kong Heat Index, which combines natural wet bulb temperature, black globe temperature and air temperature as 0.80, 0.05 and 0.15. The international WBGT standard for outdoor conditions with solar load uses 0.7, 0.2 and 0.1 — a four-fold difference in how much direct sun counts.
For someone in an air-conditioned office that difference is academic. For someone on scaffolding at two in the afternoon it is not. HeatSafe computes several indices from the same underlying station data and shows them side by side, so the gap between the official warning and the international standard is visible instead of invisible.
What the Hong Kong heat index app computes
Every formula in the app has its source written into the code beside it.
- The Hong Kong Heat Index. HKHI = 0.80 × natural wet bulb + 0.05 × globe + 0.15 × air temperature, the definition published by the Observatory and used by the Labour Department as the basis for its work heat warnings.
- WBGT, outdoor and indoor. The ISO 7243:2017 outdoor formula with solar load (0.7 / 0.2 / 0.1) and the indoor or no-sun form (0.7 / 0.3), matching the original Yaglou and Minard definition and Japan's JIS Z 8504.
- The US heat index. The National Weather Service heat index from Rothfusz (1990), including the two adjustments the NWS currently applies, computed in Fahrenheit internally and returned in Celsius.
- Real instrument data, not just estimates. The Observatory operates heat stress measurement systems at ten locations that measure directly rather than modelling, and the app reads that feed — deriving globe temperature from solar position where no direct reading exists.
- Official warning levels, clearly labelled. The Labour Department's thresholds are shown — HKHI 30 amber, 32 red, 34 black — with an explicit note that the app estimates from the same formula and that only the Labour Department's own announcement is an actual warning.
- Worker mode. For outdoor work: your current exposure conditions (direct sun, no shade, indoors without air conditioning, heat source or non-breathable protective clothing), a rest countdown, and a work log.
- Sun against shade, and the better hours. The forecast marks the hottest period and the better times to go out, and separates direct sun from having something over your head — quantifying how many heat-stress levels the sun alone adds.
- Hydration points and cooling centres. Nearby drinking water points and cooling centres, from government data.
It publishes the formulas, and it drops what it cannot stand behind
The clearest signal of the app's approach is what it removed. A UTCI implementation exists in the codebase and is deliberately not displayed, because the version implemented is a linearised approximation whose humidity coefficient is too weak, which makes it read systematically low in Hong Kong's hot and humid conditions. The reasoning is written out in the source: an index known to run low should not sit next to three verifiable ones, because a user cannot tell which number to trust and the bad one drags down the credibility of the rest.
That is an unusual decision — removing a feature because it would be misleading — and it is the reason to trust the numbers that remain.
Who it is for
Outdoor workers and the people responsible for them — construction, cleaning, delivery, security, gardening. Worker mode exists to turn an index into a rest decision.
Parents and carers, where the gap between the shaded reading and the direct-sun reading decides whether an outing is sensible. And anyone who has stood in Hong Kong in August, seen a modest official figure, and known it did not describe what they were feeling.
Data sources and disclaimer
Observations come from the Hong Kong Observatory, including its heat stress measurement stations. Warning thresholds follow the Labour Department's Guidance Notes on Prevention of Heat Stroke at Work. Facility data comes from the Water Supplies Department and FEHD. Formula sources are cited in the app.
The app estimates using published formulas. It is not an official warning service — only the Labour Department and the Observatory issue warnings. It is informational, not medical advice.
This app is still in development and is not yet listed on the App Store or Google Play. This page describes what has been built.
Frequently asked questions
Why does the app show more than one heat index? Because they weight direct sunlight very differently. Hong Kong's HKHI gives globe temperature a 5% weight; the international WBGT standard gives it 20%. Showing both makes the difference in the sun visible.
What is WBGT? Wet bulb globe temperature, the ISO 7243 standard for assessing heat stress. The app computes both the outdoor form with solar load and the indoor form.
Does it tell me when the Labour Department has issued a heat warning? It shows the official thresholds — HKHI 30 amber, 32 red, 34 black — and estimates using the same formula, while stating plainly that only the Labour Department's own announcement is an actual warning.
Where does the data come from? The Hong Kong Observatory, including the heat stress measurement systems it operates at ten locations, plus government facility data for hydration points and cooling centres.
Why is UTCI not shown? Because the implementation available is a linearised approximation whose humidity coefficient is too weak, so it reads systematically low in Hong Kong conditions. Rather than show a number known to be wrong, it is left out — with the reasoning written into the source.