Wet bulb globe temperature
How the weather becomes one number, and where the lines are.
Updated:
HeatCue starts with what the weather is doing where you are: air temperature, humidity, wind, cloud, and how high the sun is. It puts all of that together into one number, WBGT, which is what you see on Now.
The engine behind the bands
WBGT stands for wet bulb globe temperature. It’s a close cousin of plain wet bulb, but not the same thing. Wet bulb only answers one question, how cool sweat can get you, so it stays low in dry heat even when the air is dangerous. WBGT takes air temperature, humidity, wind and sun together and gives one number for how hard the heat is on your body. That’s why sport and workplace heat guidance is written against it. HeatCue uses the Liljegren method, which works out what a black globe and a wet wick would read in your weather. That number is what you see on Now, and the bands are set from it.
Where the lines are
Safe ends at 28 °C (82 °F). Caution runs to 30 °C (86 °F), Danger to 32 °C (90 °F), and past that it is Extreme. Those are WBGT readings, and they follow published categories used in sport and workplace guidance.
| Band | WBGT °C | °F |
|---|---|---|
| Safe | < 28 | < 82 |
| Caution | 28 – 30 | 82 – 86 |
| Danger | 30 – 32 | 86 – 90 |
| Extreme | ≥ 32 | ≥ 90 |
Each step up means the heat is harder to shed. You don’t need to remember the numbers. The band name and its shape tell you what to do.
The at-risk and dog profiles
Small bodies, older bodies, and hard effort all lose ground to heat sooner, and dogs cool only by panting. So the profiles move the lines rather than the reading, about 2 °C (4 °F) earlier for the at-risk profile and 3 °C (5 °F) for a dog. The number on screen doesn’t change. The warning just comes sooner.
Sun or shade
Standing in open sun is hotter than standing under a tree, often by several degrees. HeatCue works out both. The sun/shade button on Now switches between them, and it’s off at night because the two are the same once the sun is down.
HeatCue shows conditions and general guidance. It is an awareness tool, not medical advice.