Wet bulb temperature
The ceiling on what sweat can do, and the line behind the Limit band.
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Wet bulb temperature is the lowest temperature your skin can reach by sweating. WBGT, the number on the Now screen, is built partly from it, but wet bulb has a job of its own: it marks the hard ceiling on what sweat can do, the line behind the Limit band.
The name comes from a simple experiment. Wrap a wet cloth around a thermometer bulb and move air past it. Water evaporates, the bulb cools, and the reading settles at the wet bulb temperature. Your skin runs the same experiment all day.
Sweat is your cooling system
Your body needs to stay near 37 °C (98.6 °F). When you warm up, you sweat. Every drop that evaporates carries heat away from your skin.
Humidity slows that down. When the air already holds plenty of water, sweat sits on your skin instead of evaporating. You keep sweating, but you stop cooling.
Why a humid 31 °C (88 °F) hits harder than a dry 40 °C (104 °F)
A dry 40 °C (104 °F) sounds worse than a humid 31 °C (88 °F). For sweating it is the other way around: in dry air, sweat evaporates fast, so you shed heat even on a very hot day. Dry heat still has its own danger, though. It pulls water from you fast, so dehydration catches up with you quickly. That’s why WBGT still flags a dry scorcher as dangerous.
At 31 °C (88 °F) with high humidity, evaporation nearly stops. Heat builds inside you with nowhere to go. That is why wet bulb can warn you on days the plain forecast calls mild.
The 35 °C (95 °F) ceiling
Past this wet bulb, even a healthy person resting in deep shade with plenty of water gains heat faster than their body can shed it. The Limit article covers how that number was found.
Conditions that severe are still very rare, and the bands ask you to act long before them.
Where HeatCue comes in
You don’t have to track any of this yourself. HeatCue works out WBGT for wherever you are, tells you in plain words whether you’re fine to be out or should take it easy, and keeps watching so you know when the heat turns. It even knows the difference between standing in the sun and standing in the shade.
HeatCue shows conditions and general guidance. It is an awareness tool, not medical advice.