Read the heat index on a humid day
Humidity stops sweat from cooling you. The heat index is the 'feels like' temperature once that brake fails.
Tools used in this guide
The job
Sweat cools by evaporation; humid air slows it, so 35 °C at 80 % feels far hotter than the same temperature dry. The NWS Rothfusz regression is the standard 'feels like' for heat.
Temperature + humidity
Enter temperature and relative humidity. From 80 °F (~27 °C) up the full regression applies; below it a simpler average is used. The result comes with its danger band — caution, danger, extreme danger.
Worked example
95 °F (35 °C) at 60 % RH indexes to about 113 °F — the danger band, heat stroke risk with prolonged exposure. Drop the humidity to 30 % and the same 95 °F feels like ~94 °F. Humidity is the whole story.
Where it stops
Shade and light wind are assumed; direct sun can add up to 15 °F. Valid 60–140 °F. It estimates felt heat, not a medical threshold — listen to your body before the number.