๐ฅต Heat Index Calculator
Enter the air temperature (ยฐC) and relative humidity (%) to calculate the "feels like" heat index using the formula adopted by the US National Weather Service (NWS), along with a caution level. Useful as a quick reference during hot, humid weather.
How to use
- Enter the current air temperature (ยฐC).
- Enter the current relative humidity (%) โ the feels-like temperature and caution level are calculated automatically.
How the calculation works
The heat index (apparent temperature), used by the US National Weather Service (NWS), expresses how hot it actually feels given temperature and humidity. High humidity slows the evaporation of sweat, so the body sheds heat less easily and the same temperature feels hotter. This tool uses the Rothfusz regression adopted by the NWS: the temperature is converted to Fahrenheit, put through the equation, and the result converted back to Celsius. The NWS adjustments for low and high humidity are applied. NWS risk levels โข 27โ32ยฐC: caution โข 32โ39ยฐC: extreme caution (heat illness possible) โข 39โ52ยฐC: danger (heat illness likely) โข 52ยฐC and above: extreme danger
Worked example
32ยฐC, 70% humidity โ feels like about 40.4ยฐC (danger) 35ยฐC, 50% humidity โ about 40.7ยฐC (danger) 30ยฐC, 60% humidity โ about 32.8ยฐC (extreme caution) At similar temperatures, humidity makes a large difference.
Things to be aware of
- Japan's heatstroke alerts use the WBGT (wet-bulb globe temperature) index, which also accounts for sunlight and reflected heat and uses different thresholds (31 or more is "danger").
- The formula assumes shade and a light breeze. In direct sunshine it can feel up to about 8ยฐC hotter.
- People feel heat differently. Older adults and children in particular should drink water and rest often whatever the number says.
FAQ
What is the heat index?
It's a measure of how hot it actually feels to the human body, combining air temperature and humidity. High humidity slows sweat evaporation, making it feel hotter than the air temperature alone would suggest.
Is this the same as WBGT (Wet Bulb Globe Temperature)?
No, they're different indices. WBGT also factors in solar radiation and wind speed, and is the standard used in Japan for heat-stroke prevention guidance. The Heat Index calculated here is a simpler US-originated index based only on temperature and humidity.
What are the caution level thresholds based on?
This tool uses the US National Weather Service's Fahrenheit thresholds (80ยฐF, 90ยฐF, 103ยฐF, 125ยฐF), converted to Celsius. These differ from Japan's WBGT-based heat-stroke guidance, so treat the result as a general reference.