Heat Index Calculator
Humid air stops sweat from evaporating, so the same air temperature cools you less. The calculator applies the National Weather Service heat index equation and flags the risk band.
Result
- Heat risk
- Extreme caution - heat cramps and exhaustion possible
How the calculation works
The calculator applies the official NWS heat index (Rothfusz regression):
- Below 27 °C (80 °F) a simple Steadman formula is used instead of the regression.
- Corrections apply at the edges: very dry air (RH < 13%) and very humid air (RH > 85%).
- The index assumes shade and light wind. NWS notes that full sunshine can push the apparent temperature up to 8 °C (15 °F) above the index value - the sunny-trail reality is hotter than the number.
- Risk bands follow NWS categories: caution from 27 °C (80 °F), extreme caution from 32 °C (90 °F), danger from 39 °C (103 °F), extreme danger from 52 °C (125 °F) - all on the index, not the air temperature.
Heat pairs with dehydration: the hiking water calculator raises drinking rates for the same temperature and humidity, and the wind chill calculator covers the cold end of the same "feels like" spectrum.
Planning estimate only - not safety advice.
Frequently asked questions
What is the heat index actually measuring?
How hot the air feels to a human body, not a thermometer. Evaporating sweat is how we shed heat, and humid air slows evaporation, so at 32 °C (90 °F) with high humidity your body is under the stress of a much higher temperature. The NWS equation bundles those effects into one number.
When does heat become dangerous for hiking?
From about 32 °C (90 °F) on the index the calculator flags extreme caution: heat cramps and exhaustion become possible with hard effort. Above 39 °C (103 °F) heat exhaustion is likely - plan around the middle of the day, not through it.
Why is the number different from my weather app?
Some apps show "feels like" values that mix heat index with wind or sunshine models. This calculator publishes the official NWS heat index as defined - see the sunshine question below for the one real-world factor it deliberately leaves out.
Does sunshine make the heat index worse?
Yes. The official index assumes shade and light wind; sunshine is the big absent variable, and NWS notes that full exposure can raise the apparent temperature by up to 8 °C (15 °F) over the index value. When planning summer hikes, read the number here as the shaded baseline, then add margin for exposed sections - and schedule the exposed climbs outside midday.
Does wind change the heat index?
Below about 32 °C (90 °F) wind cools you; in real heat it mostly makes things worse by blowing hot air at you. The NWS index does not take wind into account, which is a fair planning simplification for hot days.