Heat Index Chart: How Hot It Really Feels
Air temperature is only half the story. When humidity is high, your body can't cool itself as efficiently — this chart shows the felt temperature for each combination of heat and moisture.
Heat Index Reference Chart
Felt temperature in °C for a given air temperature (rows) and relative humidity (columns). Values follow the Rothfusz heat index equation used by the US National Weather Service.
| Air temp | 40% humidity | 50% humidity | 60% humidity | 70% humidity | 80% humidity | 90% humidity | 100% humidity |
|---|---|---|---|---|---|---|---|
| 27°C | 27°C | 27°C | 28°C | 28°C | 29°C | 30°C | 31°C |
| 29°C | 29°C | 30°C | 32°C | 34°C | 36°C | 39°C | 42°C |
| 32°C | 33°C | 35°C | 38°C | 41°C | 45°C | 50°C | 55°C |
| 35°C | 37°C | 41°C | 45°C | 50°C | 57°C | 64°C | 72°C |
| 38°C | 43°C | 48°C | 54°C | 62°C | 70°C | 80°C | 91°C |
| 41°C | 50°C | 57°C | 65°C | 75°C | 86°C | 98°C | 112°C |
Heat Index vs. Actual Temperature
The number on the thermometer measures the kinetic energy of the air — the heat index estimates the temperature your body perceives. On a 35°C day at 40% humidity the two readings are close. But at 90% humidity, that same 35°C day feels like 64°C, because sweat can no longer evaporate and carry heat away from your skin.
This is why two cities with identical forecast temperatures can feel completely different: dry heat is far more tolerable than humid heat.
Why Humidity Matters
The human body sheds heat mainly through the evaporation of sweat. Relative humidity is the percentage of moisture the air is already holding compared to its maximum at that temperature. As humidity approaches 100%, the air has little capacity left to absorb more moisture, so sweat stays on your skin and your core temperature rises.
High humidity also reduces how much you notice thirst, so dehydration can set in before you feel it. Drink water regularly on humid days even when you don't feel thirsty.
Staying Safe in High Heat
- Move strenuous activity to the early morning or evening, when the heat index peaks are lowest.
- Drink water before, during and after outdoor time — aim for regular small sips rather than large amounts at once.
- Wear light, loose, light-coloured clothing that lets sweat evaporate.
- Take breaks in shade or air conditioning; fans alone are not effective above roughly 35°C in high humidity.
- Watch for heat exhaustion signs: heavy sweating, weakness, dizziness, nausea. Move to a cool place and cool down immediately.
Where to Check the Felt Temperature
WeatherNow's feels like value combines the current temperature with humidity and wind to give you the same practical reading as this chart, updated every few minutes. Open the “Today's Highlights” panel on any city page or the homepage to see it alongside the UV index, wind and rain chance.
This guide is general information, not medical advice. If you or someone else shows signs of heat stroke — confusion, hot dry skin, fainting — seek emergency help immediately.
Heat Index FAQ
What is the heat index?
The heat index is an estimate of how hot the air feels to the human body when air temperature and relative humidity are combined. The higher the humidity, the less effectively sweat evaporates, so the same temperature feels much hotter.
Why does humidity make heat feel worse?
Your body cools itself by sweating, and sweat only cools you when it evaporates. In humid air, evaporation slows down, so heat builds up — 32°C at 90% humidity can feel like 50°C or more.
When is the heat index dangerous?
The US National Weather Service treats a heat index above about 39°C as dangerous, with heat exhaustion likely during prolonged activity, and above 51°C as extreme danger, where heat stroke is possible within minutes of exposure.
Is the heat index the same as 'feels like'?
Similar in spirit, but not identical. 'Feels like' on WeatherNow also accounts for wind chill in cold weather and wind speed in general, while the heat index only applies in warm, humid conditions.