What the Heat Index Actually Tells You
The heat index answers a question the thermometer can't: given today's humidity, how hot does it really feel? Human bodies shed heat mainly by evaporating sweat, and humid air throttles that process. So two 90°F days can feel worlds apart — one dry and tolerable, one muggy and dangerous. This calculator takes your air temperature and relative humidity and returns a single feels-like value using the National Weather Service Rothfusz regression, the same method behind official heat advisories.
A Worked Example, Band by Band
Suppose it's 90°F with 70% relative humidity. The Rothfusz regression returns a heat index of roughly 105°F — a full 15 degrees above the air temperature, purely because the humid air is stalling your sweat. That 105°F lands right at the edge of the NWS Danger band, where heat exhaustion becomes likely and heatstroke is possible with prolonged exposure. The bands run: 80–90°F Caution, 90–103°F Extreme Caution, 103–124°F Danger, and 125°F and above Extreme Danger. Drop the humidity to 40% at that same 90°F and the heat index falls back near 91°F — the same thermometer reading, a completely different risk level.
Remember: This Number Assumes Shade
The heat index is calibrated for a person in the shade with a light breeze. Step into direct sunlight and the effective value can rise by as much as 15°F — so a shaded 100°F reading can feel like 115°F on an open field, a paved parking lot, or a tennis court at noon. If you're deciding whether outdoor work or a sports practice is safe, treat the calculator's output as the floor for a sunny day, not the ceiling.
Heat Index Calculator
How to Use This Calculator
- Enter Your Air Temperature (°F): Type in the current air (dry-bulb) temperature from your weather app or thermometer. The heat index formula is built for readings of 80°F and above.
- Enter Your Relative Humidity (%): Enter the relative humidity as a percentage. On muggy days this is the number doing most of the work — it's what pushes the feels-like value above the thermometer reading.
- Click Calculate: Hit Calculate to run the Rothfusz regression. Your feels-like temperature and its NWS danger band appear instantly below.
- Read the Danger Band: Check which band you land in — Caution, Extreme Caution, Danger, or Extreme Danger — and remember to bump the risk up a level if you'll be in direct sun.
How It Works
Air temperature alone doesn't tell you how hot it actually feels. Your body cools itself by evaporating sweat, and when the air is already loaded with moisture, that evaporation slows down. The heat index folds both numbers — temperature and relative humidity — into a single feels-like value using the National Weather Service Rothfusz regression.
The basic rule:
- Heat Index — Rothfusz regression equation (NWS) — A multi-term polynomial in dry-bulb temperature and relative humidity, fit to a biometeorological model of a walking adult in the shade. Low-heat and high-humidity edge cases use separate adjustments so the curve stays realistic.
One thing to keep in mind: the heat index assumes you're in the shade with a light breeze. Stand in direct sunlight and the effective value can climb by as much as 15°F above what this calculator reports.
Tips & Considerations
- On a dry day humidity barely matters, but above roughly 60% RH it drives the feels-like number more than the temperature does — always enter it accurately.
- The published heat index assumes shade. In full sun, mentally add up to 15°F before deciding whether an activity is safe.
- For outdoor work and sports, use the danger band to trigger action: more water and rest breaks in Extreme Caution, and consider stopping in the Danger band.
- Children, older adults, and pets reach dangerous core temperatures at lower heat index values than a healthy, acclimatized adult — plan for the most vulnerable person present.
- When the feels-like value pushes into Danger or higher, shift strenuous activity to early morning or evening rather than pushing through the midday peak.
Frequently Asked Questions
What is the heat index?
The heat index is the apparent or feels-like temperature — how hot the combination of heat and humidity feels to the human body. At 90°F and 70% relative humidity it feels like about 105°F, because the humid air keeps your sweat from evaporating and carrying heat away.
Why does humidity make it feel hotter?
Your primary cooling system is sweat evaporating off your skin. Evaporation pulls heat with it. When relative humidity is high, the air is already close to saturated, so sweat evaporates slowly or not at all. The heat has nowhere to go, your body temperature creeps up, and the same air temperature feels significantly hotter than it would on a dry day.
What heat index is dangerous?
The NWS uses four bands. 80–90°F is Caution (fatigue possible with prolonged exposure), 90–103°F is Extreme Caution (heat cramps and heat exhaustion likely), 103–124°F is Danger (heat exhaustion likely and heatstroke possible), and 125°F and above is Extreme Danger (heatstroke highly likely). Risk rises sharply once you pass roughly 103°F, especially during exertion.
Does direct sun change the heat index?
Yes. The Rothfusz regression is calibrated for shade. Full, direct sunshine can add up to about 15°F to the effective heat index. A reading of 100°F in the shade can feel like 115°F standing in an open field or on a paved court at midday.
What temperature and humidity should I enter?
Use the current air (dry-bulb) temperature in °F and the relative humidity as a percentage — both come from any weather app or a basic thermometer-hygrometer. The formula is designed for temperatures at or above 80°F; below that, humidity has little effect on how hot it feels, so the heat index roughly equals the air temperature.
Is this the same as the wet-bulb temperature?
No. Wet-bulb temperature is a direct physical measurement of the lowest temperature achievable by evaporation, and a wet-bulb near 95°F is survival-threatening. The heat index is a comfort-and-risk index derived from air temperature and relative humidity. They're related concepts but different numbers.