What is the heat index?
The heat index is the apparent temperature — what 90°F with 70% humidity actually feels like on human skin. The thermometer reads one number; your body reads another. When the air is muggy, sweat doesn't evaporate efficiently, so your built-in cooling system stalls. The heat index quantifies how much hotter the conditions feel than the dry-bulb temperature suggests.
This is the same number your local TV meteorologist reads on summer evenings. It's the number the National Weather Service uses to issue heat advisories. It's also the number that decides when outdoor work has to stop, when football practice gets called off, and when public-cooling centers open. The math is identical across all of them: the Rothfusz regression, derived in 1990 from Robert Steadman's earlier apparent-temperature tables.
If the dry-bulb temperature is the air's temperature, the heat index is your body's temperature interpretation of that air. They diverge fast once humidity climbs above 50%.
How to use the Heat Index Calculator
Two inputs, instant output. The calculator handles °F and °C natively and applies the NWS edge-case adjustments automatically.
- Enter the air temperature. Pick °F or °C from the dropdown.
- Enter the relative humidity (0 to 100).
- Read the heat index, displayed in both °F and °C.
- Check the caution tier below the number: Caution, Extreme Caution, Danger, or Extreme Danger. Each corresponds to a specific NWS heat-stress threshold.
If your inputs fall outside the formula's validity range — below 80°F or below about 40% RH — the calculator switches to a simpler Steadman approximation and notes that the result is less meaningful. The Rothfusz regression was fit to summer conditions, not winter, and it produces nonsensical numbers if you push it outside its training range.
The formula behind the number
The full Rothfusz regression is a nine-term polynomial fit to Steadman's 1979 apparent-temperature data. It's not pretty, but it's accurate.
HI = −42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH
− 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH
+ 0.00085282·T·RH² − 0.00000199·T²·RH²
T is air temperature in °F. RH is relative humidity as a percent (0–100).
Two corrections apply at the edges of the validity range:
- Low-humidity adjustment: when RH is below 13% and the temperature is 80–112°F, the base formula overestimates how hot it feels. Subtract a small correction. (Phoenix would otherwise read too high.)
- High-humidity adjustment: when RH is above 85% and the temperature is 80–87°F, the base formula slightly underestimates. Add a small correction.
Worked example: a 90°F afternoon (32.2°C) with 70% relative humidity — typical mid-summer for Atlanta, Houston, or Memphis.
- T = 90, RH = 70
- HI = −42.379 + 2.04901523(90) + 10.14333127(70) − 0.22475541(90)(70) − 0.00683783(90²) − 0.05481717(70²) + 0.00122874(90²)(70) + 0.00085282(90)(70²) − 0.00000199(90²)(70²)
- HI ≈ 106°F (41°C)
The thermometer says 90. Your body reads 106. The difference is sixteen degrees — entirely because the air is full of water and your sweat is sitting there doing nothing.
The NWS caution tiers
The thresholds below come from National Weather Service guidance. They apply to healthy adults in shade or indoor conditions. Vulnerable groups — elderly, very young, on certain medications, or with cardiovascular conditions — face the same risk one tier lower than listed.
| Heat Index (°F) | Heat Index (°C) | Tier | What it means |
|---|---|---|---|
| 80 – 90 | 27 – 32 | Caution | Fatigue possible with prolonged exposure or physical activity. Hydrate, take breaks. |
| 90 – 103 | 32 – 39 | Extreme Caution | Heat cramps and heat exhaustion possible. Limit outdoor exertion. Watch for dizziness or nausea. |
| 103 – 125 | 39 – 52 | Danger | Heat exhaustion likely; heat stroke possible with continued activity. Outdoor work should pause. |
| Above 125 | Above 52 | Extreme Danger | Heat stroke imminent. Stop outdoor activity. Active cooling needed. |
The 90°F at 70% example above lands in Danger territory. If you've ever wondered why a fairly normal-looking summer day in the South can put people in the ER, this is why. The thermometer reads moderate; the body reads dangerous.
The highest heat index ever recorded was around 178°F (81°C) in Dhahran, Saudi Arabia, in 2003 — air temp 108°F with a dew point of 95°F. The Pacific Northwest heat dome of 2021 hit roughly 125-130°F heat index, which sits at the edge of what humans can survive without active cooling for more than a few hours.
Why humidity makes heat dangerous
Your body has exactly one cooling mechanism when the air is hotter than your skin: sweating. Sweat absorbs heat as it evaporates — about 580 calories of heat per gram of water that goes from liquid to vapor. That's the only way you shed metabolic heat in a hot environment.
Evaporation needs somewhere to evaporate into. If the air is already nearly saturated with moisture, sweat doesn't go anywhere. It pools on your skin, dripping but not cooling. You keep producing it — losing water and electrolytes — while your core temperature climbs.
This is why dry heat is genuinely more tolerable than humid heat, even at higher thermometer readings. 105°F at 15% humidity (Phoenix) is uncomfortable but survivable; sweat evaporates fast, cooling effectively. 95°F at 80% humidity (New Orleans) actually carries higher heat-stress risk despite the lower air temperature.
This is also why fans don't help much above a certain point. A fan increases evaporative cooling — but only if there's room for evaporation. Above about 95°F and 60% RH, a fan can actually accelerate heat stress by blowing hot air over skin without enabling more sweat evaporation.
Heat index vs. dew point vs. wet-bulb
Three numbers describe the same atmospheric reality differently:
Heat index — the felt temperature given heat and humidity. Combines both into one "feels like" number. Best for everyday awareness.
Dew point — the temperature at which water condenses. Isolates the humidity component. Best for comparing comfort across different temperatures.
Wet-bulb temperature — the temperature a wet thermometer reads. The most physically meaningful for heat-stress thresholds; a wet-bulb temperature above ~95°F (35°C) is the theoretical limit of human survivability.
For most purposes, heat index is the right tool. Dew point is better when you want to compare days at different temperatures (since heat index changes with temperature even at the same moisture level). Wet-bulb is the metric of choice in industrial heat-safety standards.
What the heat index doesn't account for
The number assumes shade and minimal physical activity. Real outdoor conditions can be much worse:
- Direct sun adds up to 15°F to the perceived temperature. The NWS heat index chart and the calculator both assume shade. If you're standing in full sun on dark pavement, expect significantly higher felt temperatures than the heat index suggests.
- Physical exertion raises metabolic heat output by 4-8x resting. The heat index assumes you're sitting still. A construction worker or distance runner at the same heat index faces a much higher actual heat load.
- Clothing matters a lot. Dark, heavy fabric absorbs solar radiation. Tight clothing limits convective cooling. Football pads and helmets are roughly equivalent to adding 10-20°F to the effective heat index.
- Acclimatization takes 1-2 weeks of repeated exposure. Someone who lived through last August handles 95°F + 75% RH better than someone who flew in from Seattle yesterday. The heat index applies to "an average healthy adult" — your personal threshold may be higher or lower.
- Wet-Bulb Globe Temperature (WBGT) is the better metric for outdoor work and athletics because it incorporates radiant heat (sun), wind, and humidity. OSHA and most US high-school athletic associations use WBGT for go/no-go decisions.
Related calculations
Heat is one variable in your local weather equation. To get a fuller picture:
- Dew Point Calculator — the absolute-moisture counterpart. Better for comparing comfort across different temperatures.
- Wind Chill Calculator — the cold-season analog. NWS 2001 formula with frostbite-time warnings.
- Temperature Converter — quick °F ↔ °C if your AC and your weather app disagree on units.
Frequently asked questions
Is the heat index the same as "feels like" temperature?
Yes, in summer. Most weather apps use the term "feels like" to refer to whichever apparent-temperature formula is most appropriate for current conditions — heat index when it's hot, wind chill when it's cold. In winter, "feels like" means wind chill; in summer, it means heat index. Same idea, different formula behind it.
Why does the heat index sometimes match the air temperature?
When relative humidity is low enough, the heat index is approximately equal to the air temperature — sweat evaporates fast enough that the felt temperature isn't worse than the actual temperature. This is why arid climates can read comfortable at temperatures that would be brutal in a humid climate. Below about 40% RH, heat index and dry-bulb temperature converge.
Can the heat index be lower than the air temperature?
Technically, yes — at very low humidity (single-digit RH), the corrected Rothfusz formula reports a heat index slightly below the air temperature, reflecting how efficiently sweat evaporates in extremely dry air. The difference is small, and most weather services just report the air temperature in those conditions.
Why is the formula so complicated compared to wind chill?
Heat index models human physiology (sweat evaporation rate, skin temperature, internal heat production) rather than pure physics. Wind chill models convective heat loss from skin — a simpler thermodynamic process. The Rothfusz regression has nine terms because it's fitting Steadman's full physiological model, which itself integrates over a dozen variables. Wind chill's NWS formula has four terms.
What heat index triggers a National Weather Service heat advisory?
Most NWS regions issue a Heat Advisory when the heat index is forecast to reach 100°F (some regions use 105°F) for at least two consecutive hours. An Excessive Heat Warning is typically issued for forecast heat indices of 105–110°F or higher. Specific thresholds vary by region because acclimatization differs — what's normal in Phoenix is dangerous in Seattle.
Does the heat index apply indoors?
Yes — air temperature plus humidity affect indoor comfort the same way they affect outdoor comfort. An indoor space at 85°F with 75% RH has a heat index around 95°F, which is uncomfortable and potentially dangerous for vulnerable people without air conditioning. Indoor heat is a major mortality risk during heat waves precisely because building heat indices can stay dangerously high even after outdoor temperatures drop overnight.
Why do some weather apps show a higher "feels like" than your calculator?
Two possible reasons. First, some apps add a sun-exposure adjustment when the forecast assumes clear skies and direct sunlight — that can add 5-15°F. Second, some apps blend wind chill and heat index logic with a continuous model rather than the discrete NWS formulas, which can produce slightly different numbers near the boundaries. This calculator uses the pure NWS Rothfusz regression with the official low-humidity and high-humidity corrections.