Heat Index: How Hot, Humid Air Becomes a Felt Temperature
The heat index is the temperature your body perceives when you combine air temperature with humidity — the single number that drives every US National Weather Service heat advisory. This guide explains where the Rothfusz formula comes from, why humidity wrecks your body's cooling system, a worked example through to a Danger-band reading at 96°F and 65% humidity, what the four NWS risk bands actually mean for outdoor activity, and the populations and situations where heat index is the wrong tool and WBGT or wind chill is right.
What the heat index actually tells you
The heat index is the temperature your body perceives when you combine air temperature with humidity — a single number that captures how badly hot, sticky air is going to wear you out. It is not the actual air temperature, which a thermometer always reads directly, and it is not the temperature in the sun. It is a model of human perception in the shade, built so that public-health agencies have one number they can put on a forecast and have ordinary people understand it. The heat index calculator on this site uses the official US National Weather Service Rothfusz regression — the same equation that drives every NWS heat advisory and most "feels like" temperatures you see in weather apps in the United States.
The number matters because heat illness is a function of felt temperature, not air temperature. A 95°F afternoon in Phoenix at 15% humidity is a different physiological problem than 95°F in New Orleans at 75%. The Phoenix afternoon has a heat index in the low 90s; the New Orleans afternoon has a heat index of about 122°F and is well into the NWS Danger band. Knowing that gap is the difference between a long walk and a 911 call.
Where the formula comes from
The arithmetic the calculator runs was published by Lans P. Rothfusz at the NWS Southern Region Headquarters in 1990. Rothfusz did not invent the concept of an apparent temperature — Robert Steadman had already built a biometeorological model in 1979, treating the human body as a heat-exchange problem and computing the air temperature that would produce the same physiological stress at a reference humidity of 20%. Steadman's model is correct but inconvenient: it is a thick lookup table, not a formula. Rothfusz fit a nine-term multiple regression to that table so the apparent temperature could be computed on a pocket calculator. The regression is what every national weather service in North America still uses three decades later.
The full equation, with T in degrees Fahrenheit and RH in percent, is:
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
Two small corrections apply at the corners. When the humidity drops below 13% at temperatures of 80–112°F, the NWS subtracts an adjustment that knocks a few degrees off the result — dry air evaporates sweat efficiently, and the unadjusted regression slightly over-states the felt temperature. When the humidity climbs above 85% at temperatures of 80–87°F, a smaller positive adjustment is added. Below about 80°F the formula is bypassed entirely and the NWS uses a simpler Steadman estimator that, for most inputs, returns something close to the air temperature. Humidity simply has too little perceived effect when the air is not hot to begin with. The heat index calculator applies all three pieces of that logic so the answer matches what the NWS publishes for the same inputs.
Why humidity makes hot air feel worse
Your body has one main strategy for unloading excess heat: it sweats, and the evaporating sweat carries heat away as latent heat of vaporisation. About 2,260 joules leave the skin for every gram of water that evaporates, which is a substantial cooling budget when the air is dry enough to absorb it. The catch is that evaporation rate depends on the difference between the water-vapour pressure at your skin and the water-vapour pressure of the surrounding air. The drier the air, the steeper the gradient and the faster the evaporation.
Crank the relative humidity up and the gradient collapses. At 100% RH the air is fully saturated, sweat cannot evaporate, and your only remaining heat-loss channels are radiation and convection — neither of which works once the air is warmer than your skin. This is why a 95°F day at 15% humidity feels manageable and a 95°F day at 80% humidity is dangerous. The first is a sweat-evaporates problem; the second is a sweat-just-runs-off problem, and physiologically those are different worlds. The heat index reduces that physiology to one number on a scale you already know.
Worked example: 96°F at 65% humidity
Take an air temperature of 96°F and a relative humidity of 65% — a routine summer afternoon in Houston, Tampa or New Orleans. Plugging into the Rothfusz equation, term by term:
The constant is -42.379. The linear-T term is 2.04901523 × 96 = 196.705. The linear-RH term is 10.14333127 × 65 = 659.317. The cross term is -0.22475541 × 96 × 65 = -1402.493. The T² term is -0.00683783 × 9216 = -63.018. The RH² term is -0.05481717 × 4225 = -231.602. The T²·RH term is 0.00122874 × 9216 × 65 = 736.084. The T·RH² term is 0.00085282 × 96 × 4225 = 345.886. The T²·RH² term is -0.00000199 × 9216 × 4225 = -77.479. Sum: about 121°F. Neither corner adjustment applies (RH is between 13% and 85%, and T is over 87°F), so 121°F is the final answer.
96°F with 65% humidity therefore feels like 121°F — a 25-degree gap, and well inside the NWS Danger band. That is the number that triggers excessive-heat warnings, school athletics policy modifications, OSHA outdoor-worker protections, and the public messaging you see on city emergency dashboards in the South in August. Type those same inputs into the heat index calculator and you get 121°F to one decimal place — this is the same arithmetic, just done for you.
What the risk bands mean in practice
The NWS publishes four bands and a small set of recommended behaviours for each one. The boundaries are not arbitrary — each one is set near a physiological threshold documented in the heat-illness literature.
Caution: 80–90°F
Fatigue is possible with prolonged exposure or physical activity. This is the band most healthy adults can ignore for short outings but should respect on long runs, day hikes, or anything that requires sustained exertion. Increase water intake, take breaks in the shade, and acclimatise gradually if you have been indoors.
Extreme Caution: 90–103°F
Heat cramps and heat exhaustion become possible; heatstroke is unlikely without prolonged exposure or heavy exertion. This is when outdoor-worker employers should be rotating crews and providing scheduled water breaks, and when school football practices typically move to early morning or evening. Most US summer afternoons in humid regions sit in this band from June through September.
Danger: 103–125°F
Heat cramps and heat exhaustion are likely; heatstroke is possible with prolonged exposure or exertion. This is when NWS issues an Excessive Heat Watch or Warning depending on duration, when cooling centres open in major cities, and when outdoor athletics typically cancel. Anyone over 65, anyone with cardiovascular disease, and anyone on diuretics is at materially elevated risk.
Extreme Danger: 125°F+
Heatstroke is highly likely with continued exposure. This is the band where infants, the elderly, and outdoor workers without active cooling are at acute medical risk within an hour or two. Heat index readings in this band are uncommon in the continental US but appear regularly in the Persian Gulf, the lower Mississippi delta during heat waves, and Pacific Northwest events like the 2021 dome.
All four bands assume shade with a light breeze. In direct sunlight the effective heat index can be 10–15°F higher — enough to push an Extreme Caution day into Danger or a Danger day into Extreme Danger. The category, not the precise number, is the part worth remembering.
How heat index differs from related "feels like" numbers
The heat index is one of several apparent-temperature measures, and they are not interchangeable. Knowing which one a number comes from prevents most arguments about which weather app is "right."
Wind chill is the cold-weather analogue, applicable when air is cold and wind is stripping heat from exposed skin. It uses temperature and wind speed (not humidity) and is calibrated for temperatures below about 50°F. Wind chill and heat index never overlap in the same forecast, which is why most weather apps switch from one to the other at a seasonal cut-off. The wind chill calculator handles that side of the year.
Apparent temperature (Australian Bureau of Meteorology) is a Steadman descendant that incorporates wind speed and net radiation along with temperature and humidity. It generally produces lower numbers than the NWS heat index in still air because the wind term is subtractive in hot-and-humid conditions.
AccuWeather RealFeel and similar proprietary indices add solar radiation, cloud cover, and sometimes precipitation rate to the mix. They are plausibly more "realistic" for outdoor exposure but they are closed formulas — you cannot reproduce them with public data, which is part of why the NWS does not use them.
Wet-bulb globe temperature (WBGT) is the standard for occupational and athletic heat stress. It combines air temperature, humidity, wind, and solar radiation into a measurement that approximates the cooling capacity of the environment. WBGT thresholds drive US military, OSHA, and most NCAA athletic training decisions because it correlates more tightly with exertional heat illness than the heat index does. The heat index is the right number for the general public; WBGT is the right number when sustained outdoor exertion is on the line.
How to use the result
Knowing the heat index is most of the value; what you do with it is the rest. A few practical applications:
Plan the day around the peak. The heat index typically peaks 2–3 hours after the air temperature peaks, around 4–6 pm in summer. If the calculator says the peak will be over 103°F, move outdoor work or exercise to dawn or after sunset. Trying to push through a Danger-band afternoon is the single most common path to a preventable hospitalisation.
Hydrate proactively, not reactively. Thirst lags dehydration by an hour or more in hot weather. If the heat index will exceed 90°F during the day, drink before you head out and every 20–30 minutes while outside. Plain water is fine for anything under two hours; longer exposures need electrolyte replacement because plain water alone can dilute serum sodium and cause hyponatraemia.
Size the air conditioning the room actually needs. The heat index determines the load on a cooling system because the system has to remove latent heat (water vapour) along with sensible heat (air temperature). A room that needs a 9,000-BTU unit at 95°F and 30% RH may need 12,000 BTU at the same temperature with 70% humidity. The BTU calculator covers room-sizing arithmetic; if you are converting between Celsius and Fahrenheit for either input, the temperature converter handles the unit work.
Adjust expectations for direct sun. The Rothfusz formula assumes shade. Standing or walking in full sun adds roughly 10–15°F to the felt temperature, so a 95°F heat index in shade is effectively 105–110°F in sun. Wide-brim hats and light long sleeves materially reduce the radiative load and are the cheapest meaningful intervention available.
Watch overnight lows. Heat waves kill primarily when overnight temperatures stay above ~80°F and the body cannot recover between days. Single-day peaks are uncomfortable; multi-day events with hot nights are dangerous. The NWS threshold for an Excessive Heat Warning typically requires both a daytime heat index over 105°F and a night-time low temperature over 80°F for two or more consecutive days.
Common mistakes that distort the answer
Confusing relative humidity with dew point. The Rothfusz formula takes relative humidity (a percentage). If the source you are reading reports dew point in degrees instead, you need to convert before using the calculator. A 70°F dew point at 90°F gives about 52% RH; the same 70°F dew point at 80°F gives about 71%. The heat-index answers differ by 10°F.
Using outdoor heat index to plan indoor air conditioning. The heat index is an outdoor metric — it tells you how the weather feels. If your AC is set to 76°F at 50% indoor humidity, the indoor heat index is 76°F regardless of what is happening outside. Indoor humidity is a function of the building envelope, the AC system, and human activity inside the space.
Reading the number to the decimal place. The Rothfusz regression has an absolute error of about ±1.3°F against Steadman's underlying table, and the input measurements typically come from a sensor a mile away. Reporting the answer as "121.4°F" implies precision the system does not have. The band is the message; the decimal is noise.
Assuming the formula works in cool weather. Below 80°F the regression is mathematically defined but physiologically meaningless — humidity does not change perceived temperature appreciably when the air is not hot. The calculator falls back to the Steadman estimator below the NWS threshold, which is why a 70°F input at 100% humidity returns ~70°F rather than something dramatic.
When the heat index is not the right tool
The heat index is built for healthy adults in the shade making everyday decisions about hot weather. Three populations need a different framework.
Outdoor workers and athletes. Sustained exertion in heat raises core body temperature far faster than passive exposure does. OSHA and most athletic governing bodies use WBGT thresholds for activity modification because heat exhaustion and exertional heatstroke can occur at heat-index values that are merely uncomfortable for sedentary observers. If you are coaching, training, or supervising outdoor labour, the heat index is a starting point; WBGT is the operational metric.
Infants and the elderly. Both groups have reduced ability to thermoregulate — infants because the sweating reflex is still developing, the elderly because sweat output declines with age and many common medications (beta blockers, diuretics, antihistamines) blunt the response further. The general-public bands understate risk for both groups by roughly one category. A Caution-band afternoon for a healthy adult can be an Extreme Caution event for a 78-year-old on hypertensives.
Anyone in direct sun for an extended period. The shade assumption is load-bearing. Construction workers, agricultural workers, marathon spectators and beach-goers are all routinely exposed to a felt temperature 10–15°F above what the nominal heat index predicts. Shade, hats, and light-coloured clothing close most of that gap.
How to read the calculator output
The heat index calculator returns five pieces of information for any valid input. The primary result is the heat index in your chosen unit system, rounded to one decimal place — the number to put on a forecast. The first breakdown row reports how much hotter the air feels than the thermometer reads, which is the most intuitive way to communicate humidity's contribution. The second row repeats the heat index in the other unit system so you can quote the same result in °C and °F without redoing the calculation. The remaining rows show the air temperature and humidity you entered (a sanity check against typos) and the NWS risk band with the recommended public-health response.
Below about 27°C (80°F) the calculator flags the answer as outside the practical domain of the formula and effectively returns the air temperature, because humidity does not change perceived temperature appreciably in cool weather. Above the NWS thresholds, the corner adjustments kick in automatically — there is nothing to tune. Type your numbers, read the band, plan accordingly.
Frequently asked questions
Is the heat index the same as the temperature in the sun?
No. The heat index is explicitly a shade measurement — the NWS Rothfusz formula assumes a subject in the shade with a light wind. Direct sunlight adds roughly 10–15°F to the felt temperature on top of whatever the heat index says, depending on the angle of the sun, cloud cover, and how much skin is exposed. A 95°F shade heat index can be 105–110°F in full sun, which is part of why the recommended advice for hot days always includes shade and a hat. Apparent-temperature indices like AccuWeather RealFeel try to incorporate solar load directly; the NWS heat index does not.
Why does my weather app show a different "feels like" number?
Three reasons usually explain the gap. First, most US apps use the NWS Rothfusz formula but feed it slightly different inputs — humidity from a nearby airport sensor instead of the exact point you are standing at, or an hourly average instead of the instantaneous value, which can shift the answer by a degree or two. Second, some international apps use the Australian Bureau of Meteorology's apparent temperature instead, which adds wind speed and tends to produce lower numbers in still air. Third, proprietary indices like AccuWeather RealFeel use closed formulas that incorporate solar radiation and cloud cover, so their answers can diverge by 10°F or more in full sun. The heat index calculator on this site applies the published NWS formula directly to the inputs you enter, so the answer matches the NWS reference value exactly.
At what heat index should I cancel outdoor exercise?
The NWS Danger band starts at 103°F, which is where heat exhaustion becomes likely and heatstroke is possible with sustained exertion — most coaches and outdoor employers treat this as the threshold for moving practice indoors or rescheduling. Below 90°F, healthy adults can usually exercise outdoors with normal hydration. The 90–103°F Extreme Caution band warrants scheduled water breaks, light clothing, and shifting hard workouts to early morning or evening. Athletic governing bodies that take this seriously use wet-bulb globe temperature (WBGT) rather than heat index because WBGT incorporates solar load and correlates more tightly with exertional heat illness, but the heat index is a defensible starting point for individual decisions.
How is heat index different from wet-bulb temperature?
Heat index is a perceptual model — it asks, given this temperature and humidity, what air temperature in dry conditions would feel the same? The answer comes from a multiple regression fit to Steadman's biometeorological model. Wet-bulb temperature is a thermodynamic measurement — it is the temperature an actual wet surface (a thermometer wrapped in a wet wick, or a sweating human) would equilibrate to as water evaporates from it. Wet-bulb is the harder physical limit: once wet-bulb exceeds 35°C (95°F) for several hours, a healthy human in the shade cannot shed metabolic heat fast enough to survive, regardless of how much they drink. Heat index has no such hard limit because it is a felt-temperature scale, not a survivability metric. Most operational heat-stress thresholds (OSHA, military, NCAA) use wet-bulb globe temperature (WBGT), which incorporates wet-bulb, air temperature, and solar load.
Why does the formula stop working below 80°F?
Because humidity does not change perceived temperature appreciably when the air is not hot. At 75°F your body's sweat-evaporation cooling system has comfortable headroom even at 90% humidity — there is little perceptual difference between 75°F dry and 75°F humid, beyond mild stickiness. The Rothfusz regression was fit to Steadman's data in the regime where heat stress actually matters, which starts around 80°F. Below that threshold the NWS uses a simpler Steadman estimator that effectively returns the air temperature, and the calculator on this site flags the result as outside the practical domain. If you want a cold-weather analogue that incorporates the wind effect, use the wind chill calculator instead.
Does the heat index account for elderly people or infants?
Not directly — the formula was calibrated for healthy adults. The general-public risk bands understate risk for infants, the elderly, and anyone with reduced thermoregulatory capacity (people on beta blockers, diuretics or antihistamines; people with cardiovascular disease; people who are pregnant). The rule of thumb most public-health departments use is to treat the bands as one category more severe for these groups: a Caution-band afternoon is effectively Extreme Caution, an Extreme Caution afternoon is effectively Danger. Heat-related mortality during US heat waves is concentrated overwhelmingly in adults over 65 and infants under 12 months, which is why cooling centres exist and why door-to-door wellness checks ramp up during sustained heat events.
What humidity makes 100°F dangerous versus tolerable?
At 100°F, the heat index hits the 103°F Danger threshold at roughly 40% relative humidity. Below that, the air is dry enough that sweat evaporation still works and the felt temperature stays in the high-90s to low-100s. Above 40% RH, every additional 10 points of humidity adds roughly 8–10°F to the heat index: 100°F at 50% feels like ~120°F, at 60% it feels like ~131°F (Extreme Danger), at 70% it feels like ~144°F. The relationship is non-linear because the cross terms in the Rothfusz regression dominate at high T and high RH. This is why low-humidity desert heat (Phoenix, Las Vegas) and high-humidity coastal heat (Houston, New Orleans) are different public-health problems even when the thermometer reads the same.
Can I use the heat index calculator with Celsius inputs?
Yes. The calculator accepts either °F or °C — the NWS formula is defined in Fahrenheit, but the calculator converts your input to Fahrenheit, computes the heat index, then converts the answer back to your chosen unit. The math is identical; the only difference is the display. Useful reference points in metric: the 80°F threshold below which the formula is bypassed is 27°C. The Caution-band ceiling at 90°F is 32°C. The Extreme Caution ceiling at 103°F is 39°C. The Danger ceiling at 125°F is 52°C. Most of the world outside the US thinks in Celsius, and any temperature converter handles the unit work in both directions.
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