Heat Index Calculator

Heat Index and Actual Heat Safety

The physiology the number encodes

Humans dump roughly 80% of exercise heat through evaporating sweat, and evaporation rate depends on the air's remaining moisture capacity. The Rothfusz regression — a fit to Robert Steadman's 1979 'apparent temperature' physiological model — quantifies that: 92 °F at 40% humidity feels like 94, but at 75% humidity it's 112, because damp air has largely shut the evaporative valve. The model assumes a healthy adult in shade with a light breeze; sun, dense urban surfaces, and poor fitness all push effective stress higher than the published number.

The categories and who they're really for

NWS bands — Caution (80–90), Extreme Caution (90–103), Danger (103–124), Extreme Danger (125+) — calibrate to prolonged exposure or exertion. The vulnerable populations move through them earlier: adults over 65, young children, outdoor workers, and anyone on medications affecting thermoregulation (diuretics, beta-blockers, antihistamines among them). Heat is the deadliest weather category in the US — exceeding hurricanes, floods, and tornadoes combined in typical years — and most fatalities occur indoors without AC during multi-day events, when overnight lows never let bodies recover.

Working and training in it

OSHA's heat guidance keys to the index: above 91 °F triggers hydration and rest-break protocols; above 103, strenuous outdoor work needs rescheduling or aggressive work-rest cycles. Acclimatization is the underused defense — bodies adapt substantially over 10–14 days of gradual exposure, which is why the year's first heat wave and unacclimated new workers dominate casualty statistics. The warning signs that demand immediate action: confusion, cessation of sweating, and nausea (heat stroke — call 911, cool aggressively) versus heavy sweating with weakness and cramps (heat exhaustion — shade, fluids, rest).

The heat index grid: temperature × humidity

The interaction is the story: dry 100 °F heat reads close to its thermometer value, while humid 90 °F heat outruns it. Cells at 103+ sit in the NWS Danger category; the ✱ marks combinations common in Gulf-state summers.

The dashes aren't missing data — those combinations essentially don't occur naturally, because air that hot can rarely hold that much moisture. The grid's practical lesson is the left-right gradient: at 95 °F, the difference between a dry-heat afternoon (40% RH, feels like 98) and a humid one (70%, feels like 118) is the difference between uncomfortable and hazardous, on identical thermometer readings.

Recognizing heat illness before it recognizes you

The progression has a recognizable grammar. Heat cramps — painful muscle spasms during or after exertion — are the early warning, usually salt depletion talking. Heat exhaustion presents as heavy sweating with weakness, dizziness, headache, nausea, and cool clammy skin: the body still fighting, and losing ground. The response is immediate — shade or AC, fluids with electrolytes, loosened clothing, cool water on skin — and it works within 30–60 minutes; symptoms persisting past an hour warrant medical care. Heat stroke is the phase change: core temperature above 103 °F, hot skin that may have stopped sweating, confusion, slurred speech, possible loss of consciousness. It is a 911 call, full stop, and the minutes before EMS arrival belong to aggressive cooling — ice packs at neck, armpits, and groin, or immersion if available — because neurological damage tracks time spent above the threshold. The counterintuitive marker worth memorizing: in serious heat illness, less sweating is worse news, not better.

What people get wrong

• Reading the index as a shade-and-sun number. It's shade-only by definition; direct sun adds up to 15 °F. Ballfields, beaches, and rooftops run a full category worse than the forecast graphic implies.
• Hydrating on thirst. Thirst lags dehydration by 1–2% of body weight in heat — enough to degrade performance and thermoregulation. Scheduled drinking (about 8 oz per 15–20 min of work) beats reactive drinking, and multi-hour exertion needs electrolytes, not just water.
• Assuming fans always help. Above roughly 95 °F air temperature with low sweat evaporation, a fan blows heat into the body faster than it evaporates sweat — the EPA specifically cautions against fans as a primary cooling strategy in extreme heat without AC.
• Skipping acclimatization. The first heat wave of the season and the first week of a new outdoor job dominate heat-casualty statistics. Bodies adapt substantially over 10–14 days of graduated exposure — starting at full intensity on day one is how experienced workers and athletes get hurt.

Cooling a home through a heat wave, triaged

Because most heat fatalities happen indoors, the home playbook matters as much as the outdoor one. With AC, the guidance is simply to use it — heat-wave mortality studies consistently find working air conditioning is the single strongest protective factor, and 78 °F of cooling protects nearly as well as 68 while costing far less. Without AC, the levers rank by physics: block solar gain first (closed blinds on sun-facing windows cut indoor heating dramatically; exterior shading beats interior), ventilate on the right schedule (windows open during the cool overnight hours, sealed shut once outdoor temperature passes indoor), move sleeping to the lowest floor since heat stratifies upward, and use evaporative tricks — damp sheet, cool shower before bed — that work with the body's own system. Fans follow the caveat above: below ~95 °F they help; above it, without sweat to evaporate, they can hurt. And the interventions with the best evidence are social, not mechanical: checking on isolated older neighbors during multi-day events, and spending the peak afternoon hours in any air-conditioned public space — library, mall, designated cooling center — which resets the body's cumulative heat load even if home stays warm.

Athletes, workers, and the WBGT ladder

Organized athletics increasingly runs on wet-bulb globe temperature rather than heat index, and the distinction matters if you supervise anyone outdoors. WBGT weighs direct sun, wind, and radiant ground heat that the shade-based index ignores; the widely adopted athletic bands halt conditioning around WBGT 90–92 °F, mandate work-rest ratios and unlimited water below that, and shift practices to mornings during acclimatization weeks — the first two weeks of preseason, when the large majority of exertional heat-stroke deaths in US sports occur. Occupationally, OSHA's national emphasis program keys inspections to heat-index triggers (91 °F for hydration-and-shade protocols, 103 for high-hazard rescheduling), and several states now enforce their own outdoor-work heat standards with mandated breaks. For anyone running a jobsite or a practice, the two cheap interventions with outsized records are a buddy system — early confusion is invisible to its owner and obvious to a partner — and genuine acclimatization schedules for anyone new, returning, or facing the season's first wave.

Where the formula comes from

The chain runs Steadman → Rothfusz → your forecast. R.G. Steadman's 1979 papers built a full physiological model of "apparent temperature" — a clothed, walking human of standard dimensions, balancing metabolic heat against evaporative capacity. Because the model is a table, not an equation, Lance Rothfusz of the NWS fitted a nine-term regression to it in 1990; that regression, with the low-humidity and high-humidity adjustment terms the NWS specifies, is exactly what this calculator computes, accurate to ±1.3 °F against Steadman's table. Its domain starts at 80 °F, which is why lower inputs are declined. The danger categories are NWS operational thresholds; the observation that heat kills more Americans than any other weather category comes from NOAA's weather-fatality statistics, and the overnight-low mortality connection is a consistent finding in the urban heat-island epidemiology literature.

Frequently asked questions

What's the difference between heat index and wet-bulb temperature?

Heat index estimates perceived heat for a person in shade; wet-bulb globe temperature (WBGT) adds direct sun, wind, and radiant load — the standard for athletics and military training. WBGT above 90 °F halts most organized sports; heat index has no such direct measurement basis but is what forecasts publish.

How much should I drink in high heat index conditions?

Working or exercising: about a cup (8 oz) every 15–20 minutes, not chugged hourly — and past 2+ hours, add electrolytes, since water alone can dilute sodium dangerously (hyponatremia). Thirst lags need in the heat; schedule drinking rather than waiting for it.

Why do heat waves kill more at night?

Bodies repair heat stress during cool overnight hours. When lows stay above ~80 °F — increasingly common in urban heat islands — that recovery never happens, and cumulative strain compounds daily. Overnight lows are the mortality signal epidemiologists watch, more than daytime highs.

What's a dangerous heat index for pets?

Dogs cool almost entirely by panting — far less efficient than sweating — so their danger threshold sits well below ours, especially for flat-faced breeds. Above ~90 heat index, limit exertion; asphalt adds a separate burn hazard (7-second back-of-hand test). Cars are the extreme case: cabin temperatures climb 20 °F in 10 minutes even with windows cracked.

Why does 90 °F feel worse on the coast than 100 °F in the desert?

Evaporation. At 20% desert humidity, sweat flashes off and carries heat with it, so 100 °F reads near 96 on the index. At 75% coastal humidity, 90 °F reads 109 because sweat drips instead of evaporating — the cooling system runs but accomplishes little. Your thermometer measures the air; the index measures what your physiology can do about it.