The full NWS Rothfusz regression, including the low- and high-humidity adjustments that most implementations leave out.
Your body sheds excess heat mainly by sweating, and sweat only cools you when it evaporates. Evaporation carries away a great deal of energy — that is what makes it effective. But the air can only absorb so much water vapour, and the more of that capacity is already used, the slower evaporation goes.
At high humidity sweat runs off you instead of evaporating. You lose the fluid without getting the cooling, which is the worst of both. This is why 35 °C at 20% humidity is unpleasant while 35 °C at 80% humidity is genuinely dangerous.
The NWS heat index is the Rothfusz regression, a nine-term fit to Steadman's 1979 tables:
HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R²
with T in Fahrenheit and R as relative humidity in percent. Two adjustments apply at the edges, and they are the part most calculators skip. When humidity is under 13% and temperature is between 80 and 112 °F, a correction is subtracted. When humidity is above 85% and temperature is between 80 and 87 °F, a smaller correction is added.
There is also a threshold rule. The regression is not appropriate below about 80 °F, so the NWS procedure computes a simpler expression first, averages it with the air temperature, and only switches to the full regression if that result reaches 80. This calculator follows that procedure exactly.
90 °F at 70% relative humidity gives a heat index of 105.9 °F — nearly 16 degrees above the air temperature, and squarely in the NWS "Danger" band where heat exhaustion is likely with prolonged exposure.
The same 90 °F at 30% humidity comes out close to the air temperature. Identical thermometer reading, completely different physiological situation.
| Heat index | Category | What it means |
|---|---|---|
| 80–90 °F (27–32 °C) | Caution | Fatigue possible with prolonged exposure or activity |
| 90–103 °F (32–39 °C) | Extreme caution | Heat cramps and heat exhaustion possible |
| 103–125 °F (39–52 °C) | Danger | Heat exhaustion likely; heat stroke possible |
| Above 125 °F (52 °C) | Extreme danger | Heat stroke highly likely |
The heat index is defined for shady, light-wind conditions. Full sunshine can add as much as 15 °F (8 °C) to the effective value, which means a 105 °F heat index in the shade is closer to 120 °F standing in the sun. Nothing in the equation accounts for this, and it is the single largest source of underestimation in practice.
Heat exhaustion: heavy sweating, cold and clammy skin, weak fast pulse, nausea, dizziness, muscle cramps. Move to a cool place, loosen clothing, sip water, apply cool wet cloths. It usually resolves with rest and fluids.
Heat stroke is a medical emergency. Body temperature above 40 °C, hot skin that may be dry because sweating has stopped, confusion, slurred speech, possible loss of consciousness. Call emergency services immediately and cool the person aggressively while waiting. Do not give fluids to someone who is confused or unconscious.
The transition from exhaustion to stroke can be rapid. Skin that stops sweating in extreme heat is the warning sign that matters most.
Older adults sweat less and feel thirst less reliably. Young children have a higher surface-area-to-mass ratio and depend on adults to manage their exposure. People taking diuretics, beta blockers, antihistamines or some antipsychotics may have impaired thermoregulation. Outdoor workers and athletes are at risk simply through exposure and exertion, and acclimatisation — which takes one to two weeks — matters a great deal.
Temperature is what a thermometer reads. Heat index combines it with humidity to describe how hot it actually feels to a human body, because humidity determines how well sweat can evaporate — and sweat evaporating is your main cooling mechanism. At 90 °F, the heat index is about 90 at 30% humidity and about 106 at 70%. Same reading, very different physiological stress.
Because your body sheds heat primarily by evaporating sweat, and evaporation slows as the air approaches saturation. At high humidity sweat drips off rather than evaporating, so you lose fluid without getting the cooling benefit. That is the worst combination — dehydration without heat loss — and it is why humid heat causes far more heat illness than dry heat at the same temperature.
No, and this matters. The heat index is defined for shady conditions with light wind. Direct sun can add as much as 15 °F (8 °C) to the effective value, so a heat index of 105 in the shade is closer to 120 in full sun. Nothing in the equation captures this, which makes it the single largest source of underestimation when people use the figure to judge outdoor conditions.
Heat exhaustion involves heavy sweating, cold clammy skin, a weak rapid pulse, nausea and dizziness — it usually responds to rest, cooling and fluids. Heat stroke is a medical emergency: body temperature above 40 °C, skin that is hot and often dry because sweating has stopped, confusion, slurred speech, possible unconsciousness. Skin that stops sweating in extreme heat is the critical warning sign. Call emergency services and cool the person aggressively while waiting.
Because many implement only the main nine-term regression and skip the two edge adjustments the NWS specifies — a subtraction below 13% humidity and an addition above 85% humidity in a narrow temperature band. Some also apply the regression below 80 °F, where the NWS procedure calls for a simpler formula averaged with the air temperature instead. This calculator follows the full published procedure including both adjustments.
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