Heat Index Calculator
The heat index (also called apparent temperature or "feels-like" temperature) combines air temperature with relative humidity into a single value that reflects what the human body actually experiences. Humid air slows sweat evaporation and cuts heat loss — 30 °C with 80 % humidity can feel like 38 °C or more. This tool uses the US National Weather Service (NWS / NOAA) Rothfusz 1990 regression and the standard five-band heat-stress classification.
Enter a valid temperature and a relative humidity between 0 and 100 %.
Feels-like temperature (heat index)
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Within the NOAA Rothfusz formula range (apparent temperature ≥ 27 °C / 80 °F).
Heat-stress risk band (NOAA)
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Formula (NOAA Rothfusz 1990 regression)
HI = −42.379 + 2.04901523 T + 10.14333127 R − 0.22475541 T·R − 0.00683783 T² − 0.05481717 R² + 0.00122874 T²·R + 0.00085282 T·R² − 0.00000199 T²·R²
T is in °F and R is relative humidity (%). Very dry (R < 13 %) or very humid (R > 85 %) air triggers the NWS correction terms. Hong Kong summers — high humidity combined with 32 °C air — easily land in the "extreme caution" band; watch for signs of heat illness.
Formula
HI = −42.379 + 2.04901523 T + 10.14333127 R − 0.22475541 T·R − 0.00683783 T² − 0.05481717 R² + 0.00122874 T²·R + 0.00085282 T·R² − 0.00000199 T²·R² (T in °F, R in %)
- · Source formula: Rothfusz, L.P. (1990) NOAA NWS Technical Attachment SR 90-23, a regression of Steadman (1979)’s apparent-temperature table.
- · Adjustments: subtract ((13 − R)/4) × √((17 − |T − 95|)/17) when R < 13 % and 80 °F ≤ T ≤ 112 °F; add ((R − 85)/10) × ((87 − T)/5) when R > 85 % and 80 °F ≤ T ≤ 87 °F.
- · Below ~80 °F (27 °C) apparent temperature the regression overshoots; the NWS uses a linear blend HI = 0.5·(T + 61 + 1.2(T − 68) + 0.094 R) instead.
- · NWS five-band classification: < 80 °F safe, 80–90 °F caution, 90–103 °F extreme caution, 103–125 °F danger, ≥ 125 °F extreme danger.
- · The Hong Kong Observatory publishes its own "HK Heat Index" (HKHI) which uses black-globe temperature, wind and solar radiation — this universal NOAA formula needs only temperature and humidity, so values are similar but not identical to HKHI.
- · The formula assumes shade and a light breeze; direct sun can raise the actual perceived temperature by another 6–8 °F (≈ 3–4 °C).
Frequently asked
How does the heat index differ from air temperature?
Air temperature measures the air itself; the heat index combines it with humidity to reflect the temperature the body actually experiences. The body cools itself by sweating, and humid air slows sweat evaporation — the higher the humidity, the less effective cooling becomes, so it feels hotter. For example, 32 °C at 50 % humidity feels close to 32 °C, but at 90 % humidity it climbs above 42 °C — well into the NOAA "danger" band. Hong Kong summers average around 80 % humidity, which is why low-30 °C days already feel oppressive.
When should I be especially careful about heat stroke?
Once the heat index reaches the NOAA "extreme caution" band (32–39 °C) or higher, shorten outdoor activity, take 10–15 minutes of shade rest every hour, and rehydrate often (electrolyte drinks help). Elderly people, those with chronic illness, children and outdoor workers are most vulnerable. Warning signs of heat illness include dizziness, nausea, pallor, racing pulse and (importantly) sweating that stops — cool the person down and seek medical help immediately. When the heat index hits "danger" (39–51 °C) or "extreme danger" (≥ 51 °C), the Hong Kong Observatory and Labour Department typically issue Very Hot Weather Warnings or Heat Stress at Work Warnings — follow the official advice and stop strenuous outdoor work.
Why does the tool say "outside the formula range" below ~27 °C?
The Rothfusz regression was fitted to the hot end of Steadman’s apparent-temperature table (≥ 80 °F / ~27 °C). At lower temperatures the humidity effect on heat dissipation is small and the regression overshoots, so the US National Weather Service swaps in a simpler linear blend: HI = 0.5 × (T + 61 + 1.2·(T − 68) + 0.094 R). This tool falls back to that formula and marks the result as "outside the range" — it still gives a sensible number for cool weather, but there is no official heat-stress band attached to it.
Is the Hong Kong Observatory’s HKHI the same as this heat index?
Not exactly the same. The HKO’s HKHI is a WBGT-style index that combines black-globe temperature, wind speed, solar radiation, air temperature and humidity — it is measured at outdoor sites and cannot be reproduced from a single closed-form formula. The NOAA Rothfusz formula needs only air temperature and humidity, which is why it is by far the most common consumer "feels-like" calculation (it is what the US NWS, Weather.com and Wikipedia use). For occupational risk assessment in Hong Kong, defer to the Labour Department’s Heat Stress at Work Warning and the HKO’s published HKHI values.
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