What is a heat index?

The heat index is the apparent or feels-like temperature your body experiences when air temperature and humidity combine. As humidity rises, sweat evaporates more slowly, so your body cannot shed heat as efficiently. The US National Weather Service publishes heat index in °F using the Rothfusz regression formula, and uses it to issue Heat Advisories (heat index ≥ 100 °F) and Excessive Heat Warnings (≥ 105 °F).

How do you calculate heat index?

Heat index is calculated from two inputs: air temperature and relative humidity. The NWS uses the Rothfusz regression — a 9-term polynomial in T and R derived from the 1979 Steadman tables. For temperatures above 80 °F it returns the apparent temperature in degrees Fahrenheit. Our heat index formula calculator runs the full equation plus the low-humidity and high-humidity correction terms automatically, so results agree with the official NWS chart at weather.gov.

Is heat index the same as feels like temperature?

Yes — in the United States, 'heat index' and 'feels like temperature' refer to the same quantity (when conditions are hot). Other terms refer to different methods: 'humidex' is the Canadian equivalent with a slightly different formula; 'wet bulb globe temperature (WBGT)' adds sun and wind effects; 'apparent temperature' is the technical name. All measure how hot it feels, but each uses different inputs and formulas, so the numbers can disagree by several degrees.

What heat index is dangerous?

NWS uses four caution bands. Caution: 80-90 °F — fatigue with prolonged exposure. Extreme Caution: 90-103 °F — heat cramps or heat exhaustion possible. Danger: 103-125 °F — heat exhaustion likely, heatstroke possible. Extreme Danger: 125 °F and above — heatstroke imminent. The NWS issues a Heat Advisory at sustained heat index of 100 °F+ and an Excessive Heat Warning at 105 °F+ for two or more consecutive days. Infants, the elderly, and people with chronic illnesses reach these thresholds at lower temperatures.

How does humidity affect heat index?

Humidity controls how fast sweat can evaporate from your skin, and sweat evaporation is your body's primary cooling mechanism. At 100 °F with 20% humidity the heat index is about 102 °F. At the same 100 °F with 80% humidity the heat index climbs to 158 °F — a 56-degree jump from humidity alone. That's why summer days in the humid southeastern US feel hotter than dry desert days at the same air temperature.

What is a normal heat index?

Below 80 °F there is effectively no heat-index effect — your body cools efficiently and the heat index equals the air temperature. Between 80-90 °F (the Caution band) heat-related illness is rare for healthy adults. Most US summer afternoons in coastal and southern states sit in the 85-100 °F heat index range. A heat index above 105 °F is uncommon and triggers an Excessive Heat Warning. Above 130 °F is exceptionally rare in inhabited areas worldwide.

How accurate is the Rothfusz heat index formula?

The Rothfusz regression matches the underlying 1979 Steadman tables to within ±1.3 °F across the typical heat-index range. The full NWS implementation adds two correction terms — for very low humidity (RH below 13%) and very high humidity (RH above 85%) — to bring the result back in line with the published chart. Our heat index Rothfusz regression calculator applies both adjustments, so values match weather.gov to the nearest degree.

Does heat index work in Celsius?

Yes, but the Rothfusz formula is defined in degrees Fahrenheit. Our heat index calculator Celsius mode converts your temperature input to °F, runs the regression, then converts the result back to °C. The answer is identical to international chart values (e.g., Environment Canada's humidex chart). Quick conversion: a heat index of 105 °F is approximately 41 °C, and 130 °F is roughly 54 °C.

What is the difference between heat index and humidex?

Humidex is the Canadian counterpart to the US heat index, used by Environment and Climate Change Canada. The math differs: humidex is based on dewpoint and vapor pressure (Masterton-Richardson formula), while heat index uses relative humidity (Rothfusz). For typical summer conditions the two values agree within 2-3 °C, but at extreme humidity humidex usually reads a few degrees higher. Canada uses humidex; the US, UK, and Australia use heat-index style formulas.

What is the difference between heat index and wet bulb globe temperature (WBGT)?

Heat index uses only air temperature and humidity, and assumes shade with light wind. Wet Bulb Globe Temperature (WBGT) uses four inputs — air temperature, humidity, wind speed, and solar radiation — and is the standard for athletic, military, and industrial heat-safety decisions. WBGT is usually 5-15 °F lower than heat index in direct sun and is preferred when sun and wind matter (sports practices, outdoor work). Heat index is sufficient for general public heat-risk communication.

What heat index triggers an NWS Heat Advisory?

The US National Weather Service issues a Heat Advisory when heat index is forecast to reach 100 °F (38 °C) or higher for at least two hours during a single day. An Excessive Heat Warning is issued when heat index will reach 105 °F (41 °C) or higher for two or more consecutive days, or 110 °F+ for any duration. Local NWS offices may adjust thresholds for climate — northern offices issue at lower levels, southern offices at higher levels.

How do I use heat index for outdoor work safety?

OSHA recommends work-rest ratios based on heat index. Below 91 °F: normal work. 91-103 °F: rest in shade 15 min per hour, hydrate every 15-20 min. 103-115 °F: 30 min rest per hour, mandatory buddy checks every 30 min. Above 115 °F: minimize outdoor exertion. For acclimatized workers add 10 °F headroom; for unacclimated workers (first 5 days) reduce thresholds by 10 °F. Provide cool drinking water, shaded rest areas, and training in heat-illness symptoms.

Which groups are most at risk at moderate heat index?

Infants and children under 4 — smaller body mass loses heat faster, dehydration accelerates. Adults over 65 — reduced thirst sensation, blunted sweat response. People with chronic conditions: cardiovascular disease, diabetes, obesity, mental illness. Medications matter: diuretics, beta-blockers, antihistamines, and antipsychotics impair sweating or hydration. Athletes during the first week of summer training are at high risk until acclimatized. For these groups, treat each NWS caution band as one level more severe than the chart label.

Does direct sun change the heat index?

Yes. The published heat index assumes shade and light wind. Direct sunlight adds up to 15 °F (8 °C) to your apparent temperature, depending on cloud cover, time of day, and surface reflectivity. So a 95 °F heat index in shade equals about 108-110 °F in direct sun — a jump from Extreme Caution into Danger. Always interpret heat index as the shaded reading and add headroom for sun exposure. For sports and military training use WBGT, which incorporates solar radiation directly.

How long does heat acclimatization take?

Healthy adults acclimatize to hot weather over 7-14 days of progressive exposure — sweating starts earlier, sweat volume increases, and sweat becomes less salty. Unacclimatized workers are at 3-5× higher heat-illness risk during the first week of a heat wave or a new job. Strategy: increase outdoor exposure by 20% per day, starting at cooler hours, and reach full duration by day 7. Reacclimatization is needed after 4+ days off in cool weather (e.g., vacation, illness).

Can I use heat index indoors?

Yes. Indoor heat index matters during heat waves, especially if air conditioning fails or in non-AC dwellings. An indoor reading of 95 °F + 70% relative humidity puts the heat index in the Danger band (~120 °F) within hours, which is fatal for vulnerable groups. Public health agencies (CDC, NHS, Health Canada) operate cooling centers when forecast outdoor heat index exceeds 95-100 °F for several consecutive days, because urban indoor temperatures track outdoor heat with a lag.

Is heat index different in the UK, Canada, and Australia?

The underlying physics is universal but each country uses slightly different metrics. UK: Met Office uses heat index style 'feels like' values in summer advisories; Heat-Health Alert system has Yellow/Amber/Red levels tied to temperature thresholds. Canada: humidex (Masterton-Richardson formula) is the standard, with advisories from Environment and Climate Change Canada at humidex 40+. Australia: BOM uses apparent temperature (Steadman formula) and issues heatwave warnings at varying state-level thresholds. Our heat index calculator uses the US NWS Rothfusz formula but the conversions and risk bands transfer closely.

What is the highest heat index ever recorded?

The highest reliably recorded heat index is around 178 °F (81 °C) at Dhahran, Saudi Arabia in July 2003, with air temperature of 108 °F and dew point of 95 °F. The highest in the United States is roughly 172 °F at Appleton, Wisconsin, July 1995. Persian Gulf coastal cities (Bandar Mahshahr, Iran; Jacobabad, Pakistan) routinely hit heat indices in the 150-165 °F range in July and August. Climate projections suggest wet-bulb temperatures exceeding human survival limits will become more frequent in tropical regions by 2050.

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Heat Index Calculator

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thermostat Conditions

Temperature Unit

Air Temperature (°F)

Use a shade reading; direct sun adds up to 15 °F.

Relative Humidity (%)

From a weather app, hygrometer, or local forecast.

device_thermostat Heat Index (Feels Like)

Apparent Temperature

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°F

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NWS Caution Band

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Safety Advice

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NWS Advisory Status

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lightbulb Tips

•Below 80 °F: heat index = air temp (no humidity effect)
•Direct sun adds up to 15 °F over the shaded heat index
•NWS Heat Advisory at 100 °F+; Excessive Heat Warning at 105 °F+
•Acclimatization takes 7-14 days — first week of heat is highest risk
•For sports in direct sun use WBGT instead of heat index