What is the heat index?

The heat index, also called the 'feels like' temperature or apparent temperature, is a measure of how hot it feels when relative humidity is combined with the actual air temperature. High humidity reduces the body's ability to cool itself through sweat evaporation, making the air feel hotter than the actual temperature.

How is the heat index calculated?

The NWS uses the Rothfusz regression equation: HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R², where T is temperature in °F and R is relative humidity in percent. Adjustment factors apply at low humidity or high temperature extremes.

At what heat index is it dangerous?

The NWS defines four heat danger categories: Caution (80–90°F / 27–32°C) — fatigue possible with prolonged activity; Extreme Caution (91–103°F / 33–39°C) — heat cramps and heat exhaustion possible; Danger (103–124°F / 40–51°C) — heat cramps and heat exhaustion likely, heat stroke possible; Extreme Danger (125°F+ / 52°C+) — heat stroke highly likely.

What is the difference between heat index and temperature?

Temperature is the actual air temperature measured by a thermometer. Heat index accounts for how humidity affects the body's ability to cool itself. On a 95°F day with 60% humidity, the heat index can reach 114°F — nearly 20 degrees hotter than the thermometer reads. The heat index is always equal to or higher than the actual temperature.

Why does humidity make it feel hotter?

The human body cools itself primarily through sweat evaporation. When humidity is high, the air is already saturated with water vapor, so sweat evaporates more slowly — reducing the cooling effect. In low humidity, sweat evaporates quickly and the body cools efficiently. In high humidity, this cooling mechanism fails, causing core body temperature to rise faster.

What is dew point and how does it relate to heat index?

Dew point is the temperature at which air becomes fully saturated and water vapor condenses into dew. Unlike relative humidity (which changes with temperature), dew point is an absolute measure of moisture in the air. A dew point above 60°F (16°C) feels humid; above 70°F (21°C) feels oppressive; above 75°F (24°C) is dangerous. Dew point can be used directly to calculate heat index or converted to relative humidity first.

What are the symptoms of heat exhaustion?

Heat exhaustion symptoms include heavy sweating, cool pale clammy skin, fast and weak pulse, nausea or vomiting, muscle cramps, tiredness and weakness, dizziness, headache, and fainting. Move to a cool place, loosen clothing, apply cool wet cloths, and sip water. If symptoms worsen or last longer than an hour, seek emergency medical help.

What is the difference between heat exhaustion and heat stroke?

Heat exhaustion is a serious but less severe condition where the body overheats but still sweats. Heat stroke is a medical emergency where the body's temperature regulation fails entirely — body temperature exceeds 104°F (40°C), and sweating may stop. Heat stroke symptoms include hot dry skin, confusion, loss of consciousness, and rapid strong pulse. Call 911 immediately for heat stroke.

Does the heat index apply in the shade?

The NWS heat index is calculated for shady, light wind conditions. Direct sunlight can increase the heat index by up to 15°F (8°C) above the calculated value. So on a day with a heat index of 100°F in the shade, direct sun exposure may feel like 115°F. Always factor in sun exposure when assessing heat risk.

Who is most at risk from high heat index?

High-risk groups include older adults (65+), infants and young children, people with chronic conditions (heart disease, kidney disease, obesity), people taking certain medications (diuretics, antihistamines, blood pressure medications), outdoor workers, and athletes. People who are not acclimatized to heat are also at significantly higher risk during the first 2 weeks of a heat wave.

NWS Formula · Humidity · Heat Safety

Heat Index
Calculator

Calculate the real feels-like temperature using the official NWS Rothfusz formula. Enter temperature with relative humidity or dew point — get heat index and danger level instantly.

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Heat Index Calculator

NWS Rothfusz equation

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Air Temperature valid ≥ 80°F

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95°F

Relative Humidity 0–100%

60%

Air Temperature valid ≥ 80°F

°F

95°F

Dew Point must be ≤ temp

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70°F

Actual Temp

95°

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Feels Like

—°

Heat Index

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Safety Reference

Heat Index Risk Levels

Official NWS heat danger categories and recommended precautions.

Heat Index (°F)	Heat Index (°C)	Category	Health Effects	Action
80–90°F	27–32°C	Caution	Fatigue possible with prolonged activity	Stay hydrated; take breaks in shade
91–103°F	33–39°C	Extreme Caution	Heat cramps & heat exhaustion possible	Limit strenuous activity; monitor closely
103–124°F	40–51°C	Danger	Heat exhaustion likely; heat stroke possible	Avoid strenuous outdoor activity
125°F+	52°C+	Extreme Danger	Heat stroke highly likely	Stay indoors in AC; call 911 if symptoms appear

⚠️ Direct sunlight can add up to 15°F (8°C) above the calculated heat index. Values shown are for shade with light wind.

Reference Chart

Heat Index Chart (°F)

Temperature (columns) × Relative Humidity (rows) = Heat Index °F. Color = risk category.

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Direct Sun Adds Up to 15°F More

The NWS heat index is calculated for shady, light-wind conditions. In direct sunlight, add up to 15°F (8°C) to the heat index value. A 100°F heat index in the shade could feel like 115°F in the sun — potentially life-threatening.

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Heat Exhaustion vs Heat Stroke

Heat exhaustion: Heavy sweating, cool pale clammy skin, fast weak pulse, nausea, dizziness, muscle cramps. Move to cool area, apply cold wet cloths, sip water.

Heat stroke (emergency): Body temp ≥104°F (40°C), hot dry skin OR profuse sweating, confusion, rapid strong pulse, loss of consciousness. Call 911 immediately — do not give fluids. Cool the person by any means available.

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Staying Safe in High Heat Index

Hydration: Drink water before you feel thirsty — 1 cup (8 oz) every 20 minutes during activity in heat. Avoid caffeine and alcohol which accelerate dehydration.

Acclimatization: It takes 10–14 days for the body to adjust to working in heat. Limit strenuous outdoor activity during heat waves, especially in the first 2 weeks. Schedule activity for early morning or after sunset when heat index is lowest.

How the Heat Index Is Calculated

The heat index was developed by Robert G. Steadman in 1979 and later adapted by the National Weather Service using the Rothfusz regression equation. It quantifies how hot it feels to the human body when relative humidity is factored in alongside the actual air temperature — because humidity directly impairs the body's primary cooling mechanism: sweat evaporation.

The NWS Rothfusz Equation (°F)

HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R² Where: T = Temperature (°F), R = Relative Humidity (%) Valid when: T ≥ 80°F AND RH ≥ 40% Adjustment (low RH, T 80–112°F, RH < 13%): − [(13 − R)/4] × √[(17 − |T − 95|)/17] Adjustment (high RH, T 80–87°F, RH > 85%): + [(R − 85)/10] × [(87 − T)/5]

The Metric Version (°C)

HI = −8.78469475556 + 1.61139411T + 2.33854883889R − 0.14611605TR − 0.012308094T² − 0.0164248277778R² + 0.002211732T²R + 0.00072546TR² − 0.000003582T²R² Where: T = Temperature (°C), R = Relative Humidity (%)

When the Formula Applies

The Rothfusz equation is valid when air temperature is at or above 80°F (27°C) and relative humidity is at or above 40%. At lower temperatures or very low humidity, the simplified Steadman formula or a straight temperature reading is more appropriate. The formula also assumes shady conditions with light wind — direct sun or strong wind will alter the perceived temperature.

Converting Dew Point to Relative Humidity

If you have the dew point temperature instead of relative humidity, our calculator converts it automatically using the Magnus formula:

RH = 100 × exp((17.625 × Td) / (243.04 + Td)) ÷ exp((17.625 × T) / (243.04 + T)) Where: T = air temp (°C), Td = dew point (°C)

Why Humidity Makes Heat More Dangerous

The human body regulates core temperature primarily through sweat evaporation. When sweat evaporates from skin, it carries heat away — a process called evaporative cooling. This mechanism is highly efficient in dry air but fails progressively as humidity rises.

The Dew Point Threshold

Dew point is a more reliable discomfort indicator than relative humidity because it measures absolute moisture in the air rather than how saturated the air is relative to temperature. A dew point below 55°F (13°C) feels comfortable to most people. Between 60–65°F (16–18°C) it feels humid. Above 70°F (21°C) it feels oppressive, and above 75°F (24°C) it becomes dangerous — the body's cooling system struggles to keep up even at rest.

Heat Acclimatization

The human body can adapt to working in heat through a process called heat acclimatization, which takes 10–14 days of gradual exposure. Acclimatized individuals sweat more efficiently, earlier, and in greater volume — lowering core temperature more effectively. Newcomers to a heat wave are at significantly higher risk than acclimatized individuals even at the same heat index level.

The Urban Heat Island Effect

Urban areas can experience heat index values significantly higher than surrounding rural areas due to the urban heat island effect — dark asphalt and concrete absorb and re-radiate heat, reducing nighttime cooling and elevating daytime temperatures by 2–10°F (1–6°C) above rural surroundings. This compounds the heat index risk for urban populations during heat waves.

Frequently Asked Questions

The heat index is the "feels like" temperature that combines air temperature with relative humidity to show how hot it actually feels to the human body. High humidity reduces the efficiency of sweat evaporation — the body's primary cooling mechanism — making it feel significantly hotter than the thermometer reads.

The NWS uses the Rothfusz regression equation — a multi-variable polynomial formula using air temperature (°F) and relative humidity (%). It's valid when temperature is ≥80°F and humidity ≥40%. Adjustment factors apply at very low humidity or for temperatures in the 80–87°F range with very high humidity. The formula was validated against human physiological responses to heat and humidity.

The NWS defines four levels: Caution (80–90°F / 27–32°C) — fatigue possible; Extreme Caution (91–103°F / 33–39°C) — heat cramps and exhaustion possible; Danger (103–124°F / 40–51°C) — heat exhaustion likely, heat stroke possible; Extreme Danger (125°F+ / 52°C+) — heat stroke highly likely. Direct sunlight can add up to 15°F above these calculated values.

Temperature is what the thermometer reads — the actual air temperature. Heat index factors in humidity to show how hot it feels to the human body. On a 95°F day with 60% humidity, the heat index can reach 113°F — 18 degrees hotter than the thermometer reads. The heat index is always equal to or higher than actual temperature.

The body cools itself mainly through sweat evaporation. When humidity is high, the air already holds a lot of water vapor, so sweat evaporates slowly — reducing the cooling effect. In dry air, sweat evaporates quickly and you cool efficiently. In humid air (RH above 70–80%), this cooling mechanism becomes severely impaired, causing core temperature to rise faster.

Dew point is the temperature at which air becomes fully saturated — water vapor condenses into dew. Unlike relative humidity (which changes with temperature), dew point is an absolute measure of moisture. Above 60°F (16°C) feels humid; above 70°F (21°C) feels oppressive; above 75°F (24°C) is dangerous. Our calculator converts dew point to relative humidity automatically to compute the heat index.

Heat exhaustion symptoms include heavy sweating, cool pale clammy skin, fast and weak pulse, nausea, dizziness, muscle cramps, tiredness, headache, and possible fainting. Move to a cool place, loosen clothing, apply cool wet cloths to skin, and sip water. If symptoms worsen or the person loses consciousness, call 911 immediately.

Heat exhaustion: the body is overheating but still sweating; not life-threatening if treated promptly. Heat stroke: body temperature exceeds 104°F (40°C), temperature regulation fails, sweating may stop, confusion or loss of consciousness occurs. Heat stroke is a life-threatening emergency — call 911 immediately and cool the person by any means available (cold water immersion is most effective).

The NWS heat index is calculated for shady conditions with light wind. In direct sunlight, the heat index can feel up to 15°F (8°C) higher. So if the calculated heat index is 103°F in shade, standing in direct sun may feel like 118°F — potentially crossing into extreme danger territory. Always account for sun exposure when assessing heat risk outdoors.

High-risk groups include adults 65+, infants and young children, people with heart disease, kidney disease, or obesity, those taking diuretics, antihistamines, or blood pressure medications, outdoor workers, athletes, and people without air conditioning access. People not yet acclimatized to heat (first 2 weeks of a heat wave) are also at significantly elevated risk even at moderate heat index levels.

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