NOAA Heat Index Calculator (Feels Like Temperature)

The heat index — also called ‘apparent temperature’ or ‘feels like’ temperature — is a measure of how hot it actually feels to the human body when air temperature is combined with relative humidity. At high humidity, sweat evaporates more slowly, reducing your body’s ability to cool itself. For example, at 90°F with 90% humidity, it can feel like 122°F. The heat index was standardized by NOAA meteorologist Robert Steadman in 1979.

Your body cools itself primarily through sweat evaporation. When humidity is high, the air is already saturated with moisture, so your sweat can’t evaporate as efficiently — meaning heat stays trapped against your skin instead of being carried away. Dry heat allows rapid evaporation and faster cooling. This is why Phoenix at 110°F with 10% humidity can feel more manageable than New Orleans at 92°F with 90% humidity.

Actual temperature (dry-bulb temperature) is what a standard thermometer measures in the shade — a pure measure of air warmth. The heat index combines that temperature with relative humidity to estimate what a human body actually experiences. At low humidity, the heat index can actually be lower than air temperature. At high humidity, it can be dramatically higher — sometimes 20–40°F above the actual reading.

Yes and no. ‘Feels like’ is a general term used by weather apps that often blends multiple factors: heat index in summer (when heat and humidity are the concern) and wind chill in winter (when wind and cold are the concern). Technically, the NOAA heat index is calculated only for temperatures at or above 80°F and relative humidity at or above 40%. Below those thresholds, a simplified formula or wind chill applies.

The modern heat index is based on work by Robert Steadman, who published a comprehensive analysis of apparent temperature in 1979 in the Journal of Applied Meteorology. NOAA meteorologist Lans Rothfusz later developed the regression equation in 1990 that is widely used today — the 9-term polynomial formula (Rothfusz Regression Equation) that powers most modern heat index calculators, including this one.

The heat index is expressed in degrees — either Fahrenheit (°F) or Celsius (°C) — representing the equivalent temperature a human body perceives. It is not a separate unit; it is just temperature expressed on the same scale as air temperature, but adjusted to reflect the additional stress caused by humidity.

For temperatures at or above 80°F, NOAA uses the Rothfusz Regression Equation — a 9-term polynomial that factors in temperature (T) and relative humidity (RH). For lower temperatures, a simpler equation is applied. Two adjustments modify the result: a correction for low humidity (RH < 13%) which subtracts from the total, and a correction for high humidity (RH > 85%) which adds to it. The formula was derived from Steadman’s original physiological model.

No — the official NOAA heat index formula does not account for wind. It is calculated assuming shaded, calm-air conditions. Wind can feel cooling when temperatures are moderate because it speeds up sweat evaporation, but when air temperature exceeds body temperature (~98.6°F), wind actually makes things worse by blowing hot air against you. Some advanced heat stress models like WBGT (Wet Bulb Globe Temperature) do incorporate wind.

The official NOAA heat index is calculated for shaded conditions only. Direct sunlight exposure adds significant radiant heat on top of the calculated value. NWS guidance states that full sun can increase the effective heat index by up to 10–15°F (5.5–8.3°C). This is why our calculator includes an optional sunlight adjustment — outdoor athletes in direct sun face substantially higher real-world heat stress than the base formula shows.

Humidity meaningfully affects the heat index starting around 80°F (27°C). Below that threshold, the difference between humid and dry air is relatively small for most people. Above 80°F, the impact of humidity grows exponentially — at 90°F, the difference between 30% and 90% humidity can represent a 35°F swing in how hot it feels. At 100°F+, even moderate humidity levels create extreme danger conditions.

Yes. When relative humidity is very low (below approximately 30–40%), the heat index can actually register lower than the air temperature. This happens because dry air allows sweat to evaporate rapidly, cooling the skin efficiently. At 100°F with only 15% humidity, the NOAA heat index is approximately 96°F — four degrees below actual temperature. This is why dry desert heat (Phoenix, Las Vegas) can feel more survivable than humid heat at lower temperatures.

Relative humidity (RH) is the percentage of moisture in the air relative to how much the air could hold at that temperature — it’s always 0–100%. Dew point is the actual temperature at which air becomes saturated and condensation forms. Dew point is often considered more accurate for measuring human discomfort: a dew point above 65°F feels humid; above 75°F feels oppressive. Unlike RH, dew point doesn’t change with temperature, making it a more stable measure of absolute moisture.

According to NOAA and NWS classifications: 80–90°F (Caution) — fatigue possible; 90–103°F (Extreme Caution) — heat cramps and exhaustion possible; 103–124°F (Danger) — heat cramps and exhaustion likely, heat stroke possible; 125°F+ (Extreme Danger) — heat stroke highly likely. For physically active people, outdoor workers, and those not acclimatized to heat, danger thresholds are effectively 5–10°F lower than these baseline values.

Heat cramps are painful muscle spasms caused by electrolyte loss — the mildest form. Heat exhaustion occurs when the body is overwhelmed by heat, causing heavy sweating, weakness, dizziness, and nausea — serious but treatable with rest, shade, and hydration. Heat stroke is a medical emergency where core body temperature exceeds 104°F, sweating stops, and organ damage begins. Heat stroke has a fatality rate of 10–80% depending on how quickly treatment begins.

Absolutely. Heat stroke is primarily a function of heat INDEX, not air temperature alone — and physical activity dramatically increases internal body heat production. A runner at 85°F with 85% humidity (heat index ~101°F) combined with moderate training effort faces conditions equivalent to resting in 118°F+ heat. Military units and sports medicine organizations routinely see heat casualties at air temperatures as low as 78°F when combined with high humidity and sustained exertion.

Early signals to watch for: (1) Stopping sweating despite still feeling hot — most critical sign; (2) Throbbing headache; (3) Nausea or vomiting; (4) Rapid or weak pulse; (5) Cool, pale, or clammy skin (exhaustion) OR hot, red, dry skin (heat stroke); (6) Confusion, slurred speech, or unusual behavior. Any combination of these is a call-911 situation. Never try to ‘push through’ these warning signs.

Highest risk groups: outdoor and construction workers, athletes and military personnel, infants and children under 4, adults over 65, people with chronic conditions (heart disease, obesity, diabetes), those taking diuretics or antihistamines, and anyone not yet acclimatized to the heat. Statistically, the first 3 days of a heat wave kill the most people, because the population hasn’t yet adapted and air conditioning usage peaks before infrastructure can respond.

Heat is the deadliest weather hazard in the United States, killing more Americans annually than hurricanes, tornadoes, and floods combined. The CDC reports approximately 1,300–2,000 direct heat deaths per year in the US, though research suggests actual heat-related mortality (including deaths where heat is a contributing factor) may be 5–10 times higher. Urban areas with heat island effects and populations with limited access to air conditioning are most vulnerable.

Yes. Nighttime heat index values are especially dangerous because they prevent the body from recovering from daytime heat stress. When overnight lows don’t drop below 80°F — common in cities like Miami, Houston, and New Orleans in July and August — the body never gets a recovery window. Sustained elevated nighttime heat index is one of the leading risk factors for heat fatality during extended heat waves.

At a heat index of 100°F (Extreme Caution), high-intensity outdoor training is not recommended for most people. If you must train, reduce intensity to 50–60% of normal, limit sessions to under 45 minutes, train in early morning or evening, take 10-minute cool-down breaks, and always train with a partner. Fully acclimatized elite athletes may manage moderate training at this threshold, but non-acclimatized individuals face genuine medical risk.

Physical exertion generates significant metabolic heat internally, on top of external heat stress. The human body at rest generates about 80 watts of heat. Moderate running generates approximately 700–1,000 watts — up to 12x more. This internal heat load adds the equivalent of 7–18°F to your effective heat stress, which is why our calculator includes an activity adjustment. At heat index 95°F, a hard training run can push your effective heat stress past 115°F.

The safest outdoor training windows in US summers are before 8am and after 7pm, when solar radiation is minimal and temperatures begin dropping. In the Southeast and Gulf Coast states (Florida, Louisiana, Mississippi, Alabama), humidity can keep overnight heat index values above 90°F even after midnight — meaning early morning sessions are safer than late evening ones when the ground and buildings are still radiating stored heat from the afternoon.

General NWS and ACSM guidance: drink 16–20oz (2–2.5 cups) of water 2 hours before outdoor training; 8oz 20 minutes before; and 6–12oz every 15–20 minutes during activity. For sessions over 60 minutes in high heat, switch to electrolyte drinks. After training, drink 16–24oz for every pound of body weight lost. Critically: thirst is not a reliable indicator — you’re already dehydrated by the time you feel thirsty.

Full heat acclimatization takes 10–14 days of progressive heat exposure. The most significant physiological changes — increased plasma volume, earlier sweating onset, higher sweat rate, and reduced cardiovascular strain — occur primarily in the first 5–7 days. Protocol: start at 50% of normal training intensity and duration, increase by 10–15% every 2–3 days. Acclimatized athletes can sustain higher work rates at the same heat index than non-acclimatized individuals.

Yes — always apply broad-spectrum SPF 30+ sunscreen. Concerns that sunscreen blocks sweating are largely a myth. Studies show sunscreen does not significantly impair thermoregulation or sweat rate in healthy adults. Solar radiation from unprotected skin exposure adds to your overall heat load (sunburn reduces the skin’s ability to sweat effectively) — so sunscreen actually helps your cooling system by protecting the skin’s integrity.

The most consistently dangerous heat index states are Louisiana, Mississippi, Alabama, Florida, and Texas — due to their combination of high temperatures AND high humidity from the Gulf of Mexico. Arizona and Nevada hit extreme temperatures but with dry heat, making absolute heat index values lower. In recent years, the Pacific Northwest (Washington, Oregon) has become increasingly dangerous due to extreme heat waves paired with populations and infrastructure historically unequipped for it.

The highest reliably recorded heat index in the US occurred in Dhahran, Saudi Arabia context, but within the US, some of the most extreme readings have occurred in the Gulf Coast region. On July 13, 1995, during the Chicago heat wave that killed 739 people, some locations recorded heat index values above 120°F. In July 2023, parts of Texas and the Gulf Coast recorded heat index values exceeding 115°F during the historic heat dome that broke records across the southern US.

Houston’s danger comes from its unique combination of factors: temperatures regularly reaching 95–100°F in summer, relative humidity of 70–85% from the Gulf of Mexico, a massive urban heat island effect, intense solar radiation, and a large working-class population that must work outdoors. The heat index in Houston regularly exceeds 110°F in July and August. Texas leads the US in heat-related worker deaths, with Houston-area counties accounting for a disproportionate share.

Humid heat is generally more physiologically dangerous for sustained activity because it impairs the body’s primary cooling mechanism (sweat evaporation). However, extreme dry heat becomes equally dangerous above approximately 108–110°F, at which point air temperature alone overwhelms cooling capacity regardless of humidity. Short-term dry heat exposure (like stepping out in Phoenix) can be managed longer than humid heat at equivalent heat index values because sweat still evaporates.

A heat dome forms when high-pressure atmospheric conditions trap hot air over a region, acting like a lid. This sustained system prevents the normal cycling of air, causing temperatures to build day after day without overnight recovery. Heat domes amplify the heat index by simultaneously driving air temperature up and, in humid regions, preventing the moisture from dissipating. The 2021 Pacific Northwest heat dome (Portland reached 116°F) and the 2023 Texas heat dome are recent examples of catastrophic heat dome events.

The heat index estimates how hot it feels to a clothed, shaded person at rest based on temperature and humidity. Wet bulb temperature (WBT) measures the lowest temperature achievable by evaporating water into air — it directly represents the physiological limit of the body’s cooling ability. Scientists consider sustained wet bulb temperatures above 35°C (95°F) unsurvivable even for healthy people in shade, as the body can no longer shed heat. WBT is considered more accurate for extreme heat mortality risk assessment.

Wet Bulb Globe Temperature (WBGT) is the gold standard heat stress metric used by the US military, OSHA, and professional sports organizations. Unlike the heat index, WBGT accounts for temperature, humidity, wind speed, and solar radiation simultaneously. WBGT is measured using three thermometers: dry bulb (air temp), wet bulb (humidity), and black globe (solar radiation). Military and NCAA athletic guidelines use WBGT thresholds — not heat index — to restrict training activities.

Several factors create this difference: (1) Relative humidity — the dominant factor (NYC at 90°F/65% RH vs. Denver at 90°F/20% RH feels completely different); (2) Urban heat island effect — cities with dense concrete and asphalt absorb more heat; (3) Solar angle and elevation — Denver’s altitude means stronger UV; (4) Wind patterns — coastal cities often have sea breezes that provide some cooling; (5) Acclimatization — locals are physiologically adapted to their region’s specific heat profile.

Yes — and faster than temperature alone. A 2024 study from UC Berkeley found that the heat index in Texas has risen much faster than raw temperatures due to increased atmospheric moisture. Warming oceans and warmer air hold more water vapor, raising baseline humidity. The study’s authors note that a 3°F rise in temperature can feel like a 10°F increase in heat index in humid regions. Climate projections indicate that dangerous heat index days (above 103°F) could become 2–3x more frequent in the Gulf South by 2050.

No — sweating is your body’s cooling system working correctly. Sweating is the intended, healthy response to heat. The danger signal is when you stop sweating despite still being in the heat. Cessation of sweating in a hot environment means your body has become so dehydrated or overwhelmed that it can no longer produce sweat — a pre-heat stroke emergency. Profuse sweating during exercise is normal and healthy; sudden absence of sweating in the heat is not.

Alcohol makes heat danger significantly worse, not better. Alcohol is a diuretic — it causes your kidneys to excrete more fluid, accelerating dehydration. It also dilates blood vessels near the skin, making you feel temporarily cooler while actually increasing fluid loss and reducing blood pressure. Alcohol impairs your body’s ability to regulate temperature and dulls your perception of heat stress warning signs. Drinking alcohol during a heat wave meaningfully increases your risk of heat-related illness and death.

For heat exhaustion: no — cooling quickly is appropriate. For confirmed heat stroke (where core temperature exceeds 104°F), aggressive cooling is critical and must begin immediately. Cold water immersion (ice bath) is the fastest and most effective treatment and is safe when administered correctly. The old caution against ‘cooling too quickly’ has been largely dismissed by modern sports medicine research. Time to cooling is the single biggest predictor of survival in heat stroke cases.

The heat index is a standardized estimate based on an ‘average’ adult body in shade with light wind and no physical exertion. Actual perceived heat varies significantly based on age (older adults are less efficient at thermoregulation), fitness level (fit individuals sweat more efficiently), acclimatization, clothing, body composition, medication use, and individual physiology. The heat index is a useful baseline, but athletes under exertion, children, and older adults should treat the thresholds as conservative minimums, not maximum limits.