Apparent Temperature Calculator

What Apparent Temperature Measures and Why It Differs from Air Temperature

Apparent temperature -- also called "feels like" temperature -- is the temperature the human body perceives based on the combined effect of air temperature, relative humidity, wind speed, and solar radiation. A thermometer measures the energy state of the air; the body, however, exchanges heat with its environment through conduction, convection, radiation, and evaporation simultaneously. When any of these pathways is altered by wind, moisture, or sunlight, the physiological experience of heat or cold changes independently of the air temperature reading. At 28°C with 80% relative humidity and no breeze, sweat evaporation slows dramatically and the apparent temperature can reach 38–40°C; the same 28°C with 15% humidity and a 20 km/h wind may feel closer to 24°C.

The most complete standard formula for apparent temperature is the Australian Bureau of Meteorology version of the Steadman (1994) model: AT = Ta + 0.348e − 0.70ws + 0.70(Q/(ws+10)) − 4.25. Here Ta is dry-bulb temperature (°C), e is water vapour pressure in hPa derived from relative humidity, ws is wind speed at 10 m height in m/s, and Q is net radiation absorbed per unit body surface area in W/m². This formula is used for all official "feels like" outputs by the Australian Bureau of Meteorology and is the only standard index that accounts for all four thermal factors simultaneously.

Wind Chill: The Cold-Weather Index (NWS 2001)

Wind chill is the apparent temperature used by the US National Weather Service when air temperature is at or below 10°C (50°F) and wind speed exceeds 4.8 km/h (3 mph). The current formula, adopted in 2001 after replacing a 1945 model calibrated on Antarctic expeditions with frozen water containers, was derived from human subject trials: WC = 35.74 + 0.6215T − 35.75V^0.16 + 0.4275TV^0.16 (with T in °F and V in mph). The V^0.16 exponent reflects the measured physics of how wind strips the warm boundary layer from skin -- the effect grows sub-linearly, so doubling wind speed from 20 to 40 mph does not double the cooling effect. According to the NWS Wind Chill Chart, at −20°F with a 30 mph wind, exposed skin can develop frostbite in under 10 minutes.

A common misconception is that wind chill represents a temperature that objects can be chilled to. This is incorrect -- a car engine, pipes, or a glass of water will not freeze faster because of wind chill; they do not generate body heat and therefore do not benefit from the insulating boundary layer that wind destroys. Wind chill is a physiological index calibrated to the human body's heat loss rate, not a physical temperature. The formula is only valid for humans walking at approximately 5 km/h in calm to moderate winds up to 100 mph; at very high wind speeds the formula overestimates cooling for stationary persons.

Heat Index: The Hot-Weather Index (Rothfusz Regression)

The Heat Index, used by the US National Weather Service when temperature is at or above 27°C (80°F) and relative humidity is at or above 40%, is based on the Rothfusz (1990) regression of the Steadman (1979) apparent temperature tables. The full polynomial is: HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R² (T in °F, R in % RH). Two adjustments apply: if RH < 13% and 80°F < T < 112°F, subtract a dryness correction; if RH > 85% and 80°F < T < 87°F, add a high-humidity correction at moderate temperatures.

The Heat Index can produce dramatically higher values than air temperature at extreme humidity. The NWS Heat Index Chart shows that at 38°C (100°F) and 100% relative humidity, the Heat Index reaches approximately 67°C (153°F) -- a theoretically lethal apparent temperature. In practice, air this hot and saturated is not found outdoors except briefly during extreme events; the 2003 European heat wave, which caused approximately 70,000 excess deaths, involved peak Heat Index values of 45–50°C in cities not acclimatised to such conditions. On such days, outdoor air quality also deteriorates; use our Air Quality Index (AQI) calculator to check whether ozone or particulate matter compounds the heat risk. On such days, outdoor air quality also deteriorates;

Humidex: The Canadian Heat Index

Humidex, developed by Environment Canada meteorologists Masterton and Richardson in 1965, uses dew point temperature as its humidity input rather than relative humidity percentage. The formula is: H = T + 0.5555 × (6.11 × e^(5417.753 × (1/273.16 − 1/(273.16+Td))) − 10), where Td is the dew point in °C. Because dew point directly measures the absolute amount of water vapour in the air (unlike relative humidity, which expresses saturation percentage and changes with temperature), Humidex provides a stable measure of muggy discomfort that does not require temperature to interpret. Environment Canada classifies Humidex above 30 as "some discomfort," above 40 as "great discomfort; avoid exertion," and above 45 as "dangerous." The highest Humidex ever recorded in Canada was 53, in Windsor, Ontario, on 20 June 1953.

Solar Radiation: The Factor Most Calculators Ignore

The Steadman BOM formula includes a radiation term -- 0.70 × (Q/(ws+10)) -- that represents the heat load from sunlight absorbed by the body. At Q = 0 (shade or night), this term vanishes. In full Australian summer sun with light clothing (Q ≈ 790 W/m²), the radiation term adds approximately 4–6°C to the apparent temperature at 5 m/s wind. In dark clothing in direct sun (Q ≈ 1000 W/m²), the addition can reach 7–8°C. This is why BOM apparent temperature values in outdoor summer conditions consistently exceed heat index values calculated for the same temperature and humidity in a shaded weather station context.

The wind term in the denominator -- ws + 10 -- means wind reduces the radiation effect as well as the base temperature: at 10 m/s wind, the radiation addition at Q = 790 is halved compared to calm conditions. This interaction between wind and solar load explains a counterintuitive observation: on a very sunny day, a stronger breeze can provide disproportionate cooling relief beyond what temperature and humidity alone would predict. The Australian Bureau of Meteorology thermal stress guidance specifically notes that apparent temperature values above 40°C are associated with increased heat-related illness presentations in Australian emergency departments, and values above 46°C correspond to conditions observed during major heat mortality events.

Apparent Temperature Danger Thresholds and Health Guidance

Both heat and cold extremes of apparent temperature are associated with documented mortality increases. For heat, the Intergovernmental Panel on Climate Change notes that apparent temperatures above 40°C for more than 6 hours present serious risk of heat exhaustion and heat stroke, with risk rising sharply above 46°C especially for elderly individuals. For cold, the NWS wind chill thresholds flag −27°C (−17°F) as the level at which frostbite can occur in 30 minutes on exposed skin, and −48°C (−55°F) as the threshold for frostbite in 5 minutes.

Apparent temperature risk also changes with altitude: the same air temperature feels colder at elevation because lower atmospheric pressure reduces the insulating air mass and wind strips heat faster at altitude. Acclimatisation substantially modifies individual response: a person who has lived in Phoenix for 10 years will experience the same apparent temperature of 42°C very differently from a recent arrival from a cool climate. The CDC heat safety guidance emphasises that acclimatisation takes 7–14 days and requires gradual exposure; the most dangerous period for heat illness is the first hot spell of the year before any acclimatisation has occurred. Similarly, the most dangerous cold exposures are those that catch people inadequately dressed after a warm spell, which is why the NWS issues Wind Chill Watches and Warnings as precautionary alerts even when cold snaps are brief.