Wind chill, heat index and dew point calculator

The NWS formula applies at 50 °F (10 °C) or colder with wind of at least 3 mph (4.8 km/h); outside those limits this calculator shows the air temperature. Environment and Climate Change Canada reports wind chill only when the temperature is 0 °C or below and the wind is 5 km/h or more. At 5 °C in a 20 km/h wind the index is 1.1 °C.

Environment and Climate Change Canada gives these times for exposed skin: 10 to 30 minutes from −28 °C, 5 to 10 minutes from −40 °C, 2 to 5 minutes from −48 °C, and under 2 minutes at −55 °C or colder. Air at −20 °C with a 30 km/h wind has a wind chill of −32.6 °C, inside the 10 to 30 minute band.

The NWS classifies a heat index of 80–90 °F (26.7–32.2 °C) as caution, 90–103 °F as extreme caution, 103–124 °F as danger and 125 °F (51.7 °C) or higher as extreme danger. Air at 96 °F (35.6 °C) with 65% humidity has a heat index of 121 °F (49.4 °C), in the danger band where heat stroke is possible.

The NWS La Crosse office describes a dew point of 55 °F (12.8 °C) or lower as dry and comfortable, 55–65 °F as sticky with muggy evenings, and 65 °F (18.3 °C) or higher as oppressive. Air at 25 °C and 60% relative humidity has a dew point of 16.7 °C (62 °F), in the sticky range.

No. The humidex is Environment and Climate Change Canada's index, a different formula that also combines temperature and humidity, so the two give different numbers for the same weather. ECCC rates a humidex of 30–39 as some discomfort, 40–45 as great discomfort, and 46 or more as dangerous, with heat stroke possible.

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 6 worked examples whose answers come from independent sources; for example, “0 °F with 15 mph wind” is checked against NWS wind chill chart: 0 °F, 15 mph → −19 °F (formula −19.40).

NWS — Wind chill chart and formula (2001); NWS Weather Prediction Center — The heat index equation (Rothfusz 1990 with adjustments); Environment and Climate Change Canada — Wind chill index and frostbite risk; O. A. Alduchov and R. E. Eskridge, “Improved Magnus form approximation of saturation vapor pressure”, J. Appl. Meteor. 35 (1996) 601–609; NWS La Crosse — dew point comfort guide.