How to use it
- Choose metric or imperial units.
- Enter the air temperature, the wind speed and the relative humidity.
- Read the feels-like temperature, which formula was used, and the difference from the air temperature.
Formulas
Wind chill (US National Weather Service and Environment Canada, 2001), valid for air at or below 10 °C (50 °F) and wind above 4.8 km/h (3 mph):
- metric, T in °C and v in km/h:
13.12 + 0.6215·T − 11.37·v^0.16 + 0.3965·T·v^0.16 - imperial, T in °F and v in mph:
35.74 + 0.6215·T − 35.75·v^0.16 + 0.4275·T·v^0.16
Heat index (NWS, Rothfusz regression), temperature T in °F and humidity RH in percent, used from 80 °F up:
−42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² − 0.00000199·T²·RH²
The NWS adjustments are applied: for RH below 13 percent and T from 80 to 112 °F, subtract ((13 − RH)/4)·√((17 − |T − 95|)/17); for RH above 85 percent and T from 80 to 87 °F, add ((RH − 85)/10)·((87 − T)/5). Metric input is converted to °F for this formula and the result back to °C.
Worked example
At 0 °F with a 15 mph wind, 15^0.16 = 1.5423, so the wind chill is 35.74 − 35.75 × 1.5423 = −19.4 °F, the −19 shown on the NWS chart. At 90 °F and 70 percent humidity the heat index is 105.9 °F, close to 106 on the NWS table.
Notes
Wind chill ignores humidity and the heat index ignores wind. Between the two ranges, roughly 10 to 27 °C, the result equals the air temperature.