Heat Index & Humidex Calculator

Three official 'feels like' figures for hot weather, side by side, because the country you are in decides which one the forecast quotes. The U.S. National Weather Service heat index is computed exactly as the Weather Prediction Center describes it: Steadman's simple formula first, then the Rothfusz regression once that estimate averaged with the temperature reaches 80 °F, with the corrections for very dry air (below 13 % relative humidity) and for very humid air between 80 and 87 °F. Environment Canada's humidex uses the dew point and its own vapour-pressure expression, and the Australian Bureau of Meteorology's apparent temperature adds wind, which none of the others consider. Each comes with its agency's risk band — caution, extreme caution, danger and extreme danger for the heat index; some, great and dangerous discomfort for humidex. Enter humidity as relative humidity or as a dew point and the other is shown too. The heat index agrees with MetPy and with the NWS chart (96 °F at 65 % reads 121 °F), and humidex reproduces Environment Canada's example (30 °C with a 15 °C dew point is 34). What these numbers leave out: all assume shade, so full sun can add up to 8 °C (15 °F) to the heat index; they describe a healthy adult at rest; and the heat index regression was fitted for roughly 80 to 110 °F, so outside that range the tool says it is an approximation.