Heat Index Calculator

Apparent temperature from heat and humidity.

The National Weather Service issues wind-chill and heat-index advisories against fixed thresholds, and conditions inside a city differ from the airport reading that supplies the official number. Humidity moves the heat index far more than temperature does.

American forecasts report temperature in Fahrenheit and wind in miles per hour, and the wind-chill and heat-index values published by the National Weather Service are calibrated to those units. Wind chill describes heat loss from exposed skin, not a temperature the air can actually reach.

Both figures are about the body rather than the thermometer: shade, clothing, humidity and sun exposure move the felt temperature well away from the quoted number.

Apparent temperature models how a body loses or gains heat; it is not a reading of the air. Wind chill assumes exposed skin at a certain height, heat index assumes shade and light activity, and direct sun can add ten degrees or more on top of either.

Heat Index Calculator sits with the rest of the library's everyday work, and it rests on the same outdoor-comfort arithmetic the way a manufacturer's label or a timetable does. Its answer is only as good as the assumptions standing behind it.

The quantities it works from — temperature and relative humidity — are the ones a real outdoor-comfort question actually turns on, and each carries a convention of its own, set out below.

Everyday arithmetic in the United States carries conventions that are easy to miss: dates run month before day, distances and weights follow customary units, and prices are quoted before tax. Those conventions make a surprising share of daily sums come out differently from the same question asked elsewhere.

The figures in this section follow American practice unless the subject itself is international.