Wind chill & heat index
Zero degrees feels much colder in a strong wind, and 32 degrees feels much hotter in humid air. The calculator shows the feels-like temperature and tells you which formula it used.
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How the calculation works
Up to 10 degrees and with wind above 4.8 km/h, the app works out wind chill with the formula Environment Canada and the US National Weather Service have used since 2001. It describes how cold exposed facial skin feels in the wind.
From 27 degrees, the app uses the US National Weather Service heat index, based on the Rothfusz regression, which combines temperature and relative humidity. In between, the calculator shows the air temperature, because neither formula applies there.
How to read the result
The big number is the feels-like temperature; below it you can see which formula was used.
For wind chill, the app warns from a felt minus 28 degrees: according to Environment Canada, exposed skin can then freeze within 10 to 30 minutes. For the heat index, a note appears from 32 degrees, where the US National Weather Service advises extreme caution.
Frequently asked questions
Does wind actually make it colder?
Not the air. But wind strips away the warm layer next to your skin faster, so you lose heat more quickly.
Which wind speed should I enter?
The speed at about 10 metres above ground, as given in weather reports. On ridges and summits the wind is often stronger than in the valley.
Why don't I need humidity in the cold?
Wind chill only uses temperature and wind. Humidity only counts for the heat index.