Wind chill and heat index calculator: what the air feels like
Work out the wind chill from temperature and wind speed, or the heat index from temperature and humidity, with the National Weather Service's formulas and charts.
Built by the Tribune's code from the National Weather Service's published formulas. No AI wrote this page.
Wind chill chart (°F)
Rows: wind speed in mph. Columns: air temperature in °F. Worked out with the Weather Service's formula, rounded to the nearest degree; your choice is outlined.
Heat index chart (°F)
- Caution
- Extreme caution
- Danger
- Extreme danger
Rows: relative humidity. Columns: air temperature in °F. Shading shows the Weather Service's heat index levels; your choice is outlined. Worked out with the Weather Service's equation, so a few values differ by a degree or two from its printed chart, which comes from older tables. Blank: beyond the range the equation covers.
How it's calculated
Wind chill uses the formula the National Weather Service adopted in 2001: Wind chill = 35.74 + 0.6215T − 35.75(V0.16) + 0.4275T(V0.16), where T is the air temperature in °F and V the wind speed in mph. The Weather Service defines it only for temperatures of 50°F or below and winds above 3 mph.
Heat index uses the Weather Service's equation (the Rothfusz regression, with the adjustments the Weather Service publishes for very dry or very humid air, and its simpler formula for lower values). The Weather Service notes that heat index values are for shady places with a light wind; full sunshine can raise them by up to 15°F.
Both are estimates of how the air feels on exposed skin. For warnings where you are, see your local National Weather Service forecast, and the Weather Service's cold safety and heat safety pages.
Formulas: National Weather Service wind chill chart; The Heat Index Equation (NWS Weather Prediction Center).