Heat Index vs Feels-Like Temperature

Heat index is the NWS Steadman formula from a 1979 paper. It takes air temperature and relative humidity and outputs an apparent temperature for a 5'7" 147-pound adult walking 3 mph in shade. The formula is fixed and published. The same inputs always give the same output.

Feels-like is whatever the weather app's vendor decided. AccuWeather RealFeel includes sun and wind. The Weather Channel uses heat index above 80 F and wind chill below 50 F. Google Weather uses humidex. Apple Weather uses NWS heat index plus a wind chill model. There is no industry standard.

When to use which

Use heat index for any work or sports decision. It is the formula OSHA, NWS, and the CDC reference. If you read "heat advisory" or "excessive heat warning" in a news bulletin, the threshold is in heat index.

Use feels-like for casual planning (what to wear, whether to grab a jacket). Treat it as a rough indicator with vendor-specific quirks.

For athletes and outdoor workers, use neither, use WBGT (Wet-Bulb Globe Temperature) instead. WBGT factors in sun and wind, which heat index does not.

Why the numbers differ

A typical scenario: 92 F, 65% humidity, partly sunny, light breeze.

- Heat index: 105 F (just temperature + humidity, shade assumption) - AccuWeather RealFeel: 110 F (adds sun and reduces for breeze) - Apple Weather feels-like: 105 F (defaults to heat index above 80 F) - WBGT: 88 F (wet-bulb dominated, lower number but more dangerous threshold)

Each is internally consistent with itself but tells you a slightly different story. Use the framing that matches the decision.