Heat Index, Feels-Like, and Wet-Bulb: What the Numbers Actually Mean
Three numbers, three different formulas, three different jobs. Here is when each one matters and which one your weather app is probably showing.
When the temperature outside hits 92 F, your weather app might say it feels like 105 F. The local news says the heat index is 102 F. An athletic trainer warns the wet-bulb is 84 F and practice is canceled. All three are talking about heat stress on your body, but they measure different things.
The heat index is the NWS Steadman formula. It takes air temperature plus relative humidity and tells you the apparent temperature in shade with a light wind. It is a single number designed for the general public. The formula was built from a 1979 paper by Robert Steadman that modeled a 5'7" 147-pound adult walking 3 mph in shade.
Feels-like temperature is whatever your weather app's vendor decided. AccuWeather has its own RealFeel formula that includes sun, wind, and cloud cover. The Weather Channel uses heat index for hot weather and wind chill for cold. Apple Weather uses the NWS heat index above 80 F and the JAG wind chill below 50 F. There is no standard, which is why two apps can show different feels-like numbers for the same place.
Wet-bulb temperature is the one that matters for athletes and outdoor workers. It is measured (or computed) using a thermometer wrapped in a wet cloth, the temperature it reads is the lowest temperature evaporation can cool you down to. When wet-bulb hits 95 F, no amount of sweating or shade keeps a healthy adult alive for more than about 6 hours. Most sports organizations use the Wet-Bulb Globe Temperature (WBGT), which also factors in sun and wind.
Which one do you actually need? For an afternoon walk, the heat index on your weather app is fine. For running a 10K, check WBGT, if it is above 82 F, slow your pace by 30 seconds per mile. For working an 8-hour outdoor shift, OSHA uses heat index 80 F as the trigger for water and rest breaks, and 90 F for mandatory cool-down rest.
The single most dangerous misconception is treating 100 F dry heat the same as 90 F humid heat. The heat index chart says 100 F at 15% humidity is "extreme caution" (HI 100), while 90 F at 70% humidity is "danger" (HI 106). Humidity changes the math because sweat is your body's main cooling mechanism, and high humidity slows evaporation. This is why the Gulf Coast in July is more dangerous than Phoenix in July, despite the lower air temperature.
A quick rule of thumb: if you can see your breath in cold weather, you can see the humidity in hot weather as haze. When the air is hazy and the temperature is above 85 F, check the heat index, not just the temperature. The number you find will usually be 10 to 25 degrees higher than the reading on your car's thermometer.