'Feels like' temperature is doing the work of two completely different formulas depending on the season, and nobody tells you which one is running the show.
“Feels like” sounds like a single, simple concept — the temperature, adjusted for vibes. In reality it’s two completely different equations stacked on top of each other in a trench coat, swapping out depending on the season, and most apps never tell you which one is currently driving.
The Cold-Weather Version: Wind Chill
Wind chill exists because moving air strips heat away from exposed skin faster than still air does. Your body constantly generates a thin layer of warmed air right against your skin; wind blows it away before it can insulate you, forcing your body to keep reheating that boundary layer over and over. The stronger the wind, the faster that heat gets stripped, and the colder it feels even though the actual air temperature hasn’t dropped at all.
This is why 20°F with a 30 mph wind can feel like single digits, and why wind chill only really applies below about 50°F — above that threshold, moving air doesn’t strip heat fast enough to matter, it just feels like a breeze.
The Hot-Weather Version: Heat Index
Heat index runs on a totally different mechanism: humidity. Your body cools itself primarily through sweat evaporating off your skin. High humidity means the surrounding air is already close to saturated, so sweat evaporates more slowly (or barely at all), and your body’s main cooling system stalls out. That’s why 90°F at 30% humidity is uncomfortable, but 90°F at 80% humidity is genuinely dangerous — same air temperature, wildly different ability for your body to cool itself.
You can run either number yourself: the wind chill calculator and the heat index calculator use the National Weather Service formulas and show the full charts.
Wind chill is about how fast the air steals your heat. Heat index is about how badly the air is jamming your body’s cooling system. Neither one is “the real temperature” — they’re both estimates of how your specific body will experience it.
Why Neither Number Is Perfect
Both formulas make assumptions — about clothing, about sun exposure, about individual physiology — that don’t apply to everyone equally. A “feels like 15°F” wind chill assumes exposed skin; someone in proper winter gear experiences something closer to the actual air temperature. A “feels like 105°F” heat index assumes shade; direct sun exposure can push the real experienced temperature meaningfully higher than the number on the app.
The Takeaway
“Feels like” isn’t lying to you, but it’s quietly running a different formula depending on the season — wind-driven heat loss in winter, humidity-blocked cooling in summer — and both are estimates built on assumptions about what you’re wearing and where you’re standing. Treat the number as a strong hint, not gospel, and adjust for your own actual conditions. If you’re specifically deciding whether it’s too cold to run today, here’s where that line actually sits.