Weather apps routinely show two numbers: the measured air temperature and a 'feels like' value beside it. Sometimes the gap is a degree or two; sometimes it exceeds 10 °C. That second number is not marketing decoration — it is an attempt to describe how the human body actually experiences the air, accounting for wind, humidity and radiation.
Your body is a wet, heated object in moving air
Human skin is usually warmer than the surrounding air, and it produces sweat to dump excess heat. How fast heat leaves depends on more than air temperature: wind blows away the thin insulating layer of warmed air clinging to you, while humid air slows the evaporation of sweat by already holding more water vapor. Apparent-temperature formulas model those effects.
Wind chill: cold in motion
Wind chill applies at low temperatures. On a calm day your body warms a thin shell of air next to the skin, which then acts as insulation. Wind strips that shell away continuously, forcing you to reheat fresh air. The stronger the wind, the faster heat leaves exposed skin and the greater the frostbite risk — even though the thermometer never changes.
Importantly, wind chill cools people and animals, not inanimate objects: a car or a puddle cannot be chilled below the actual air temperature by wind. It simply reaches that temperature faster. Windchill thresholds guide exposed-skin exposure times; at values near −27 °C (−17 °F), exposed skin can freeze in half an hour, and below about −48 °C (−54 °F) within minutes.
Heat index: humidity turns up the dial
In warm conditions the dominant factor is moisture. Evaporating sweat is the body's primary cooling mechanism, and evaporation only works while the air can accept more water vapor. As relative humidity climbs, sweat beads instead of drying, resting on skin while your core keeps generating heat. The heat index quantifies how hot that combination feels in shade with a light breeze.
- At 32 °C (90 °F) with low humidity the heat index can sit near the actual temperature and feel bearable.
- At the same 32 °C with 80% humidity the heat index may exceed 43 °C (110 °F), entering heat-disorder territory.
- Direct sunshine can add a further 5–10 °C (9–18 °F) on top of the published index, which is a shade value.
Why there is no single 'apparent temperature' formula
Different agencies use different models. The U.S. National Weather Service maintains separate wind-chill and heat-index tables; many international providers and consumer apps use a continuous 'apparent temperature' model, which also folds in solar radiation. Small differences between apps are therefore normal — they reflect modeling choices, not measurement errors.
Open-Meteo, which powers SkyLog's readings, provides an apparent-temperature series based on a widely used combined model, so the value you see already blends temperature, humidity, wind and radiation effects consistently across every city worldwide.
When to trust the feels-like number most
- Cold and windy: the wind-chill value governs exposed skin and layering choices.
- Hot and humid: the heat index governs pace, hydration and the need for shade and breaks.
- Clear sunny days: mentally add warmth for direct sun, especially in dry cold where a thermometer understates comfort.
- Transitions: a morning run, a windy platform or a shaded courtyard each shift local apparent conditions quickly.
Air temperature describes the world outside your body. Feels-like temperature describes the boundary where your body meets it — and that is the number your plans should start from.
Understanding the two-number forecast also settles the old argument about whether forecasts 'exaggerate'. The thermometer is accurate; your experience is also accurate. They are measuring different things, and dressing, scheduling and exercising by the feels-like value — rather than the headline air temperature — is the simplest way to stay both comfortable and safe.