What "feels like" temperature actually measures
Wind chill and heat index are not adjusted thermometer readings. They estimate how fast your body loses or fails to lose heat — two different mechanisms.
Your weather app says 0°C but feels like −6°C. Nothing about the air is actually −6°C — a thermometer hung outside would read zero regardless of the wind. What the app is describing is not the air at all. It is you, and specifically how fast you are losing heat to your surroundings.
Two different mechanisms, two different formulas
Cold and heat discomfort work in opposite directions, which is why they need separate calculations.
- Wind chill applies in the cold. Your body maintains a thin layer of warmed air against the skin; wind strips it away, so heat escapes faster. The air is not colder — your heat loss matches what still air at that lower temperature would cause.
- Heat index applies in the heat. Your body cools by evaporating sweat; humid air slows evaporation, so the cooling fails. The reported figure is the dry-air temperature that would feel equivalent.
What wind chill costs you
| Air temperature | Wind speed | Feels like | Difference |
|---|---|---|---|
| 5°C | 15 km/h | 1.7°C | −3.3°C |
| 0°C | 25 km/h | −5.9°C | −5.9°C |
| −5°C | 30 km/h | −13°C | −8°C |
| −10°C | 40 km/h | −20.8°C | −10.8°C |
| −20°C | 50 km/h | −35.4°C | −15.4°C |
Notice the gap widening as it gets colder. Wind matters more the colder it already is, because the temperature difference driving the heat loss is larger. The same 40 km/h wind is a nuisance at 5°C and a frostbite risk at −10°C.
Why the effect levels off
Wind speed enters the formula raised to the power 0.16, which is a very flat curve. Going from still air to 20 km/h makes a dramatic difference; going from 40 to 60 km/h adds comparatively little. Once the insulating layer has been stripped away, there is nothing left for extra wind to remove.
It also assumes exposed skin. Wind chill is really a statement about your face and hands — a windproof layer largely defeats the mechanism, which is why the numbers describe risk of frostbite rather than how cold you will feel while properly dressed.
Humidity and the heat index
| Air temperature | Humidity | Feels like | Risk level |
|---|---|---|---|
| 30°C | 45% | 30.3°C | Caution |
| 32°C | 60% | 37.1°C | Extreme caution |
| 35°C | 70% | 50.3°C | Danger |
| 38°C | 50% | 48.6°C | Danger |
The jump from 32°C to 35°C adds only three degrees of air temperature but thirteen degrees of felt temperature, because the humidity rose alongside it. Heat and humidity compound rather than simply add — which is why a dry 38°C is survivable work weather and a humid 35°C is genuinely dangerous.
Why humid heat is the real hazard
Sweating is the only cooling mechanism your body has once air temperature exceeds skin temperature. In dry air it works remarkably well. As humidity rises, sweat stops evaporating — it just runs off, taking almost no heat with it. At sufficiently high humidity, no amount of sweating cools you at all, and body temperature simply rises regardless of fitness or acclimatisation.
This is why humid heat waves cause more deaths than hotter dry spells, and why "but it is a dry heat" is a genuine physiological observation rather than an excuse.
Using the numbers sensibly
Treat both as risk indicators rather than temperatures. Wind chill tells you how quickly exposed skin is in trouble and how urgently to cover it. Heat index tells you whether your cooling system can keep up, and therefore whether to move activity to the morning. Neither is telling you what a thermometer would say — that is the actual air temperature, and it has not changed.
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