April, after a long cold winter. It hits 15°C and people are out in t-shirts, sitting on terraces, looking at each other with relief. September, after a warm summer. 15°C and everyone's back in jackets, saying it's getting cold. Neither group is wrong. The temperature is the same. Almost everything else is different.
The objective part: conditions vary a lot at 15°C
The same air temperature can produce a wide range of feels-like temperatures depending on wind, sun, and cloud cover. A still, sunny 15°C day with no wind can feel close to 20 to 22°C. The same 15°C with 35 km/h wind and thick cloud can feel like 9°C.
| Conditions | Air temperature | Feels-like |
|---|---|---|
| Sunny, no wind | 15°C | ~20-22°C |
| Sunny, 25 km/h wind | 15°C | ~12°C |
| Overcast, no wind | 15°C | ~14°C |
| Overcast, 35 km/h wind | 15°C | ~9°C |
| Rain, 20 km/h wind | 15°C | ~10°C |
That's a 13-degree spread in felt temperature, all at 15°C. So when people disagree about whether 15°C is warm or cold, they're often not comparing the same conditions.
The subjective part: acclimatisation is real
Your body adjusts to the temperature range it's been experiencing. After months of sub-10°C weather, 15°C genuinely does feel warm because your baseline has shifted. Your thermoregulation adapts, your clothing habits have adjusted toward heavier layers, and the contrast is noticeable. By September you've been operating in 20-28°C for months, and 15°C represents a significant step down from that. The temperature is identical. The context is not.
This isn't a psychological trick or a matter of attitude. Thermal acclimatisation is a documented physiological process. Your body actually processes temperature differently depending on what it's been used to.
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How KorteBroekAan.nl handles it
The site calculates feels-like temperature for whatever the current conditions are, which handles the objective part. For the subjective part, the Weakling to Legend slider lets you shift the threshold for clothing recommendations to match your own tolerance. If you run cold and 15°C on a sunny day still feels chilly to you, moving toward the Weakling end adjusts the advice. If you've been doing cold-water swimming all winter and think 15°C is basically summer, the Legend setting is there.
Related reading: why 20 degrees is not always shorts weather explores the same variability at a temperature most people assume is straightforwardly warm. The full Weather Explained section covers all the factors that contribute to the spread.
Further reading
Related articles in the Weather Explained section: