15°C in full sun, no wind, a bench against a south-facing wall. It feels pleasant. Surprisingly so. You might even roll up your sleeves. The thermometer hasn't lied, but it also hasn't told you the full story.
How solar radiation adds warmth without changing air temperature
The sun emits infrared radiation that travels through the atmosphere and is absorbed directly by your skin and clothing. The air temperature barely changes as a result; the radiation bypasses it almost entirely. This is different from how a warm room works, where the air itself is what's carrying the heat. With sunlight, the energy goes straight to you.
The amount of warmth you gain depends on the angle of the sun (high in summer, low in winter), whether the sky is clear or cloudy, and whether wind is stripping away the warmth as fast as you're absorbing it.
| Conditions | Extra perceived warmth |
|---|---|
| High summer, clear sky | +7 to +10°C |
| Spring/autumn, clear sky | +4 to +6°C |
| Low winter sun, clear sky | +1 to +3°C |
| Partly cloudy | +2 to +4°C |
| Fully overcast | ~0°C |
In midsummer on a clear, still day, the solar addition can push the perceived temperature 10 degrees above the air temperature. In winter even a bright day adds relatively little because the sun sits so low on the horizon.
Clothing colour matters here
Dark clothing absorbs more solar radiation than light clothing, so on a sunny day a black jacket can feel noticeably warmer than a white one at the same air temperature. It's a real effect, not just a perception.
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How KorteBroekAan.nl uses this
Cloud cover percentage feeds into the feels-like temperature calculation. A clear sky tilts the calculation warmer than the air temperature; heavy overcast removes the solar contribution almost entirely. This is why the site might recommend lighter clothing on a cool but sunny day that a simple temperature lookup would suggest is jacket weather.
The practical illustration: a clear 14°C day can feel like 20°C. The same 14°C overcast and breezy can feel like 10°C. The thermometer reading is identical. The experience is not.
The counterpart to this article, why shade feels colder, explains what happens when the solar warmth disappears. The full set of factors is covered across the Weather Explained section.
Further reading
Related articles in the Weather Explained section:
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Why shade feels colder
You're sitting in the sun, perfectly comfortable. A cloud passes. Suddenly you're crossing your arms. The air temperature hasn't changed at all.
Why sunshine matters for clothing advice
Two days, both 14°C. One needs a jacket, the other just a light shirt. The difference is direct solar radiation, and it matters more than most weather apps let on.
Why 15 degrees sometimes feels warm and sometimes cold
15°C in April after a cold winter feels like liberation. The same temperature in September feels like summer is over. Both reactions make complete sense.
Shorts on the terrace: sun changes everything
A terrace in the sun feels warmer than the thermometer suggests. Direct radiation can add several degrees to how warm your legs feel — but shade and wind take it away fast.