You know the feeling. Terrace in full sun, warm enough to sit without a jacket. Then a cloud drifts across and the temperature seems to drop five degrees in ten seconds. Nothing has changed in the air. What vanished was the radiant warmth coming directly from the sun.
Why the gap exists
Official temperature measurements are taken in shade, by design. The thermometer is protected from direct sunlight so it reads the actual air temperature rather than the combination of air temperature and solar radiation. That's the right way to get a consistent, comparable number. But it means the temperature in your forecast is specifically not the temperature you experience when you're sitting in direct sun.
On a clear summer day, direct sunshine adds 6 to 10 degrees of perceived warmth above the air temperature. In spring and autumn the effect is smaller, typically 3 to 5 degrees. Move into shade and that addition disappears almost instantly.
| Conditions | Air temperature | Feels-like |
|---|---|---|
| Full sun, no wind | 18°C | ~24°C |
| Shade, no wind | 18°C | ~17°C |
| Full sun, light breeze | 18°C | ~21°C |
| Shade, light breeze | 18°C | ~15°C |
On an 18°C day the difference between sitting in sun and sitting in shade can be 6 to 9 degrees. That's the difference between shorts being comfortable and reaching for a light jacket.
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The wind complication
Wind interacts with shade in an interesting way. In the sun, a breeze takes the edge off the solar warmth but still leaves you comfortable. In shade, the same breeze has nothing to offset, so it just cools you. The combination of shade and wind is where a seemingly mild day starts to feel properly cold.
How KorteBroekAan.nl accounts for it
Cloud cover percentage is factored into the feels-like temperature calculation, which partially captures this effect. Full overcast approximates shade; a clear sky approximates direct sun. The transition between the two, on days with broken cloud, is harder to pin down exactly, but the site will at least give you a better estimate than the raw air temperature alone.
The article on why direct sunlight feels warmer covers the solar radiation side of this in more detail. Other factors that contribute to the full picture are in the Weather Explained section.
Further reading
Related articles in the Weather Explained section: