You check the forecast before bed: 20°C tomorrow. You step outside at 7am in a t-shirt and immediately wish you'd grabbed a jacket. The forecast wasn't wrong. You just looked at the wrong number.
How temperature moves through the day
Air temperature follows a predictable daily cycle. The minimum occurs shortly after sunrise, when the ground has been radiating heat away all night and the sun hasn't had time to warm things back up. The maximum arrives around 2 to 3pm, when the sun's heating effect has had most of the day to accumulate. By evening the temperature is already dropping, and it continues falling through the night.
| Time | Temperature relative to daily maximum |
|---|---|
| 06:00 | 5-7°C below maximum |
| 09:00 | 3-5°C below maximum |
| 12:00 | 1-2°C below maximum |
| 14:00-15:00 | Daily maximum |
| 18:00 | 1-3°C below maximum |
| 21:00 | 4-6°C below maximum |
A forecast maximum of 20°C means you might be looking at 13°C at 7am. That's a meaningful difference when you're deciding whether a light jacket belongs in the bag.
Why mornings feel even colder than the numbers suggest
Air temperature aside, mornings compound the chill in a few ways. The sun is low on the horizon, so solar radiation adds little warmth. Dew and overnight moisture in the air give the cold a damper quality. On clear nights mist sometimes forms in valleys or low-lying areas, and mist makes a cold morning feel more penetrating even at the same temperature.
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How KorteBroekAan.nl handles this
The site uses hourly temperature data rather than daily maximums. The clothing advice for 7am is based on 7am conditions, not on what it will be at 2pm. This makes a real difference for the morning commute, a run before work, or anyone trying to figure out whether the jacket stays home or comes along.
The honest summary: a 20°C day in the forecast is a 20°C afternoon. The morning is usually something else.
The flip side of this is why evenings cool down quickly. The rest of the Weather Explained section covers the other factors that shift the felt temperature around the daily cycle.
Further reading
Related articles in the Weather Explained section:
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