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I remember several years ago, the weather report on the VRT (Flemish national media) would show the temperature forecast of the coming days and it would also show the uncertainty on that temperature. I have included a picture of an example:

enter image description here

The x-axis shows the days (in Dutch) and the y-axis the temperature. The gray area around the white predicted line is, I presume, the uncertainty. As you may notice, the magnitude of the uncertainty fluctuates with a period of a day. The uncertainty seems the smallest at the hottest time of the day, whereas at the coolest times, it seems the largest.

Why is this? Is it because of an attractor in the meteorological system? Or is there another explanation?

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This is typical of the winter weather in the Netherlands (I live there) and Belgium as well. In a period with maximum temperatures below 0, whether it's going to be cold or very cold in the night depends mainly on two things: the presence of any clouds which preserve the 'heat' from the day, and the presence of wind (especially from the south) which transports relatively warm air from elsewhere. During the day, the Sun can help with warming up (but not much, since it's not high up in the air), mitigating at least the 'clouds' factor.

Here's a different graph, taken September 6th from a Dutch weather forecast site (archive link). We currently have sort-of an Indian Summer and the uncertainties on the daily highs are comparable, and on the first few days even larger than the night temperatures.

enter image description here

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  • $\begingroup$ So why is it different in when it's warmer? Do you mean in winter the heating during the day is almost the same with or without clouds? And how about wind during the day? $\endgroup$
    – Lu Kas
    Sep 6 at 15:50
  • $\begingroup$ @Lu Kas would think the difference may well be airmass (absolute) moisture. Polar airmass being much drier permits the possibility of a greater nighttime dropoff if those clouds/wind allow it. Whereas in the summer there's less room to drop because the dewpoint is nearer the temperature due to being a more consistent regional airmass. That'd match us in Florida and much of the US. Air pressure climatology may also be vital in some places (nearer high pressure = less wind and perhaps less clouds, allowing more uncertainty... so whichever season that's more common in a spot alters the pattern) $\endgroup$ Sep 6 at 21:53

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