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:
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?