Just to come back to the main topic....
Yes a cloud has a limit everywhere in the world, yes it might happen elsewhere, but meteorology is not just a matter of clouds limit.
Clouds were not created once upon a time, and then travel the world following the winds.
The thing is spa is build in the Belgian Fenns (??... fagnes in French)
This is the only circuit I know that is build in a region with these characteristics : woods surrendering the circuit, some zone surrending the circuit are very humid -> marsh. The nurb is not far away but it s just woods I think.
The circuit is also not at sea level. It s not very high either (400m) but it does also make a difference.
So, very humid in some zone, not so humid in others make it a circuit with higher probability then others.
Don t tell me Miami is the same because Miami is a tropical weather where spa is "temperated" and there s no direct influence of the see (winds, etc)
An other example of the particularity of spa, is on the length, there are circuits longer than spa which are not supposed to be wet in some area, while others remain dry.
For exemple, le mans. A team director had a cousin living in the suburb area of le mans where he knew that it was the first zone where rain would appear if it starts raining. So, during the race, the cousin was in his backyard with his phone waiting for drops. When he felt one, this team was the first one to pit and not loose time because when the rain comes, it comes fast and it rains everywhere on the circuit (generally).