This morning, I spent a good hour or so on the ground as a passenger on an airline flight from EDDL Dusseldorf to EDDH Hamburg.
We were denied start-up clearance – according to the captain’s announcement due to weather (i.e. fog/vis) at the destination. I was bored, so I checked the METAR and TAF right after the announcement and again, a few minutes for an updated METAR. The initial one reported 7000 NSW and looked ok – even for VFR – and 15 minutes later METAR and TAF were as shown below, with a slightly decreased vis and a low scattered layer at 300ft.
I was surprised that this would cause such delays! I can see how Low Visibility Procedures can cause significant delays at an airport, but the conditions as stated above don’t seem to call for that? The 300ft cloud base can’t be the problem and the temp 0500 from the TAF didn’t seem like they had actually occured during the time frame in question.
Maybe the crew based up their slot due to other reasons (in fact, we had passengers still boarding 10 min after scheduled departure)? 
Of course it could be the captain was simplifying the matter for the “great unwashed”.
It could be Euro control where slot restrictions in part caused by the weather (and possibly other factors) the French have been introducing changes which have caused havoc and which all in all translated to the “simple” explanaton that it was due to weather which we all understand. :-}
Two possible explanations
1) Bald-faced lie.
2) flow management
With TEMPO fog reducing the flow rate during a time period (remember, TEMPO means the forecaster thinks it WILL happen), ATC reduces the number of aircraft they can accept during that period. This is to minimise not only holding times, but also the peak number of aircraft in the holding stacks. If this happens during “rush hour”, aircraft can be held on the ground just because of the forecast.
My guess is flow management as well.
The local ATC will transmit a flow rate to Eurocontrol who in turn will issue slots accordingly. If the rate was set to the worst case scenario, it will correspond to the FZFG condition an inbound flow will be reduced accordingly. I reckon the pilot who did that announcement thought it was enough to tell his pax it’s due to weather, where he could have said they have an ATC slot caused by bad weatherforecasts.
