It was quite foggy in some places this morning. That is why the KNMI code handed out yellow for Drenthe, Friesland, Noord-Holland and Groningen.
Locally, there was a dense fog in these areas, so that the view could be less than 200 meters. The traffic can suffer from that, according to the weather institute.
The dense fog disappeared in the course of the morning, in Drenthe the weather warning was until 9 am.