Its been puzzling me for a while this one too, as you would indeed expect the fog to form on the park first, due to lower air temperature. Something that has just entered my head is that in fog forming conditions, the air would be relatively still, and
maybe as a result of relatively low circulation, the air quality is significantly better over the park in terms of particulate pollutants. Its often small airborne particles that cause cloud formation (condensation nucleii - silver nitrate IIRC being favoured by the 'rainmakers' of the Russian steppes & American prairies) so its possible the cleaner air in the park area could be less prone to fog formation...just a thought
