Actually, having flown yesterday both morning and afternoon, the visibility was variously reported as around 8-9 km. It was reasonably hazy, and I was grateful for the traffic service I had for the afternoon flight in the afternoon which was mostly in a westerly direction (towards low sun in haze) because spotting stuff was hard enough even knowing where to look.
I'm not saying these conditions may or may not have been contributory, just saying that conditions "up top" are not necessarily that easy to ascertain from one's garden.