I've noticed the last four or five years that the northwest sky during the summer appears darker to my eye than the southeast sky, with the Milky Way in between. I've measured the difference with my SQM and generally get about a 0.1 difference - for instance, if I get a reading of ~21.70 in the northwest sky I'll get ~21.60 in the southeast sky. I've seen this at the Golden State Star Party and the Oregon Star Party but not at sites with overall brighter skies. No doubt the summer sky has always been this way, and I just started to notice the difference a few years ago, but the more I see it the more I wonder what's going on.

Has anyone else noticed this difference and know what causes it?