Under very dark skies the transparency of the atmosphere can change noticeably over a period of minutes.

This was easily noticeable at the GSSP on Monday night when the Milky Way would ebb and flow in size and structure after midnight. It was commented on by several very experienced observers that I was hanging out with.

Differences in contrast correlated highly with my readings using a Sky Quality Meter (SQM.) Readings of the same part of the sky both away and at the MW showed variability of 0.3 magnitudes over a couple of hours.

This is largely caused by excitation of oxygen in the upper atmosphere and can vary over scales of minutes, much as an aurora display but much more subtle.