As Andrey mentioned, I have the
SQM-L. MPSAS is how the brightness of the sky is measured. In terms of sky darkness, a reading of 22.0 MPSAS with the meter pointed at the zenith would indicate the darkest skies you would encounter. While a reading of 16.0 would indicate the brightest you could encounter. MPSAS is basically a scale of sky luminance.
In my case, at our dark site property I have encountered as dark as around 21.6, but on average nights it tends to be around 21.3, which is a nice sky to observe under. You are correct that it can vary over the course of an evening as conditions in the sky change. Aiming at the summer Milky Way band will give you a lower (brighter) reading, so one has to be cognizant what is in the direction of where it is aimed. While it is a pretty good tool for getting a sense of the sky's quality at a given location at a given time, it is not something that I use on a regular basis. I can get a pretty good sense of what the sky is like easily while I am observing. Then again, I do not travel to various locations. I either observe from home, which is typically around 19.2 MPSAS on the meter or at the other property, which averages about 21.3 as I mentioned.
I don't know if you have heard of the
Bortle Scale, devised by John
Bortle back around 2000, and first published in Sky & Telescope magazine. It is a method for estimating sky quality in real time, utilizing your eyes only, based on a set of criteria that John established. It can be found here:
https://skyandtelescope.org/astronomy-r ... sky-scale/
One's
Bortle quality can vary over the course of an evening as conditions shift. While it is a useful tool, it is not foolproof or easily translated from one person to the next because of individual experience levels and personal optical acuity.
Another method utilized by observers is the colored light pollution maps. These are created by using nighttime satellite imagery with an algorithm applied to estimate the spread of light glow from populated areas across terrain. They utilize a colored scale to indicate the level of "estimated" light pollution in particular locations. These can be a broadly useful tool to locate areas that are possibly darker. However, localized conditions can vary, and there can be light sources that might impact a specific geographic location that might not be accounted for in the broader scale of the maps. Also, locations nearer water, be that lakes, streams or ponds, might be more prone to fog, which is not factored into the colored maps. Even after identifying potential sites, they still need to be vetted to make certain they live up to the assumed quality. Here is one example of the mapping technology:
https://www.lightpollutionmap.info/#zoo ... kiOjg1fQ==
There are various sites that attempt to correlate the
Bortle Scale to the colored maps, but that is not with the blessing of John
Bortle. I can tell you he does not care for such attempts to combine his scale with the
LP mapping trend. So I would take such attempts with a grain of salt. Truth be told the two types utilize differing data and methodologies, so they don't necessarily play well together.
Anyway, I hope some or all of this helps. Quite frankly, whether one has an
SQM, uses the
Bortle Scale or the colored
LP maps, the ultimate indicator of the sky quality for where you are at the time you are there, are you eyes and common sense.