SQM (Sky Quality Meter) Graphs
(see below for more information on what the numbers mean)
Some help on reading the graph. If you see readings of 20.0 or higher it means that it is a pretty clear and dark night. If you see readings below 20.0 it can mean several things: clouds, haze, smoke or the moon is out. If the graph looks choppy (20.0 one reading and 18.0 the next) it is probably due to it being partly cloudy. When the clouds roll in the number will drop and when the clouds move away the number will go back up.
Magnitude per square arcsecond (MPSAS) is the definition of brightness in magnitudes spread out over one square arcsecond
of the sky.
The "magnitudes per square arcsecond" numbers are commonly used in astronomy to measure sky darkness.
For example; if the SQM meter provides a reading of 20.00 mpsas, that would be like saying that a light of a 20th
magnitude star brightness was spread over one square arcsecond of the sky.
Each magnitude lower (numerically) means just over 2.5 times as much more light is coming from a given patch of sky. A change of 5 mags/sq arcsec means the sky is 100x brighter.
The darkest I've personally experienced with the SQM in a natural clear sky was 21.20.
Here are some examples:
By convention, this is often assumed to be the average brightness of a moonless night sky that's completely free of artificial light pollution.
This is typical for a rural area with a medium-sized city not far away. It's comparable to the glow of the brightest section of the northern Milky Way, from Cygnus through Perseus.
This is typical for the outer suburbs of a major metropolis. The summer Milky Way is readily visible but severely washed out.
Typical for a suburb with widely spaced single-family homes. It's a little brighter a remote rural site at the end of nautical twilight, when the Sun is 12° below the horizon.
Bright suburb or dark urban neighborhood. It's also a typical zenith skyglow at a rural site when the Moon is full. The Milky Way is invisible, or nearly so.
Typical near the center of a major city such as New York or Boston.
The zenith skyglow at the end of civil twilight, roughly a half hour after sunset, when the Sun is 6° below the horizon. Venus and Jupiter are easy to see, but bright stars are just beginning to appear.
The zenith skyglow at sunrise or sunset.