SQM Reading

[JemC]

just took a SQM reading of the night sky from my back garden

I got a reading of 
SQM: 19.71 (5.91)

Seeing as i don't fully understand this or what the numbers mean?

is that a good or bad reading, a below average/above average or just plain normal for suburban area

appreciate any kind of explanation to a simpleton  (layman's terms if possible)

Regards

[vlaiv]

It is describing sky brightness in magnitudes per arcsecond squared (per surface area) - 19.71 mag/arcsec^2.

That is moderate LP site, or in Bortle scale it is ~5 (yellow zone). It appears that second number you quoted is actually number on Bortle scale.

Use following chart to do the conversion:

Look at the second column to compare SQM reading to.

You can also use following site to compare:

https://www.lightpollutionmap.info/#zoom=4&lat=5759860&lon=1619364&layers=B0FFFTFFFF

Just find your location and click on it (given atlas 2015 overlay) - it should give you average SQM reading obtained from satellites and some clever algorithms. There is also option to view actual SQM readings people took. Just enable it on right hand side panel under "SQM/SQC overlay & filters".

You can also contribute to this website by posting your own SQM readings.

[Scooot]

This is a good read as well.

http://www.skyandtelescope.com/astronomy-resources/rate-your-skyglow/