I regularly use a "Sky Quality Meter" made by Unihedron.  This nifty little handheld device measures the sky brightness in visual magnitudes per square arcsecond.

Here is a plot of data taken from Xanadu Observatory.  The measurements were taken at approximately 9:00 - 10:00 p.m. local time (0400 - 0500 UTC) when possible.  I typically averaged 3 measurements.  This was not intended to be a scientific approach but the results are still interesting.  There is an obvious trend, with the dips representing times when the moon is close to full.

Note that on dark, clear evenings, a rough average of the sky background from the observatory currently appears to be approximately 19.5 magnitudes per arcsecond.  Slightly higher readings occur in the early morning hours when artificial lighting is reduced.  Lower readings on the chart above represent evenings when cloud cover or moon phase result in a brighter sky (at full moon, readings are below 15).

Click here to see a Light Pollution Map for Xanadu Observatory.  The SQM readings obtained by me are consistent with this map.  As you can see, Xanadu is certainly not situated in an ideal observing location, despite Tucson's reputation as an astronomy-friendly community.

For non-scientific comparison purposes, I have taken readings at the following locations:

• A dark site in Southern AZ (about 45 min. from Xanadu), moon ~ 53% but not yet risen -->  21.41

• Prude Ranch, site of the Texas Star Party, near New Moon --> 21.76

• Farnsworth Ranch, site of the All-Arizona Messier Marathon, near New Moon --> 21.46

• YMCA Camp Oakes, site of the RTMC Astronomy Expo, near New Moon --> 21.18

• Vekol Road site, near Casa Grande off I-8, near New Moon --> 21.34

• Starizona parking lot at new moon (alongside major road, near shopping mall) --> 17.65

• Clear Creek campground near Cottonwood, AZ, moon ~ 73%, reading ~ 90 degrees away --> 19.78

• A backyard in Claremont, CA, moon  ~ 80% but not yet risen --> 17.21

• Main Street USA in Disneyland Park, moon ~ 73% but not yet risen --> 14.97

Don't understand the above graph?

• Learn about the Magnitude system here.

• Here's a nice website that explains what "Julian Date" means.