As we saw in part 1 and part 2 of this series the typical measurements of sky brightness in Providence are between about 4.1 - 4.3 nelm (naked eye limiting magnitude) on clear nights. Here is a graph that shows a typical hazy summer night. The readings were taken on the night of July 1st into the morning of July 2nd of 2014 and are in the range that we commonly see. The dashed horizontal line is a somewhat arbitrary divider between typical and darker nights. When the sky brightness is below about 4.3 the observing is much better.
Looking at a graph of the sky brightness doesn't give an intuitive idea of what the sky actually looked like for observing. We can see this by looking at the wide angle views of the sky using the camera mounted on the roof. Here is a time lapse movie from the same night as the above graph.
Each second of video shows 5 minutes of changes in the sky above the Observatory. The entire night is compressed into one and a half minutes. The bright streaks zipping across the sky are airplanes. The lower left corner is southwest which is the direction of downtown Providence. The sky is always much brighter in the that direction due to outdoor lighting. The camera is more sensitive than the human eye and uses a filter to reduce the brightness of street lights. We can catch a glimpse of the Milky Way despite the fact that it was not visible to the human eye.
The quality of video uploaded to this blog is not that good due to aggressive compression. A better copy is here. The size is about 65 MB so it could take a couple of minutes to download using a slower connection.
We've see many nights during the year like this one when there is a lot of moisture in the atmosphere which scatters light and brightens the sky.