Above Image: Los Angeles & suburbs as seen from Griffith Observatory in 2018.
In order to subtract man-made emissions from their data, professional astronomers analyze and catalogue sky-glow. And although that information can be found by anyone, the data are from darker observatories commonly removed from strong urban sky-glow that many amateurs face.
So there's a disturbing gap in the knowledge of tangible urban sky-glow spectra in any astronomical literature, online or elsewhere. For the big cities, dated estimates of sky-glow are often found and the only assumed sky-glow emissions are the sodium and old mercury spectral lines. Just by glancing at the spectra below, amateurs need to worry about a great deal more.
Two decades into the millennium, it's time to update the info on the new spectra over urban centers. It turns out that each city has a unique blend of spectral features in their sky-glow.
UP-LIGHT CITY SPECTROGRAPHS
Using the easier overcast method, I visited other cities to secure their "up-light" signatures. For such overcast sky-glow spectra, the scattering is always independent of any wavelength and representative of the local up-light.
Is LED lighting good? Is it bad? How can you tell? Downtown Toronto's OVERCAST up-light spectrum:
The city of Kingston, Ontario, now uses LED systems for most of its city streets. But why are certain emission lines still apparent and distinct? (LED devices do NOT emit discreet spectral lines.)
Some American cities..
Chicago used some HUGE wattages just a few years ago, 1000w and 400w HPS were not unusual for downtown. The 2017 sky-glow of Chicago had a strong orange glow. However, away from the city proper it looked much whiter.
The following 2 spectra where obtained from the same spot on a warm clear night from Schiller Woods, near W. Irving Park and N. Cumberland Ave. First (top) aiming west over O'Hare airport, then east-southeast over Chicago (bottom), just 30 minutes apart, the two sky-glow spectra were noticeably different.
How bright is Vegas? It's bright alright and I know exactly what that brightness is made from; the overcast spectrum appears just slightly different from other cities since Vegas uses plenty of induction-lighting & metal-halide lamps.
CLEAR-NIGHT SPECTROGRAPHS
How about a clear night spectrum? Below is the Los Angeles sky-glow spectrum acquired from an isolated spot on Griffith Park in 2018:
L.A.'s sky-glow spectrum was not a big surprise. The city had converted its street-lights to LED back in 2014. Therefore, on nice and clear nights, a distinctly blue sky-glow is created.
Evolving Sky-Glow - Updated Spectra
Here are some before & after sky-glow spectra, showing the difference in composites. The first one is over my home. There was no LED lighting whatsoever in 2008.
