Zenithal night sky brightness is one of the most used measurements of the level of light pollution on a given place. Using the standard measurement of magnitude per squared arcsecond (mag/arcsec^2 or mpsas), higher values represent darker skies. Far from light pollution sources, it is possible to reach above 22 mpsas on clear moonless nights, during astronomical night (Sun 18° below the horizon). That is the case of a pristine sky, nowadays rare. By contrast, the values of measurements over a light polluted city centre under the same astronomical conditions may not exceed 18 or 19 mpsas.
The measurements of the night sky brightness are dependent of the presence of clouds. Every estimate of the light pollution on a given place has to attend to this factor. The presence of clouds can be inferred from the graphs. On a strongly light polluted site, artificial light is reflected off the clouds increasing the total night sky brightness, when values as low as 14 or 13 mpsas could be reached. On a dark sky site, the opposite happens: clouds block the tenuous light from the stars, the natural skyglow and the Milky Way. In those circumstances, values could reach 23 mpsas.
Our measurements are taken every night using dedicated photometers, Unihedron Dark Sky Meters (one SQM-LU model, mounted at the Astronomical Observatory of Coimbra, and one SQM-LR model, installed at Escola Superior de Saúde do Politécnico do Porto). The SQM mounted at Escola Superior de Saúde do Politécnico do Porto courtesy of Salvador Bará – Facultade de Física da Universidade de Santiago de Compostela, Galicia. Open access data. Please credit as: “Data from Raul C. Lima (CITEUC & ESS- PPorto), available from the Space Weather service SPINLab (Space-Planetary Interactions Monitoring and Forecasting Laboratory).”
The plots are produced using the free software PySQM. PySQM is a multi-platform, open-source software designed to read and plot data from Unihedron SQM-LE and SQM-LU photometers, giving as output files complying with the International Dark Sky Association (IDA) NSBM Community Standards for Reporting Skyglow Observations (http://www.darksky.org/night-sky-conservation/248). PySQM is distributed under GNU GPL, either version 3 of the License, or (at your option) any later version. See the file LICENSE.txt for details. This software has been developed by Mireia Nievas (UCM) with the invaluable help of: Jaime Zamorano (UCM), Laura Barbas (OAN) & Pablo de Vicente (OAN).
Coordinates of the SQM at ESS-PPorto: Porto, 41.17734693977744N, - 8.606102155804138, alt. 109 m