  Long-term monitoring of the evolution of the artificial night sky brightness
is a key tool for developing science-informed public policies and assessing the
efficacy of light pollution mitigation measures. Detecting the underlying
artificial brightness trend is a challenging task, since the typical night sky
brightness signal shows a large variability with characteristic time scales
ranging from seconds to years. In order to effectively isolate the weak
signature of the effect of interest, determining the potential long term drifts
of the radiance sensing systems is crucial. If these drifts can be adequately
characterized, the raw measurements could be easily corrected for them and
transformed to a consistent scale. In this short note we report on the
progressive darkening of the signal recorded by SQM detectors belonging to
several monitoring networks, permanently installed outdoors for periods ranging
from several months to several years. The sensitivity drifts were estimated by
means of parallel measurements made at the beginning and at the end of the
evaluation periods using reference detectors of the same kind that were little
or no exposed to weathering in the intervening time. Our preliminary results
suggest that SQM detectors installed outdoors steadily increase their readings
at an average rate of +0.034 magSQM/arcsec^2 per MWh/m^2 of exposure to solar
horizontal global irradiation, that for our locations translates into
approximately +0.05 to +0.06 magSQM/arcsec^2 per year.
