  Nighttime visible-light sky brightness and transparency are reported for the
Polar Environment Research Laboratory (PEARL), located on a 610-m high ridge
near the Eureka research station, on Ellesmere Island, Canada. Photometry of
Polaris obtained in V band with the PEARL All Sky Imager (PASI) over two
winters is supported by standard meteorological measurements and visual
estimates of sky conditions from sea level. These data show that during the
period of the study, October through March of 2008/09 and 2009/10, the sky near
zenith had a mean surface brightness of 19.7 mag/square-arcsec when the sun was
more than 12 deg below the horizon, reaching 20.7 mag/square-arcsec during
astronomical darkness with no moon. Skies were without thick cloud and
potentially usable for astronomy 86% of the time (extinction <2 mag). Up to 68%
of the time was spectroscopic (<0.5 mag), attenuated by ice crystals, or clear
with stable atmospheric transparency. Those conditions can persist for over 100
hours at a time. Further analysis suggests the sky was entirely free of ice
crystals (truly photometric) 48+/-3% of the time at PEARL in winter, and that a
higher elevation location nearby may be better.
