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 } ~ { } { arcsec } ^ { -2 } when the sun was more than 12 \arcdeg below the horizon , reaching 20.7 ~ { } { mag } ~ { } { arcsec } ^ { -2 } 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 ( \leq 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 \pm 3 % of the time at PEARL in winter , and that a higher elevation location nearby may be better .