Mauna Kea Sky Transparency from CFHT SkyProbe Data

Nighttime sky-transparency statistics on Mauna Kea are reported based on data from the Canada-France-Hawaii Telescope SkyProbe monitor. We focus on the period beginning with the start of MegaCam wide-field optical imager operations in 2003 and continuing for almost three years. Skies were clear enough to observe on 76% of those nights; attenuations were less than 0.2 mag up to 60% of the time. An empirical model of cloud attenuation and duration is presented allowing us to further characterize the photometric conditions. This is a good fit to the SkyProbe data and indicates that Mauna Kea skies are truly photometric (without clouds) an average of 56% of the time, with moderate seasonal variation. Continuous monitoring of transparency during the night is necessary to overcome fluctuations in attenuation due to thin clouds.