Fraction of clear skies above astronomical sites: a new analysis from the GOES12 satellite

Comparing the number of clear (cloud-free) nights available for astronomical observations is a critical task, which should be based on homogeneous methodologies. Current data are mainly based on different judgements based on observer logbooks or on different instruments. In this paper, we present a new homogeneous methodology at very different astronomical sites for modern optical astronomy, in order to quantify the fraction of available nights. The data are extracted from night-time Geostationary Operational Environmental Satellite 12 (GOES12) infrared images and compared with ground-based conditions when available. In this analysis, we introduce a wider average matrix and three-band correlation in order to reduce the noise and to distinguish between clear and stable nights. Temporal data are used for the classification. In the time interval 2007-2008, we have found that the percentages of satellite clear nights are 88 per cent at Paranal, 76 per cent at La Silla, 72.5 per cent at La Palma, 59 per cent at Mt Graham and 86.5 per cent at Tolonchar. A correlation analysis of the three GOES12 infrared bands B3, B4 and B6 indicates that the fraction of stable nights is lower by 2-20 per cent, depending on the site.