Analysis of the fraction of clear sky at the La Palma and Mt Graham sites

The amount of available telescope time is one of the most important requirements when selecting astronomical sites, as it affects the performance of ground-based telescopes. We present a quantitative survey of cloud coverage at La Palma and Mt Graham using both ground- and satellite-based data. The aim of this work is to derive clear nights for the satellite infrared channels and to verify the results using ground-based observations. At La Palma, we found a mean percentage of clear nights of 62.6 per cent from ground-based data and 71.9 per cent from satellite-based data. Taking into account the fraction of common nights, we found a concordance of 80.7 per cent of clear nights for ground- and satellite-based data. At Mt Graham, we found a 97 per cent agreement between the Columbine heliograph and the night-time observing log. From the Columbine heliograph and the Total Ozone Mapping Spectrometer-Ozone Monitoring Instrument (TOMS-OMI) satellite, we found that about 45 per cent of nights were clear, while satellite data (GOES, TOMS) are much more dispersed than those of La Palma. Setting a statistical threshold, we retried a comparable seasonal trend between the heliograph and satellite.