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.
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