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

TL;DR

This study compares ground and satellite data to quantify clear sky fractions at La Palma and Mt. Graham, aiding astronomical site selection by assessing available telescope time.

Contribution

It provides a comparative analysis of ground and satellite observations to determine clear nights at two major astronomical sites, validating satellite data with ground measurements.

Findings

At La Palma, 62.6% clear nights from ground, 71.9% from satellite.

At Mt. Graham, 45% clear nights from satellite data, with high agreement between different satellite sources.

Satellite data shows more dispersion at Mt. Graham compared to La Palma.

Abstract

The fraction of available telescope time is one of the most important requirements for selecting astronomical sites affecting the performance of ground based telescopes. A quantitative survey of clouds coverage at La Palma and Mt.Graham is presented using both ground and satellite based data. The aim of this work is deriving clear nights for the satellite infrared channels and verifying the results using ground based observations. At La Palma we found a mean percentage of clear nights of 62.6% from ground and 71.9% from satellite. Taking into account the fraction of common nights we found a concordance of 80.7% clear nights from ground and satellite. At Mt.Graham we found a 97% of agreement between Columbine heliograph and night time observing log. From Columbine heliograph and TOMS-OMI satellite we found about 45% of clear nights, while satellite data (GOES, TOMS) are much more dispersed than those ones of La Palma. Setting a statistical threshold we retried a comparable seasonal trend between heliograph and satellite.