The astroclimatological comparison of the Paranal Observatory and El Roque de Los Muchachos Observatory

The new extremely large telescope projects need accurate evaluation of the candidate sites. In this paper we present the astroclimatological comparison between the Paranal Observatory, located on the coast of the Atacama Desert (Chile), and the Observatorio del Roque de Los Muchachos (ORM), located in La Palma (Canary Islands). We apply a statistical analysis using long term databases from Paranal and Carlsberg Meridian Telescope (CAMC) weather stations. The monthly, seasonal and annual averages of the main synoptical parameters in the two sites are computed. We compare the long term trends in order to understand the main differences between the two sites. Significant differences between the two analyzed sites have been found. Temperature have increasing trends in both observatories with somewhat higher evidence at ORM. Seasonal variations of pressure at Paranal have been highly decreasing since 1989 and we do not see the same phenomenon at ORM. The two sites are dominated by high pressure. In cold seasons RH is lower than 60% at CAMC and 15% at Paranal. In warm seasons RH is lower than 40% at CAMC and 20% at Paranal. The analysis of the dew point has shown better conditions at Paranal with respect to CAMC in winter, autumn and spring before 2001, while the two sites are becoming similar afterwards. Winds at ORM are subject to pronunced local variations.