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Context. In this paper we present an analysis of wind speed, wind direction, relative humidity and air pressure taken at Telescopio Nazionale Galileo (TNG), Carlsberg Automatic Meridian Circle (CAMC; now called Carlsberg Meridian Telescope) and Nordic Optical Telescope (NOT) at Observatorio del Roque de Los Muchachos, in the Canary Islands. Aims. Data are also compared in order to check local variations and both long term and short term trends of the microclimate. Furthermore, influence of wind speed on the astronomical seeing is estimated to the aim to better understand the influence of wide scale parameters on local meteorological data. Methods. The analysis is done using a statistical approach. From each long series of data we compute the hourly, daily and monthly averages. A particular care is used to minimize any effect due to biases in case of missing of data. Wind direction is estimated by the annual percentage of the time in which the wind come from fixed directions. Results. We found that relative humidity presents a negative correlation with temperature and pressure, while pressure is correlated to the temperature. The three telescopes show different prevailing wind direction, wind speed, relative humidity and air pressure confirming differences in local microclimate. We found that seeing deteriorates when wind speed is lower than 3.3 m s −1 . Comparison in terms of wind speed and high relative humidity (> 90%) shows that TNG seems to have optimal observational conditions with respect to CAMC and NOT. Air pressure analysis shows that ORM is dominated by high pressure, that means prevailing stable good weather, as is to be expected from anticyclonic conditions. Finally, short time variations of pressure anticipate temperature variations tipically by 2-3 hours, this property vanishes in time scales higher than some hours and disappear in longer time scales.

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 data bases from Paranal and Carlsberg Automatic Meridian Circle (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 analysed sites have been found. Temperature has increasing trends in both observatories with somewhat higher evidence at the ORM. Seasonal variations of pressure at Paranal have been highly decreasing since 1989, and we do not see the same phenomenon at the ORM. The two sites are dominated by high pressure. In cold seasons, relative humidity (RH) is lower than 60 per cent at CAMC and 15 per cent at Paranal. In warm seasons, RH is lower than 40 per cent at CAMC and 20 per cent 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 the ORM are subject to pronounced local variations

We present an analysis of temperature taken at two telescopes located at the Observatorio del Roque de Los Muchachos, in the Canary Islands. More than 20 years of measurements at the Carlsberg Automatic Meridian Circle (CAMC; now called the Carlsberg Meridian Telescope) are included. Analyses of data from the Telescopio Nazionale Galileo (TNG) and CAMC are compared in order to check local variations and long-term trends. Furthermore, the temperatures at different heights are correlated with the quality of astronomical seeing. We consider a correlation between the North Atlantic Oscillation (NAO) Index and annual downtime with mean annual temperatures. The final aim of this work is to better understand the influence of wide-scale parameters on local meteorological data. The analysis is performed using a statistical approach. From each long series of data, we compute the hourly averages and then the monthly averages in order to reduce the short-term fluctuations due to the day-night cycle. Particular care is taken to minimize any effect due to biases due to missing data. Finally, we compute the annual average from monthly averages. The two telescopes show similar trends. There is an increase in temperature of about 1.0ЊC per 10 yr from the annual means, and in addition, the annual minimums increase more rapidly than the maximums. We find that a positive NAO Index reduces the increase of temperature and accelerates the decrease. Moreover, there is no evidence that a positive NAO Index corresponds to a lower number of nonobservable nights. Finally, seeing deteriorates when the gradient of temperatures between 2 and 10 m above the ground is greater than Ϫ0.6ЊC.

Context. It is known that the Canary Islands are normally affected by dominant winds flowing from north-northeast, that in some meteorological conditions, can transport sand from the Sahara desert at high altitude. The dust may affect the efficiency of the telescopes and decrease the transparency of the sky. Aims. To maximize the scientific return of the telescopes located at the Observatorio del Roque de Los Muchachos (ORM), we present an analysis of the atmospheric dust content and its effects on astronomical observations. Than B, V and I dust aerosol astronomical extinction are derived. Methods. Using a 5-year series database of data taken from a dust monitor located inside the Telescopio Nazionale Galileo (TNG) dome, we computed mean hourly and daily values of the dust content as measured with a four-channel dust monitor. Results. We detected particles of 0.3, 0.5, 1.0, and 5.0 µm. Furthermore, using a power law we derived the content of 10.0 µm particles. We found a typical local dust concentration ranging from 3 × 10 6 particles per cubic metre at 0.3 µm, to 10 3 at 5.0 µm and 10 at 10.0 µm, increasing up to 3 orders of magnitudes during the dust storms, with a relatively higher increase of 1.0, 5.0, and 10.0 µm particles. The number of local dust storm events is the same in the local winter and summer, but the average background and storm-related increases in the dust concentration in summer are significantly higher than in winter. In a uniform approximation, during the dust storms, an average height of the dust layer of 2.5 km above the telescope is inferred. Conclusions. During the sand storms, La Palma Island is affected by an almost uniform layer extending up to 5 km above the sea level. The visible extinction is dominated by particles at 1.0, 5.0 and 10.0 µm. In agreement with the results from Carlsberg Automatic Meridian Circle (CAMC), we find a typical extinction of about 0.2 mag airmass −1 during dust storms.