G. Lombardi

Canary Islands are normally interested by dominant North-East winds that, in some meteorological conditions, can transport sand at high altitude from the Sahara desert. The dust may affect the efficiency of the telescopes and decreases the transparency of the sky. In order to maximize the scientific return of the telescopes located at the ORM, we present an analysis of the atmospheric dust content and its effects on astronomical observations. B, V and I dust aerosol astronomical extinction are derived. Using a 5 years series database of data taken from the four channel TNG dust monitor, we compute a mean hourly and daily values of the dust content. We have detected particles having size 0.3, 0.5, 1.0 and 5.0 um. Using a power law we have derived the content of 10.0 um particles. We found a typical local dust concentration ranging from 3x10^6 particles per cubic meter at 0.3 um, to 10^3 at 5.0 um and 10 at 10.0 um, increasing up to 3 order of magnitudes during the dust storms, with a relative higher increase of 1.0, 5.0 and 10.0 um particles. The number of local dust storm events is the same in winter- and summertime, 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. During the sand storms La Palma Island is affected by an almost uniform layer extending up to 5 km above the sea level, down, at least the height of the telescope. The visible extinction is dominated by particles at 1.0, 5.0 and 10.0 um. In agreement with the results from Carlsberg Automatic Meridian Circle (CAMC) we find a typical extinction during dust storms of about 0.2 mag/airmass.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

This paper presents an analysis of wind speed and direction, relative humidity, and air pressure taken at Telescopio Nazionale Galileo (TNG), the Carlsberg Automatic Meridian Circle (CAMC; now called Carlsberg Meridian Telescope), and the Nordic Optical Telescope (NOT) at Observatorio del Roque de Los Muchachos, in the Canary Islands. Data are also compared in order to check local variations and both long‐ and short‐term trends of the microclimate. Furthermore, the influence of wind speed on astronomical seeing is estimated in order to better understand the influence of wide‐scale parameters on local meteorological data. The analysis is done using a statistical approach. From each long series of data, we compute the hourly, daily, and monthly averages. Particular care is taken to minimize any effect due to biases in case of missing of data. Wind direction is estimated using the annual percentage of time during which winds come from fixed directions. We found that relative humidity presents a negative correlation with temperature and pressure, while pressure is correlated with 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. Comparisons 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, which indicates prevailing stable, good weather, as is to be expected from anticyclonic conditions. Finally, short‐time variations of pressure anticipate temperature variations typically by 2–3 hr; this property diminishes in timescales higher than some hours and disappear in longer timescales.

In this paper we present an analysis of wind speed, wind direction, relative humidity and air pressure taken at TNG, CAMC and NOT at Observatorio del Roque de Los Muchachos, in the Canary Islands. Data are 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. 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. 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. 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.