El Roque de Los Muchachos Site Characteristics. II. Analysis of Wind, Relative Humidity, and Air Pressure

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Abstract
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.
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