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

Abstract

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.

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

“…The first detailed analysis of more than 10 yr of meteorological data, obtained using the TNG meteorological station at the ORM, can be found in the following two papers: Lombardi et al (2006, hereafter Paper I) and Lombardi et al (2007, hereafter Paper II). Paper I shows a complete analysis of the vertical temperature gradients and their correlation with the astronomical seeing.…”

Fraction of clear skies above astronomical sites: a new analysis from the GOES12 satellite

“…Differences in the microclimate at La Palma have been discussed in previous papers (Lombardi et al 2006, hereafter Paper I; Lombardi et al 2007, hereafter Paper II; Paper III). Paper I gives a complete analysis of the vertical temperature gradients and their correlation with the astronomical seeing.…”

An astroclimatological study of candidate sites to host an imaging atmospheric Cherenkov telescope in Romania

“…If the condensation occurs on the optical surfaces, which include the upper surface of the telescope reflector and the interior surfaces of the light concentrators, the image of the observed astronomical object degrades. Regarding the relative humidity, a value of 90 per cent was used by the NOT collaboration, by Lombardi et al (2007), and by Murdin (1985) as a threshold beyond which the observations should be stopped and the telescopes turned off. Mazin (2007) and Jabiri et al (2000) also considered that a relative humidity level higher than 90 per cent means that observing is not feasible.…”

“…Generally, an air pressure analysis is performed by the astronomical community in order to see if a certain site is dominated by high pressure, which would imply prevailing stable good weather (Lombardi et al 2007).…”