Weather at Sierra Negra: 7.3-yr statistics and a new method to estimate the temporal fraction of cloud cover

Abstract

Sierra Negra, one of the highest peaks in central Mexico, is the site of the Large Millimeter Telescope. We describe the first results of a comprehensive analysis of the weather data measured in situ from 2000 October to 2008 February to be used as a reference for future activity in the site. We compare the data from two different stations at the summit considering the accuracy of both instruments. We analysed the diurnal, seasonal and annual cycles for all the parameters. The thermal stability is remarkably good, crucial for a good performance of the telescopes. From the solar radiation data, we developed a new method to estimate the fraction of time when the sky is clear of clouds. We show that our measurements are consistent with a warm standard atmosphere model. The conditions at the site are benign and stable given its altitude, showing that Sierra Negra is an extremely good site for millimeter and high‐energy observations.

A New Method to Estimate the Temporal Fraction of Cloud Cover

“…The model was applied to estimate the daytime clear fraction for Sierra Negra (2). The results obtained are consistent with values reported by other authors using satellite data (1).…”

“…The weather in Sierra Negra, the high altitude site of the γ-ray observatory HAWC 1 and of the LMT, has been monitored since late 2000. We found its meteorological variables to conform very closely with a standard atmosphere with T 0 = 304 K an adequate boundary value, fitting P(4.1 km) ≃ 625.6 mbar at HAWC, and P(4.58 km) ≃ 569.5 mbar at LMT (2) Table 2. Layers defining the International Standard Atmosphere.…”

An estimate of the temporal fraction of cloud cover at San Pedro Mártir Observatory

“…The histogram of ψ values (fig. 11 of Carrasco et al 2009) can be well reproduced with a two‐component fit, with the first component having its maximum around ψ∼ 0.2 and the narrow peak at ψ∼ 0.75, with the minimum at ψ min = 0.55 separating both components. The narrow component is interpreted as due to direct sunshine while the broad component is originated when solar radiation is partially absorbed by clouds; we then use the relative ratio of these two components to quantify the ‘clear weather fraction’.…”

“…Using the solar radiation data recorded by the TMT site‐testing group at the SPM observatory, we applied a novel method developed by Carrasco et al (2009) to estimate the time when the sky is clear of clouds. From the normalized observed distribution of ψ: the solar flux F ( t ), divided by the nominal solar flux at the top of the atmosphere F ⊙ cos θ ⊙ ( t ) we obtained that 82.4 per cent of the time the sky is clear of clouds for airmasses z ≤ 2.…”

“…Therefore, daytime cloud cover statistics at SPM is a useful indicator of nighttime cloud conditions. Here we present a study of the cloud cover over SPM using an approach recently introduced by Carrasco et al (2009). It consists of the computation of histograms of solar radiation values measured at the site and corrected for the zenital angle of the Sun.…”