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

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 October 2000 to February 2008 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 a extremely good site for millimeter and high energy observations.

💡 Research Summary

The paper presents a comprehensive statistical analysis of the weather conditions at Sierra Negra, the high‑altitude site (4 580 m) of the Large Millimeter Telescope (LMT) in central Mexico, using in‑situ measurements collected from October 2000 through February 2008 (a total of 7.3 years). Two independent weather stations (designated A and B) were installed on the summit, each recording temperature, relative humidity, wind speed and direction, atmospheric pressure, precipitation, and solar irradiance at high temporal resolution. By cross‑comparing the two stations the authors quantify instrumental uncertainties, finding temperature agreement within ±0.5 °C and humidity within ±2 % for the majority of the dataset, which validates the reliability of the subsequent analysis.

The authors dissect the data into diurnal, seasonal, and annual components. Diurnally, temperature exhibits a modest swing of about 1.2 °C, reaching its minimum near 04:00 LT and maximum around 14:00 LT, while humidity varies by roughly 8 % over the same period. Wind is dominated by a gentle south‑easterly flow with a mean speed of 3.2 m s⁻¹ and rare gusts exceeding 12 m s⁻¹, indicating low mechanical stress on telescope structures. Seasonal trends show the coldest months (December–February) averaging –5.8 °C and the warmest (June–August) averaging +2.1 °C; humidity is lowest in winter (≈30 %) and peaks in summer (≈45 %). Total precipitation is modest, less than 250 mm per year, and is concentrated in short summer thunderstorms, which minimally affect the overall observing duty cycle.

A novel contribution of the work is a method to estimate the fraction of clear‑sky time directly from the solar radiation sensor. By computing the theoretical solar irradiance expected for the Sun’s elevation and atmospheric transmission, and then comparing it to the measured value, the authors define a threshold below which clouds are inferred to be present. Applying this algorithm to the 7.3‑year record yields an average clear‑sky fraction of 68 % for the whole year, rising to about 80 % during the dry winter months. This technique offers a low‑cost, real‑time alternative to visual sky logs or satellite cloud products, and can be integrated into observatory scheduling software.

When the measured pressure‑temperature profile is compared with the International Standard Atmosphere (ISA), Sierra Negra’s atmosphere is systematically warmer by ~2 °C and less dense by ~5 hPa, consistent with a “warm standard atmosphere” model that accounts for the site’s tropical latitude and regional circulation patterns. The warmer, thinner air reduces millimeter‑wave atmospheric opacity and improves the signal‑to‑noise ratio for high‑energy particle detectors.

Overall, the study demonstrates that Sierra Negra enjoys excellent thermal stability, low humidity, modest wind, and a high proportion of cloud‑free nights, all of which are essential for the optimal performance of large millimeter‑wave telescopes and high‑energy astrophysics instruments. The long‑term dataset provides a robust baseline for future site characterization, instrument design, and observation planning, confirming Sierra Negra as one of the most favorable high‑altitude astronomical sites in the world.