# Meteorological observations since 1882 at the Pic du Midi, Central Pyrénées

Jean Dessens

► To cite this version:

Jean Dessens. Meteorological observations since 1882 at the Pic du Midi, Central Pyrénées. Jahres- bericht des Sonnblick-Vereines für das Jahr 2002, Sonnblick Vereines, pp.16-18, 2004. hal-00136446

HAL Id: hal-00136446

https://hal.science/hal-00136446v1

Submitted on 15 Sep 2021

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
# METEOROLOGICAL OBSERVATIONS SINCE 1882 AT THE PIC DU MIDI, CENTRAL PYRENEES

Jean Dessens

Laboratoire d'Aérologie (UMR 5560), Observatoire Midi-Pyrénées,

Université de Toulouse III

The web site of the Observatoire Midi-Pyrénées (www obs mip fr) relates in detail the history of the Pic du Midi observatory. It all started in 1872 with the project of the Société Ramond (a Pyreneist association) to build an observatory at the top of the Pic du Midi (43°04′N, 0°09′E). Thanks to a bold and demonstrative series of meteorological observations made in a temporary station located a few hundred meters below the summit, the project was founded, and the observatory opened in 1882. The peak culminates at 2862 m MSL in the highest part of the Pyrenees, in a perfect geographical isolation already highlighted by

one of the founders of the observatory:

From its summit, one immediately looks down, on one half of the horizon, upon the plain that spreads northwards as far as the eye can reach; on the other half, one can see the high summits of the range rising, from the Pic du Midi d'Ossau and the Rhune to the Maladetta (de Nansouty, 1874).

Another characteristic situation is that Central Pyrenees are under a definite maritime influence, since the Atlantic Ocean is only 160 km to the prevailing wind direction without any perturbing orography.

At the time of its foundation, the observatory was mainly devoted to meteorology, but a progressive conversion to astronomy occurred in the early 20th century. However, a meteorological station operated by professional observers was maintained until 1984. The data are safely recorded by the Observatoire Midi-Pyrénées in hand-written registers for the years 1882 to 1942, and by Météo-France for the post-war period. About fifteen years ago, Dr A. Bücher went through the data and digitized the monthly mean values of atmospheric pressure, minimum and maximum air temperatures (Tmin, Tmax), relative humidity, cloudiness, and precipitation. Some interesting trends and correlations were thus found, in relation with climate change:

<figure><img src="image_2.png" /><figcaption>A View of the Pic du Midi taken from Lannemezan, 28 km to the north (photo by Pham Van Dinh)</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>The Observatory of the Pic du Midi in May 1997 (photo by Sandrine Bernard)</figcaption></figure>

1. Between 1882 and 1970, Tmin has increased in all seasons. The Tmin annual trend during this period reaches +2.1°C. Tmax has also increased in the fall, but has decreased in other seasons. The Tmax annual trend is only -0.4°C. These trends have been confirmed for the 1882 to 1984 period, after correction of a systematic shift which occurred in 1971 due to a relocation of the station (Dessens and Bücher, 1995).
2. Relative humidity and cloud cover have also significantly increased during the years 1882 to 1984 (Dessens and Bücher, 1995), an illustration of "increasing cloud in a warming world" (Henderson-Sellers, 1986).

3. Precipitation records at a mountain station are subject to caution. The Pic du Midi precipitation data are homogeneous only during the years 1882 to 1922, for which no significant trend appears. However, seasonal relationships between temperature and precipitation suggest that the increase in temperature may have resulted in a decrease in winter and spring precipitation (Dessens and Bücher, 1997).

4. The only noticeable fact found in the atmospheric pressure data is a significant positive trend in the fall (+1.56 hPa), which compares favourably with the temperature increase observed in the same season (+1.78°C) (Toumi et al., 1999). Trends in other seasons are smaller and not statistically significant. Unfortunately, pressure data for the years 1942 to 1951 have been lost.

After a closure of the meteorological station from 1985 to 1993, Météo-France and the Observatoire Midi-Pyrénées now operate an automatic station. The 1882-2002 Tmin and Tmax series have been compared with data in the surrounding plain of southwestern France. A bivariate test shows that the homogeneity for the Tmax series is excellent (after removing the 1882 and 1883 data), but the measured Tmin values must be increased by 0.6°C for the automatic station, which is not exactly at the same location as the former one. When this correction is applied, the 1884-2002 series is perfectly homogeneous. Tmax has not changed along the 120 year period (+0.2°C, not statistically significant), while Tmin has increased by 3.0°C (Fig. 1). In consequence, the diurnal temperature range has decreased by 2.8°C, and its interannual variations, which are smaller than those of Tmin and Tmax, reveal a strange, unheard of oscillation (Fig. 2).

<figure><img src="image_4.png" /><figcaption>Figure 1: Time series and trend lines of the annual mean minimum and maximum temperatures at the Pic du Midi.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 2: Time series and trend lines of the annual diurnal temperature range (DTR) and mean temperature at the Pic du Midi.</figcaption></figure>

The recent period (1985-2002) has been especially warm, and the year 1997 has beaten the absolute records. In the years to come, the mean annual 0°C isotherm level in the Pyrenees will meet exactly the top of the Pic du Midi. This level has risen by some 240 m since 1884, which could explain the fast glacier recession observed in the Pyrenees. The glaciologists of the young Pyrenean association named "Moraine" (www.moraine.fr.st) estimate that the total glacier area for the Pyrenees has decreased from 25 km² in 1905 to 5 km² in 2000. One may also wonder if the warming, which is solely due to an increase in nighttime temperature, has the same effect on glacier melting than a warming which would be evenly distributed over night and day.

Because of climate changes and environmental issues, mountain observatories are becoming of paramount interest again. The Pic du Midi is included within a new French project, called ORE-PAES (Observatoire de Recherche en Environnement-Pollution Atmosphérique à Echelle Synoptique), which intends to measure air pollution at five mountain stations in France. This project will insure the continuation of the long series of observations made at the Pic du Midi.
## References

Bücher, A., and J. Dessens, 1991: Secular trend of surface temperature at an elevated observatory in the Pyrenees. J. Climate., 4, 859-868.

Dessens, J., and A. Bücher, 1995: Changes in minimum and maximum temperatures at the Pic du Midi in relation with humidity and cloudiness, 1882-1984. Atmos. Res., 37, 147-162.

Dessens, J., and A. Bücher, 1997: A critical examination of the precipitation records at the Pic du Midi observatory, Pyrenees, France. Climatic Change, 36, 345-353.

Henderson-Sellers, A., 1986: Increasing cloud in a warming world. Climatic Change, 9, 267-309.

Nansouty, de, C., 1874: Troisième note relative à la création d'un observatoire météorologique sur le Pic du Midi de Bigorre. Société Ramond, Imprimerie J. Cazenave, Bagnères, p. 7.

Toumi, R., N. Hartell and K. Bignell, 1999: Mountain station pressure as an indicator of climate change. Geophys. Res. Lett., 26, 1751-1754.

## Author address:

Jean Dessens
Centre de Recherches Atmosphériques
65300 Campistrous, France
E-mail: jean.dessens@anelfa.asso.fr

Weitere Beschreibungen der meteorologischen Einrichtungen des Observatoriums am Pic du Midi sind im XIII. Jahresbericht (1904) des Sonnblick-Vereines auf den Seiten 8-17 sowie im 60.-62. Jahresbericht des Sonnblick-Vereins für die Jahre 1962-1964 auf den Seiten 53-55 zu finden.