Borgo Grotta Gigante meteorological station
Rete Osservatori Storici Meteorologici Italiani - ROSMI
by Renato R. Colucci
Lat. 45°42'31.6" N - Long. 13°45'53.8" E Gr. - 1°18'45.4" E M.M. Roma
Altitude 276 m s.l.m.m. - Distance from the sea in straight line: 2800 m​​
Images of the Borgo Grotta Gigante meteorological station, located on the lawn in front of the Grotta Gigante Visitor Center. Notably featured are the white-painted wooden Stevenson Screens, housing the instruments.
In the garden located in front of the Grotta Gigante visitor centre, is located the historical weather station. Its installation took place in 1966, and the observational site, which officially began data collection on January 1st, 1967, represents the only homogeneous and continuous meteorological data record for the Trieste Karst area, covering a climatological time span exceeding the minimum standard defined by the WMO.
The Borgo Grotta Gigante weather station has continued operating over the years thanks to the valuable efforts of the “E. Boegan” Cave Commission (CGEB) of the Società Alpina delle Giulie (SAG), which has carried forward the legacy of those who laid the foundations for the project in the 1960s.
In 1966, Fabio Forti, assisted by Prof. Giuseppe Nangeroni of the National Research Council (CNR), who contributed instruments provided by the institution, envisioned installing a shelter for external meteorological measurements. This served as a "benchmark" for the pioneering studies in hypogean meteorology conducted at the time in the Grotta Gigante and the nearby Grotta Doria.
Winter view of the meteorological station of Borgo Grotta Gigante after the heavy snowfall at the end of December 2005, which caused the snow cover to exceed 30 cm on the ground.
Tullio Tommasinii collaborated with Fabio Forti and oversaw the equipment until 1976. Both were supported in their work by the renowned Trieste meteorologist and Prof. Silvio Polli, who also authored the first three editions of the annual bulletin.
After Tommasini, Fulvio Gasparo, a CGEB member, continued the valuable work of data collection, verification, and organization for the preparation of the annual bulletins. Starting in the 1990s, he was joined by Roberto Zorzenon, and since 1995, also by Renato R. Colucci. Since 1998, the management of the observational site has been entrusted to Renato R. Colucci.
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The invaluable contributions of all the observers over the years must be acknowledged. Arceo Escher, caretaker of the Grotta Gigante, was responsible for daily observations and equipment monitoring until 1987. Bruno Pegan, Mauro Godina, and Giuliano Godina, tour guides at the cave, supported Escher’s work from the early years and continued the observations after 1987. Other guides have also contributed over shorter periods to the essential data collection and proper operation of the station. Among them, Fulvio Forti provided significant support to Colucci in daily data collection and site maintenance until June 2024. Some of the Grotta Gigante tourist guides, appropriately trained, continue to assist with equipment monitoring and daily data collection.
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Thanks to the efforts of all these individuals, the Borgo Grotta Gigante meteorological station today inherits a scientific legacy of great value. This legacy comprises an archive of meteorological data of fundamental importance for understanding the climate of the Karst region and for improving the assessment and comprehension of the rapid environmental changes we are witnessing. For these reasons, as of 2024, the Borgo Grotta Gigante meteorological station has been included in the Rete degli Osservatori Storici Meteorologici Italiani (ROSMI).
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Instruments and management approach
The distinctive feature of the station is its continued use of traditional mechanical instruments alongside modern automated acquisition systems. This characteristic sets the observational site apart from a typical meteorological station, allowing it to be classified as a Climatological Observatory.
For most observed parameters, at least two instruments operate in parallel (mechanical-analog and digital), ensuring data collection that maintains the continuity and homogeneity of the historical series. This approach minimizes the risk of data loss in case of hardware malfunctions, equipment failures, or electrical issues.
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The integration of digital automatic instrumentation in 2007 was part of the redevelopment of the Grotta Gigante visitor center. It included the installation of acquisition stations, the development of electrical and structural facilities, and the implementation of next-generation sensors. A key element was the establishment of the "weather room," a space dedicated to housing hardware supports, acquisition stations, a Torricellian barometer with a 1-meter tube for control, logistical equipment, and the complete paper archive of historical data.
This long-anticipated project was made possible thanks to a scientific collaboration with the Osservatorio Meteorologico dell’ARPA-FVG as part of the Interreg ADRIAMET project (now inactive), linked to the METEORISK and Interreg FORALPS projects.
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The Borgo Grotta Gigante site thus became part of the regional meteorological network of Friuli Venezia Giulia and is now integrated into the European cross-border network focused on monitoring and data collection for the prevention of "Local Severe Weather" phenomena and the study of atmospheric dynamics in the Alpine region. Furthermore, there is an active collaboration with the Alpine-Adriatic Meteorological Society for managing mechanical instruments, collecting and archiving data, and preparing the bulletin, which has always been published as a supplement to the scientific journal Atti e Memorie della CGEB.
Until 2010, the site hosted a thermo-pluviometric station on behalf of the Hydraulic Service of the Central Directorate for the Environment of the Friuli Venezia Giulia Region (formerly the National Hydrographic and Mareographic Institute). Part of the electronic instrumentation now belongs to the Rete idro-nivo-meteoclimatica della Protezione Civile della Regione FVG.
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The meteorological station of Borgo Grotta Gigante immediately after the renovation of the Grotta Gigante Visitor Center in 2007 (on the left) and the weather room, where the historical archive is housed, along with the data acquisition station and the Torricelli barometer.
Geographical location and climatic characterization
The observational site is more or less influenced by the climate of the Adriatic Sea depending on the prevailing meteorological conditions, due to the modest marginal relief of the Karst plateau.
The surrounding terrain is typically karstic, featuring numerous depressions, sinkholes, limestone outcrops, and caves. Over the past few decades, however, the surrounding vegetation has grown significantly, as has occurred across the Karst region, altering some microclimatic aspects of the entire plateau. It is important to note that until the first half of the 20th century, tall tree species were scarce, and the predominant landscape was grassland. Today, mixed forests of black pine, downy oak, hornbeam, and manna ash have replaced the grassland, and it is not uncommon to find conifers reaching 10–15 meters in height and coppice woods of 8–10 meters.
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In 2007, a statistical-climatological study was conducted in collaboration with the CNR and OSMER-ARPA to characterize the normal climate values of the central Karst region. This study updated the site’s climatology for some meteorological parameters to the reference period 1971–2000. The initiative was funded by the European Regional Development Fund through the ADRIAMET project within the Interreg IIIA Trans-Adriatic program. Some of these data are summarized in Table T and Table P.
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Based on the collected data and decadal series analysis, the climate of the BGG area, according to Köppen’s classification, is mesothermal, humid temperate, and semi-continental. The central Karst region's climate is marked by pronounced seasonality typical of its latitude.
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The minimum temperature drops below zero on an average of 59 days per year during the winter half-year, while it exceeds 30°C on 20 days during the summer. The extremes occurred in the winter of 1981–1982 (82 days below zero) and the exceptional summer of 2003 (61 days above 30°C). Overall, the normal annual thermal range between absolute minimum and maximum temperatures is 42°C (normal annual extremes: -8.0°C / 33.9°C). The absolute minimum temperature of -14.9°C was recorded on January 14, 1968, while the absolute maximum temperature of 38.8°C was recorded on August 5, 2017. January is typically the coldest month, with an average monthly temperature of 3.3°C (-0.6°C in January 1985), while August is the warmest, with an average of 21.9°C (26.0°C in August 2003).
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Table T - Average and extreme air temperature values at Borgo Grotta Gigante for the period 1971-2000.
Precipitation are typically well-distributed throughout the year and exhibits two peaks (November and June, with 142 mm and 130 mm, respectively) and two lows (February and July, with 72 mm and 83 mm, respectively). The annual total of 1,340 mm is spread over 132 days of precipitation, 99 of which record amounts exceeding 1 mm, and 7 are snowy days.
Table P - Normal average precipitation values at Borgo Grotta Gigante for the period 1971-2000.
During the winter season, depending on the meteorological conditions, the area is frequently affected by the Bora wind (ENE) and the Sirocco wind (SE). In the first case, the dry and cold continental wind can persist for several consecutive days and easily reach or exceed gusts of 100 km/h. The Sirocco, on the other hand, brings significantly milder conditions accompanied by intermittent rainfall. For these reasons, thermal variations can be substantial and rather abrupt.
Long periods of winter calm, with the formation of thermal inversions and fog, are not uncommon and are more frequent and persistent than commonly assumed. However, these phenomena mostly affect the coastal areas of the Karst and the shoreline.
During the summer, breezes dominate. In particular, the daytime sea breeze (originating from the W-SW) can penetrate several kilometers inland, moderating the maximum temperatures during the central hours of the day. The nocturnal land breeze (originating from the E-SE) is typically weaker and mainly affects the coastal zone of the Karst plateau.
Thermal inversion over the city and the Gulf of Trieste, seen from the Karst. Photo by Renato R. Colucci