Can 250 years of weather data be trusted? Inside Prague’s Clementinum Observatory
With summer approaching and the climate crisis progressing, it is likely that new temperature records will once again be recorded at Prague’s Clementinum meteorological station with more than 250 years old tradition. Nevertheless, scientists point out that data collected there should be interpreted with caution.
Encountering the whims of the weather is a universal experience. It is therefore no surprise that the weather is one of the most common conversation starters and a frequent topic of everyday small talk. However, in order not only to discuss subjective impressions of the weather but also to study it in a more objective and systematic way, people had to develop instruments that would enable accurate and consistent observations. In the Czech lands, the first systematic meteorological measurement started as early as in the 18th century in the former Jesuit College in Prague known as the Clementinum.
“Continuous measurements have been carried out here since 1775, but the origins of observation at this site go back even further, as scientists first had to refine the instruments used to measure temperature and determine the exact location where regular measurements should be taken,” explains Ilona Zusková, Head of the Department of Meteorology and Climatology at the Czech Hydrometeorological Institute.
Meteorological measurements and observations at the Clementinum were first initiated in 1752 by Joseph Stepling, the first director and founder of the Clementinum Observatory. He measured temperature, air pressure and precipitation, and in a publication from the same year he explained the scientific and practical importance of regular, long-term observations.
Stepling’s student and later successor, Antonín Strnad, began conducting truly regular and systematic measurements in 1775, as documented in his journals. To make it possible to compare Prague’s results with those from meteorological stations elsewhere in the world, observations were standardized to take place at the same times each day.
“Using the specific three observation times – 7 a.m., 2 p.m., and 9 p.m., with the evening measurement counted twice – the values are added together and divided by four to calculate the average daily temperature. This method is still used today.”
The first measurements took place in the Astronomical Tower in the centre of the Clementinum complex. However, since 1780 the observations have been moved to an annex of the southern wing, where they continue to this day.
“The issue of solar influence was addressed, and that is crucial in climatology. Measurements must be sheltered, meaning the instrument is placed inside a kind of enclosure so that it is protected from radiation, while still allowing air to circulate.”
Despite relocating the measuring equipment, the station still does not meet current placement standards, which recommend measurements to be taken approximately 2 metres above the ground, ideally above cut grass.
“In Clementinum, measurements began on the 4th floor of the Astronomical Tower, 39 metres above the courtyard. Today, measurements are taken on the 1st floor of the building behind the church, about 6.5 metres above the ground. You will therefore not find standard measuring conditions in Clementinum,” says Martin Novák from the Czech Hydrometeorological Institute in Ústí nad Labem.
Prague is often associated with various temperature records that were measured at the Clementinum station, since its long observation series makes it possible to compare data spanning more than 250 years. However, due to its unsuitable location, these data cannot be considered fully accurate.
“The vast majority of stations with long measurement series are located in cities, because they were established at universities. That is why we must also interpret rising average temperatures recorded by these stations with caution. To a significant extent, they are influenced by the expanding urban development.”
For this reason, meteorologists considered whether to adjust the measurements to international standards, but decided to continue under the same non-standard conditions so that the long-term data series would remain continuous.
“Long data series are something meteorologists could describe as a family treasure. In our case, this applies precisely to the Clementinum series, and that is why it is important to continue it.”
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