Regular meteorological measurements at Prague’s Clementinum observatory began in 1752
. These were not the first instrument-based measurements in the Czech lands, however. The age of instrument-based observations in the country begins already 40–50 years earlier. However, these were not systematic observations, they were often not published, and the records of these observations are only preserved in private letters, calendars, or parish chronicles.
Similarly, even the Clementinum measurements of air temperature and pressure are quite fragmentary until 1774. What has been preserved are records of the highest and lowest air temperatures and pressures from the individual months of the year 1752, monthly extremes and averages of barometric pressure from the years 1752 and 1769–1793, a list of morning and afternoon temperatures and average monthly temperatures for the period 1771–1793, and average monthly temperatures for selected months in the period 1769–1774.
The beginning of the Clementinum series is considered to be the year 1775
. Until 1783 the series includes gaps in observation times or entire days, but from January 1, 1784 it is continuous and entirely free of gaps as viewed according to modern criteria. Likewise, atmospheric precipitation (rainfall, snowfall) was measured as early as 1752, but regular and reliable precipitation measurements at the Clementinum only begin on May 1, 1804.
At first, air temperature and pressure were measured twice a day – in the morning (either at sunrise or, in summer, two hours after sunrise) and in the afternoon at around 3 PM. From 1800 until the end of the year 1839, observations and measurements were performed every two hours starting at sunrise and finishing at 10 PM. The basic observation times were the so-called “Mannheim times” (7 AM, 2 PM, and 9 PM), which persist until today. After 1840, observations took place every day and every hour from 10 AM until 8 PM. However, as the importance of the Clementinum as an astronomical observatory declined later in the 19th century, there were fewer meteorological observations. By the end of the 19th century, observations were performed only three times a day at the internationally agreed times of 7 AM, 2 PM, and 9 PM. The decline of the Clementinum observatory reached its peak at the beginning of the 20th century. Despite all its difficulties, the Clementinum was able to continue making meteorological observations. In the 1930s, meteorologists all over the world began to pay more attention to climate fluctuations, meaning that there was increased interest in long-term historical measurements such as those from the Clementinum.
Meteorological observations at the Clementinum continue until today – more than 250 years now
. Although the Clementinum’s measurements were and still are influenced by a number of factors (such as the location of the measuring instruments within the Clementinum complex and its position in the very center of the city), for modern science, they represent a unique and extremely valuable source of information on weather and climate conditions during modern history.