Three meteorological stations in the network of the National Institute of Meteorology and Hydrology are among the 70 stations worldwide which have been working for more than a century without interruption. They have been recognized by the United Nations World Meteorological Organization as part of the world cultural and scientific heritage. These are the stations “Obraztsov Tchiflik” founded in 1889, “Sliven” founded in 1890 and “Knezha” founded in 1910. The stations did not stop working even during the wars of the first half of the 20th century.

After their nomination the stations were examined thoroughly (methods of work, location, representativeness). The experts of WMO looked over every historical document and the Bulgarian candidates received the status of “Centennial Observing Stations”, certificates from WMO and metal plates for their designation.

“Long-term meteorological observations are part of the irreplaceable cultural and scientific heritage of mankind that serve the needs of current and future generations for long-term high quality climate records. They are unique sources of past information about atmospheric parameters, thus are references for climate variability and change assessments. To highlight this importance, WMO has a mechanism to recognize centennial observing stations. By so doing, the Organization promotes sustainable observational standards and best practices that facilitate the generation of high-quality time series data.” is what is said in WMO's message.