EURAMET reports, as a news story, the publication of the new standard EN 18097:2025 Hydrometry – Measurement of precipitation intensity – Metrological requirements and test methods for non-catching type rain gauges, which for the first time at European level defines standardised procedures for the laboratory calibration of instruments that measure rainfall without physically collecting it.
Atmospheric precipitation, such as rain or snow, is recognised by the World Meteorological Organization (WMO) as an Essential Climate Variable (ECV). Its accurate measurement is therefore crucial not only for everyday operational applications, but also for assessing climate change and its variability over time.
Traditionally, the amount of rainfall has been measured using “catching” gauges, i.e. instruments that physically collect precipitation to determine its volume or mass. However, these instruments can be affected by extreme environmental conditions, such as wind, frost or ice accumulation, and may introduce systematic errors, for example when raindrops adhere to the collection funnel.
However, due to their direct exposure to the elements, these instruments are affected by harsh environmental conditions - such as very low temperatures or high winds - and require frequent maintenance. They can also underreport precipitation levels due to raindrops sticking to the instrument’s collecting funnel.
In recent years, alternative instruments known as non-catching gauges have become increasingly widespread, including optical and acoustic disdrometers, which detect precipitation by measuring the number, size and fall velocity of individual particles. These instruments offer significant advantages, such as greater robustness and the ability to provide microphysical information on precipitation, but they also pose new challenges from a metrological point of view, as they cannot be calibrated using conventional reference flow rates.
In this context, the EMPIR INCIPIT project (Calibration and accuracy of non-catching instruments to measure liquid/solid atmospheric precipitation), coordinated by INRiM, developed traceable calibration methods for non-catching gauges. This was achieved through the design, characterisation and comparison of laboratory rainfall generators capable of reproducing, under controlled conditions, the real characteristics of precipitation events, such as drop size distribution and fall velocities.
The results of the project have been incorporated into the new EN 18097:2025 standard, which defines laboratory test procedures, criteria for the evaluation of measurement uncertainty, and a classification of instruments based exclusively on their metrological performance, without reference to the physical measurement principle.
The new standard represents an important reference for manufacturers, meteorological services and the scientific community, improving the comparability of measurements across different stations, climates, locations and time periods. In the long term, these advances will support a more effective use of remote sensing techniques, such as radar and satellite systems, with tangible benefits for weather forecasting, water resources management and climate studies.
This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.