Climatological assessment of the summer half-year (Apr-Sep) 2026 at Hartheim Forest Research Station (DE-Har)

Summary: Summer half year 2026 was to-date the warmest, sunniest and direst summer-half year since continuous measurements began at the Hartheim Forest Research Station (ICOS DE-Har) in 1978. Average air temperature during summer half-year 2026 was 18.4° which is +2.5K above the long-term average. Solar irradiance was +17% than the long-term average, and precipitation was only 56% of the long-term average. Air temperature and solar radiation exhibit clear warming and brightening trends over the period 1978-2026. Interpretation: Please note that the air temperature record was observed at 2 m below canopy inside a growing and since 2018 disintegrating forest, so the air temperature does not only reflect regional climate, but may also respond to structural and physiological changes in the forest canopy. This can not be compared to WMO standard observations. Precipitation and solar irradiance are measured above the forest and can be considered unaffected by the forest and represent regional averages in the Upper Rhine Valley. Data sources: University of Freiburg, Chair of Environmental Meteorology / ICOS, https://zenodo.org/communities/de-har/ and Imbery (2005) https://freidok.uni-freiburg.de/data/15664, with 2026 calculated using interim data. A German translation of this document is available here: Christen, A., Plapp, T., Schindler, D., & Sulzer, M. (2026). Klimatologische Einordnung des Sommerhalbjahres (Apr-Sep) 2026 an der Forstmeteorologischen Messstelle Hartheim (DE-Har) (Version 1.0). Albert-Ludwigs-Universität Freiburg. https://doi.org/10.5281/zenodo.23120702