We compared open-site and below-canopy climatic conditions from 14 different sites in Switzerland based on LWF (Long-term Forest Ecosystem Research) data. The 14 sites represent different locations, orientations and elevations, from the Jura Mountains to the southern side of the Alps, composed of deciduous, coniferous and mixed forests. Meteorological measurements were carried out under the canopy at the observation plots, and in open areas outside the forest plots. We analysed air temperature during summer 2003, in connection with the exceptionally high temperatures measured during that summer and during the 11-day August heat wave. We compared minimum and maximum daily air temperature differences between open-site and below-canopy conditions.