To calculate the Heating and Cooling degree days (HDD. CDD) we use the observed average daily temperatures (E-OBS) for 2002 until 2012 on the 25x25 km raster provided by the European Climate Assessment & Dataset project, see also Haylock, M.R., van den Besselaar, E. J. M., van der Schrier, G., and Klein Tank, A. M. G., “A European daily high ‐ resolution observational gridded data set of sea level pressure,” J. Geophys. Res., vol. 116, 2011).
We then apply an environmental lapse rate of 6.5°C per 1000 m elevation gain according to the specifications of the International Standard Atmosphere model using the digital elevation model over Europe (EU-DEM) layer on the 30x30 m grid level (European Environmental Agency, “Copernicus Land Monitoring Service - EU-DEM,” 2017. https://www.eea.europa.eu/data-and-maps/data/copernicus-land-monitoring-service-eu-dem)
Project Website: http://www.hotmaps-project.eu/