The climate index heating day refers to days on which the daily average temperature is 12 °C or less and therefore, according to this SIA definition, heating is required. The number of heating days has fallen sharply throughout Switzerland in recent decades.
Heating degree days allow conclusions to be drawn about weather-related heating energy consumption. They are defined as the sum of the daily differences between the target indoor air temperature of 20°C and the daily average temperature of all heating days in a year. Heating degree days have also decreased throughout Switzerland in recent decades.
The overall decrease in these two climate indices is due to global warming. It can be assumed that this trend will continue in the future, due to climate change.