The daily air temperature observations are thoroughly investigated. Thermometers used have always been calibrated in degree Celsius (°C), but several factors that influence the observed temperatures have changed over time. Important such factors are: the time points when the temperature was observed, the position of the thermometer, the protection of the thermometer against radiation from direct sunlight and from the surroundings and influence from the growing city (the urban heat island). Corrections for such changes have been made as far as possible and the observations have been used to develop homogenized records of daily and monthly mean temperatures back to 1756.
Detailed discussions of how the temperature observations have been made and how the homogenized series were developed are provided in the following publications.