Commonly available meteo (climatic) data have usually some uncertainties, of different kinds, which may produce very significant differences between sources, or years in a same source.
These may be:
- The quality of the data recording, care of the operators, positioning, calibration and drift of the sensors, perturbations like shadings, dirt or snow on the sensors, etc.
- The fact that many data sources are only available in Monthly values, involving a synthetic generation for getting hourly values,
- For terrestrial measurements, the presence of a not negligible horizon,
- For terrestrial measurements, the location difference (distance of measuring station),
- For Satellite data, the quality of the models used for interpreting the data, which is in continuous improvement since 20 years, and the position of the satellite (if geostationnary),
- The yearly variability, which may be supposed to have a gaussian distribution,
- The evolution of the climate
. In Europe, it seems that the irradiation has increased by as much as 5% or more since the beginning of the 21th century.
See the differences in the PVGIS data
between the old database and the more recent "Climate-SAF" database.
Another example: in Geneva, for official measurements of the ISM, the 2003-2011 average is 10% above the 1980-2002 average ! (which is probably an extreme situation).