Cloud amount and cloud types have been observed in several different ways through time, but the different observation routines are not always well documented. The original observations have been digitized back to 1756. Digitized cloudiness values are available for download, but the data are not recommended for studies of long-term trends in cloud amount because the record is judged to be homogeneous only from around 1950.
A discussion of how the cloud observations have been made, and of the reliability and homogeneity of the data, is provided in the following publication.
- Moberg A, Alexandersson H, Bergström H, Jones PD. 2003: Were Southern Swedish summer temperatures before 1860 as warm as measured? Int. J. Climatol. 23: 1495-1521 (Sections 3-4 and Appendix A)
Legend to symbols used for observations of clouds and other meteorological phenomena observed on the sky. This legend, drawn by D. E. Næzén, is found in a meteorological observation diary from Umeå in 1796. The same cloud symbols were used in Stockholm during 1784-1815. (Photo: Kurt Eriksson, National Archives. See Photo 2 in Moberg, 1998: Meteorological observations in Sweden made before A.D. 1860. Paläoklimaforschung/Palaeoclimate Research 23: 99-119).