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.