Data and Methodology
Method of Observations
Clouds are observed visually by trained observers at surface meteorological stations in units of eighths of sky (oktas) following the guidelines given by the World Meteorological Organization (WMO). The surface observations of clouds are made in the “synoptic” code, which is exchanged globally through Global Telecommunication System (GTS). Clouds are classified as high, middle or low according to the altitude of the cloud base above the surface of earth. The information about clouds in the synoptic weather report consists of total cloud amount, individual cloud layer’s cloud amount and cloud type (high, medium and low) and base height of the lowest layer of cloud. Total cloud amount recorded in the observations are the amount of sky estimated to be covered by all cloud types. The surface cloud observations have provided the long-term cloud climatologies (Warren et al. 1986, 1988; Hahn and Warren 1999), evaluation of satellite cloud observations (Rossow and Schiffer 1999) and analyzing decadal and long-term changes in cloud cover during the last 100 years or so. Since weather reports of clouds are available for several decades with no change in official observing methods, long-term variations and trends in cloud cover can be studied.