HOMOGENIZATION OF THE HELLENIC CLOUD COVER TIME SERIES - PRELIMINARY RESULTS

Sa？etak Cloud cover is an important meteorological parameter because it affects the ra- diative energy balance of the Earth and precipitation and plays a major role in the hydrological cycle. Impact of clouds on the radiative balance is twofold: cloud cover modifies the albedo of the Earth and the atmospheric long wave radiative exchange. Long records of cloud cover data are the result of human estimations made during the synoptic observation hours at each meteorological station. Currently the quantification of cloud cover has been automated due to the advent of modern remote sensing space (e.g. satellites) and ground sky monitoring techniques (e.g. sky cameras). All weather time series records may suffer from inhomogeneities; their use in climatology requires homogenization. In this work we attempt a preliminary homogenization of the cloud cover time series of the Hellenic National Weather Service (HNMS) network. Data come from 36 meteorological stations, and cover the period from 1975 to 2004. Raw data comprise (synoptic) hourly cloud cover observations which we subjected to quality control before producing daily and then monthly average cloud cover time series. For this homogenization exercise we used the HOMER software tool