1. Harokopio University of Athens, Department of Geography, El. Venizelou 70, Athens 17672, Greece
2. National Observatory of Athens, Institute of Astronomy, Astrophysics, Space Applications and Remote Sensing, I. Metaxa & Vas. Pavlou, Penteli 152 36, Greece
3. CNRS, Ecole Normale Superieure, Department of Geosciences, Paris 75231, France