1. Department of Optics and Quantum Electronics, University of Szeged, H-6701 P.O. Box 406 Szeged, Hungary, Research Group on Laser Physics of the Hungarian Academy of Sciences, H-6701 P.O. Box 406 Szeged, Hungary, Hilase Developing, Producing, Servicing and Trading Ltd., Department of Inorganic and Analytical Chemistry, University of Szeged, H-6720 Szeged, Dóm tér 7., Szeged, Hungary, and Hungarian Meteorological Service, P.O. Box 39 H-1675, Budapest, Hungary