Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland

now at: Dept. of Physics and Atmospheric Science, Dalhousie University, Halifax, B3H 3J5, Canada and at Environment Canada, Toronto, M3H 5T4, Canada

Finnish Meteorological Institute, Climate Change, P.O. Box 503, 00101 Helsinki, Finland

Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland

Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland

now at: Dept. of Biophysics, Graduate School of Science, Kyoto University, Sakyo Kyoto 606-8502, Japan

Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland

Institute of Physics, University of Tartu, Ülikooli 18, 50090 Tartu, Estonia

Finnish Meteorological Institute, Climate Change, P.O. Box 503, 00101 Helsinki, Finland

Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland

Finnish Meteorological Institute, Climate Change, P.O. Box 503, 00101 Helsinki, Finland

School of Physical and Chemical Sciences, North-West University, Private Bag x6001, Potchefstroom 2520, South Africa

Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland