1. Dept. of Atmospheric Sciences, University of Washington, Seattle WA 98195, USA
2. Dept. of Nuclear Physics, Lund University, S-223 62 Lund, Sweden
3. Dept. of Physics, University of Helsinki, SF-00170, Helsinki, Finland
4. Dept. of Meteorology, Stockholm University, S-106 91 Stockholm, Sweden
5. Dept. of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA