Climatic Character of Hail in Tibet

With the method of correlation analysis, power spectrum and cluster analysis, the climatic character of hail in Tibet and the weather background were analyzed by using the daily data of hailing days in Tibet from 1981 to 2008 and the weather map data from 2007 to 2008.Resultsshow that the average annual hail days have a downward trend and a significant sign of 2 to 3 years of quasi-periodic changes in Tibet.The distribution of hail-rich zone is zonal along high altitude and mountains and the hail events occur more frequently in the north than in the west and east.The average annual hail days are 20.8 days in Tibet, and the biggest center locates at Naqv.The seasonal change of hail days is obvious in Tibet, and hail often occurs from March to October.Most hails happen from June to September.Tibet is a typical summer-hail district.There is little hail happens in the morning or at night, instead, it happens mainly at 12 to 20 o'clock.There is a positive correlation among the hail, altitude and thunderstorm.The happening of hail is closely connected with the synoptic system, such as vortex, shear line, westerly trough and west pacific subtropical high.