Abstract:Two icing cases happened on 500-kV high voltage transmission lines in Hubei Province during February 25-26, 2008 and January 5-9, 2009. Study showed that the circulation patterns in both cases are similar: there were two troughs and a ridge in high latitudes, a short-wave trough in low latitudes at 500 hPa, a low-level jet at 700 hPa, and a surface cold front. The low-level jet and surface cold front were conducive to the occurrence of freezing rain and snows. Also, icing processes involved precipitation, certain temperature and humidity conditions. These conditions provided good potential and thermal conditions for the formation of icing. A warm layer existed in the vertical temperature structure of the atmosphere in the 2008 case, but not in the 2009 case. This resulted in the difference in formation mechanisms of the two cases.