Large-area power transmission line icecoating accidents occurred in Zhejiang Province early 2008. And the upper air sounding data are used due to the lack of representative icecoating observations. The configuration of temperature and humidity factors at various altitudes in the lower atmosphere is also obtained. It reveals that combination of continuously low temperature and corresponding high humidity fits the investigation of transmission line incidents so well that it can be regarded as a basic criterion for icecoating prediction and early diagnosis. The evaluation indicates that, from middle January to early February in 2008, the continuous occurrence of low temperature accompanied by high humidity was the main factor of power transmission line icecoating accidents. In such severe air conditions, the correlation between intensity of icecoating and altitude is proved to be positive in the lower boundary layer. Comparisons with the similar historic weather records point out that, the unprecedented severe meteorological conditions resulted in the extremely severe ice-coating accidents in early 2008.