Abstract Traditionally, transmission lines have been operated based on static thermal rating, which is determined by fixed and conservative meteorological data. It causes underutilization of potential transmission capacity. To overcome this limitation, this paper presents seasonal time scale, which can provide flexible ratings. Seasonal meteorological parameters are uncertain. The uncertainty can directly impact the conductor temperature, and eventually impact transmission capacity of the lines. In order to account for the uncertainty, this paper presents an uncertainty analysis model. This model consists of three parts: establishment of probability distribution models, Monte Carlo simulation, and operation risk assessment. In the case study, the actual meteorological data for 7 years in the Weihai is utilized, and is divided into four subsets by spring, summer, autumn and winter. The uncertainty analysis model is tested on an actual 47 km Wei-Kun line and a 4-bus power system. Results show that potential transmission capacity can be extracted if seasonal meteorological parameters are taken into account.