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The maximum allowable carryingcurrent capacity of overhead transmission lines highly depends on the meteorological parameters but traditionally the rigid and harsh setting of meteorological parameters makes the carryingcurrent capacity conservative. In order to make the carryingcurrent capacity setting satisfy the actual meteorological situation, this study utilizes the hourly data from 94 automatic meteorological stations over the southern Hebei to drive the wire energy balance equation for the analysis of carryingcurrent capacity. It helps to improve the transmission capacity of the transmission lines in the southern Hebei that the constant carryingcurrent capacity is changed into seasonally varied carryingcurrent capacity. Based on the analysis of the sensitivity of allowable maximum carryingcurrent capacity to the variation of ambient wind speed and air temperature, the study has determined the probability density distribution of ambient air temperature and wind speed, obtained the regional probability distribution of allowable maximum carryingcurrent capacity, and evaluated the transmission capacity at several safety operation levels. The results show that at the 95% safety operation level, the maximum carryingcurrent capacity of the LGJ400/35 transmission line in the southern Hebei can be set to 945, 774, 795 and 1012 A in spring, summer, autumn and winter, respectively, which is significantly higher than the traditional carryingcurrent capacity. Thus, currentcarrying capacity of the overhead transmission lines driven by actual meteorological data on the seasonal scales improves the transmission capacity of the lines.