Diurnal variations in the formation and development of convective storms over contiguous North China during the warm season were studied using reflectivity from six China Next Generation Weather Radars between 2008 and 2011. Our results, including temporal and spatial analysis of hourly storm frequency through the warm season, and inter-month comparisons during June, July and August, indicate that most storms initiate over the northwestern mountains in the afternoon as a result of solar heating, with the highest frequency in June, and the lowest in August. Storms propagate from the mountains to the southeastern foothills and plains, with the highest rates occurring in June, and the lowest in August. In the late afternoon, there is a remarkably high storm frequency over the foothills and plains, which indicates a significant topographic control on the southeastward propagation and intensification of storms during the warm season. Storm activity occurs mainly on the plains through the night, with the highest frequency in July and the lowest in June, as a result of a favorable nocturnal convection mechanism. The region-averaged hourly storm frequencies for the warm season, and also for each month in JJA, are all bimodally distributed, with peak frequencies occurring in late afternoon and during the night, with the highest frequency recorded in the late afternoon during June and July, but at night in August. In general, the mean storm frequency is highest during the day and night in July, and lowest from afternoon to evening in August, but from nighttime to the next morning in June.