Abstract: Observing and studying clouds is important for the development of numerical models, conducting climate change estimations, and studying weather modification. Observations obtained from a ground-based Ka-band millimeter-wave cloud radar deployed in Jian’ou, northern Fujian, China, between 2019 and 2022 were studied to advance our understanding of the structural characteristics of clouds and their temporal variations in the mountainous areas along the southeast coast of China. The characteristics, seasonal patterns, and diurnal variations of cloud occurrences, along with their vertical structures and parameters, were studied in this area. In addition, the causes underlying these observed characteristics and variations were determined based on atmospheric water vapor and thermodynamic conditions. The findings indicated the following: (1) cloud occurrence exhibited seasonal variations, with the highest occurrence observed in spring, followed by summer and winter, and the lowest during autumn. Low and high clouds dominated during spring and summer, whereas low and mid-level clouds predominated during autumn and winter. (2) radar reflectivity factors of high clouds were generally larger than those of low clouds during spring and summer. In summer, ice-phase and liquid-phase cloud layers exhibited the largest radar reflectivity factors, followed by spring and autumn; the smallest reflectivity factors were observed during winter. Cloud base height and top height were highest during summer, followed by spring and autumn, respectively, while they were lowest during winter. The thickest cloud layer and largest cloud gap distance also occurred during summer, followed by spring and autumn, respectively, whereas the thinnest cloud layer and smallest cloud gap distance were observed during winter. (3) Diurnal variations in cloud vertical structures and parameters were significant in different seasons, and their differences were also apparent. Compared with spring, summer exhibited earlier and stronger convection activities and a longer lifespan, resulting in larger and longer-lasting variations in cloud parameters during the afternoon. Diurnal variations in cloud occurrence, intensity, and other cloud parameters were less pronounced during autumn and winter compared with spring and summer. However, the diurnal variation trend of clouds in autumn was similar to that in summer, while the diurnal variation trend of clouds in winter was similar to that in spring. (4) Average specific humidity, horizontal wind field, and convective available potential energy showed significant positive correlations with the seasonal and diurnal variations in cloud radar-observed cloud-related changes in the local area. These correspondence relationships can provide reference for improving parameterization schemes of numerical models and guiding weather modification.