Abstract: Urban heat island effect (UHI) has become an important ecological and environmental problem that hinders the sustainable development of cities. However, few studies explored the seasonal variations of UHI in the cold regions of China, which limits the comprehensive understanding of UHI in different climate zones. Taking Changchun City of Jilin Province in northeast China as an example, Landsat 8 and GF2 images were selected as main remote sensing data, and four surface urban heat island intensity indices were proposed to characterize the urban thermal environment. The spectral unmixing, profile line analysis, and regression models were used to explore the impacts of potential driving factors on the monthly and seasonal variations of UHI. The results showed that: (1) The spatial pattern of the urban thermal environment in Changchun exhibited significant monthly and seasonal variations, with the highest land surface temperature (LST) of 39.44 ℃ in August and the lowest of -18.60 ℃ in December. The UHI effect was prominent in winter and summer, but not obvious in spring and autumn. (2) The intensity of UHI as expressed by different indices had great differences. Based on the index characterized by land use type, the intensity of UHI could reach as high as 10.56 ℃ in July. However, some months (February, March, and November) showed the reverse UHI with temperature in rural area being higher than that in urban area. (3) Vegetation and impervious surface area were important driving factors for the spatial and temporal variations of urban thermal environment. There was seasonal variation in the influencing mechanism, with the most significant correlation between UHI intensity and the two factors in the summer, and no effect in winter. (4) The LST of urban function zones showed monthly variations. Specifically, the industry zone had the highest LST. However, the human activities such as straw burning resulted in the highest LST in the croplands in some months. Our results elucidate the driving mechanisms of monthly and seasonal variations in the urban thermal environment, which is of great significance for the regulation of thermal environment ecology in cold regions.

Key words: urban heat island effect, land surface temperature, monthly variation, driving factor, cold region