The century-long search for the precise mechanisms responsible for urban heat islands continues, while urban warming worsens in many megacities. Most studies have focused on mean temperature, daily and annual temperature ranges and urban heat island intensity. We hypothesize that an analysis of the changes in the characteristics of the complete daily and annual temperature cycles, including not only the mean temperature and temperature ranges (amplitudes) but also the maximum and minimum temperatures and the phases, can provide more information on urban warming phenomena. Through a detailed analysis of long-term observations in Hong Kong, we found that the difference in the daily cycle between urban and rural stations is very distinct, whereas the annual cycles are much more similar, suggesting that the urban environment has a greater effect on the daily cycle than on the annual cycle. The daily phase has shifted a total of 1.77 h later over the last 130 years (1.36 h per century) in the urban area of Hong Kong according to the Hong Kong Observatory (HKO) data. The annual phase change at HKO reflects the globally observed phenomenon that the annual phase advances or seasons onset earlier.
Urban warming Daily temperature cycle Annual temperature cycle Phase lag
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This work was funded by a RGC CRF project (HKU9/CRF/12G) of the Government of the Hong Kong SAR, China and a grant awarded by the Key Laboratory of Eco Planning and Green Building, Tsinghua University, MOE, China. We thank two anonymous reviewers for their valuable comments and suggestions. We would also like to thank Mr. Shun Chi-ming, Director of Hong Kong Observatory, and Dr. LEE Tsz-cheung for their help.
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