d. Heatwave definitions
We defined a very hot day as one with the daily maximum temperature exceeding 33 °C and a very hot night as one with the daily minimum temperature exceeding 28 °C (HKO 2022). A daytime heatwave event was defined as a period with at least three consecutive very hot days in a summer season spanning June to September. A nighttime heatwave event was considered to consist of at least three consecutive very hot nights.
We evaluated five heatwave indices (Table 1): hot day frequency (HDF), heatwave duration (HWD), heatwave temperature (HWT), hot night frequency (HNF), and nighttime heatwave duration (NHD). These indices can effectively represent the trends of magnitude, intensity, and duration of daytime and nighttime heatwaves. Moreover, the spatial plots of each index can reflect the spatial expansion of hot areas.
Table 1. Heatwave Index Definitions