## 4 Climatology and Diversity of Hong Kong QBIE Temperature Events
### Diversity of QBIE Events  
As shown in Figure 4, the occurring frequency, duration, intensity, and peak dates of QBIE events, estimated from daily minimum, mean, and maximum temperatures, can vary significantly. These differences suggest the existence of distinct types of QBIE events, which are determined by their specific characteristics. For example, a QBIE heat wave that \begin{table}
\begin{tabular}{l c c c c c c} \hline  & **Heat Wave** & **Heat Wave** & **Heat Wave** & **Cold Surge** & **Cold Surge** & **Cold Surge** \\  & **by Minimum** & **by Mean** & **by Maximum** & **by Minimum** & **by Mean** & **by Maximum** \\  & **Temperature** & **Temperature** & **Temperature** & **Temperature** & **Temperature** & **Temperature** \\ \hline Average event & 2.16 & 1.85 & 1.92 & 2.76 & 2.70 & 2.69 \\ counts per year & & & & & & \\ Average event & & & & & & \\ intensity (\({}^{\circ}\)C) & 2.06 & 1.99 & 2.21 & \(-\)2.24 & \(-\)2.09 & \(-\)2.34 \\ Average event & & & & & & \\ duration (days) & 10.96 & 11.16 & 10.49 & 10.67 & 10.49 & 10.08 \\ Average event & & & & & & \\ days per year & 23.68 & 20.62 & 20.10 & 29.53 & 28.33 & 27.08 \\ (days) & & & & & & \\ \hline \end{tabular}
\end{table} 
Table 2: Climatological statistics of QBIE events based on the HKO station records from 1885 to 2022, where data from 1940 to 1946 are not included. is identified solely from daily maximum temperature indicates that the event occurs only during daytime; conversely, an event identified only from daily minimum temperatures occurs at night only. Temperature extremes experienced during daytime and nighttime could have very different impacts on the environment and human health [33; 34]. Thus, it is important to examine the diversity of QBIE temperature events and understand their climatological features. In this study, we classified all QBIE temperature events into six different categories, including (1) weak nighttime (WNT), (2) strong nighttime (SNT), (3) weak daytime (WDT), (4) strong daytime (SDT), (5) combined daytime and nighttime (CDN), and (6) other QBIE events. Specifically, a WNT QBIE event is defined as one that is observed only in the daily minimum temperature series, but not in the daily mean and maximum temperature series. An SNT QBIE event is defined as one that appears in both the daily minimum and mean temperature series, but not in the daily maximum temperature series. Conversely, a WDT QBIE event is defined when it is observed only in the daily maximum temperature series, but not in the daily minimum and mean temperature series. An SDT QBIE event is defined if it is observed in the daily mean and maximum temperature series, but not in daily minimum temperature series. A CDN QBIE event is defined if it is observed in all the daily minimum, mean, and maximum temperature series. Events that do not fit into the above five categories are classified as other QBIE events (Table 3). Overall, CDN events are the most common type of QBIE events in Hong Kong. CDN heat waves (134 events) take up over one-third of all QBIE heat wave events (392 events), while CDN cold surges (208 events) account for 41% of all QBIE cold surge events (513 events). While WNT, SNT, and WDT events have similar event counts, SDT events occur less frequently, which suggests that nighttime events are slightly more common than daytime events (Figure 5a). This is also reflected in the comparisons of average event days among the various types of QBIE events (Figure 5d). In terms of event intensity, CDN events are, on average, stronger than all other four types, with intensities of 2.15 \({}^{\circ}\)C for heat waves and 2.28 \({}^{\circ}\)C for cold surges. WNT events (1.31 \({}^{\circ}\)C for heat waves and 1.40 \({}^{\circ}\)C for cold surges) and WDT events (1.59 \({}^{\circ}\)C for heat waves and 1.46 \({}^{\circ}\)C for cold surges) exhibit the lowest intensities, as expected (Figure 5b). Further, the average event durations appear to be approximately proportional to the event intensities (Figure 5c).