Abstract: Using 315 d of hourly observation data collected by 19 automatic weather stations in 2022, the stations are classified into three categories, namely urban (six stations), rural (eight stations), and coastal stations (five stations) to analyze (i) the diurnal variation characteristics of an urban heat island (UHI) in Haikou City, northern Hainan Island and (ii) the effects of wind speed, precipitation, and humidity on the UHI. main findings are as follows: (1) the order of annual average temperature for the three station categories is rural station > urban station > coastal station from morning to afternoon, and it is reverse, i.e., coastal station > urban station > rural station, from night to early morning. The UHI presents a “V”-shaped diurnal pattern, with two turning points at 09:00–10:00 BJT(Beijing time) and 17:00 BJT. A weak “urban cool island (UCI)” (0.6 °C) phenomenon is observed during the day, while a pronounced UHI (1.2 °C) phenomenon occurs at night. The occurrence of this UCI phenomenon is closely related to sea breeze effects. (2) The strongest UCI intensity occurs in spring (−1.1 °C), followed by winter (−0.6 °C), summer (−0.4 °C), and autumn (−0.2 °C). In contrast, the strongest UHI intensity occurs in autumn (1.6 °C), followed by summer (1.4 °C), winter (1.1 °C), and spring (1.0 °C). Additionally, the onset of UHI in the late afternoon during summer and autumn occurs 2 h earlier than during winter and spring. (3) The UHI (or UCI) becomes more pronounced under Level 1 wind speed and no-rain conditions, with maximum intensities reaching 1.5 °C (−1.0 °C) and 1.9 °C (−0.9 °C), respectively. Under Level 2–4 and Level 5–7 wind speeds, the UHI intensities decrease to around 1.2 °C and 0.9 °C, respectively, while the maximum UHI intensity stabilizes around 1.2 °C under other precipitation conditions. Notably, the duration of the UHI increases significantly with increasing wind speed and precipitation. (4) The UHI and UCI are most pronounced under comfortably dry conditions, with intensities reaching approximately 2.5 °C and −0.9 °C, respectively. The UHI intensity decreases by about 0.3 °C with each successive increase in the humidity level, while the UCI intensity also begins to decline from the humid level onward as humidity further increases. (5) During the daytime, the maximum amplitude of temperature difference between urban and coastal stations is generally slightly larger than that of the UCI, and an increase in this temperature difference may provide some indication of sea breeze precipitation. (6) Detailed analysis of the effects of wind speed, precipitation, and humidity conditions, categorized into high and low thresholds, shows that UHI intensity is most pronounced at low thresholds (i.e., low wind speed, no rain, and low humidity). As more variables move into the high-threshold category, the UHI intensity gradually decreases. (7) Compared with non-sea breeze days, sea-breeze days are characterized by more pronounced UHI and UCI features and a later onset of UHI. (8) The spatial distribution characteristics indicate that high UHI intensity areas are mainly concentrated along the coastal regions at night. The temperature of the northwest coastal stations is similar to that of coastal urban stations. The differences in UCI intensity among urban stations are relatively small in summer and autumn.