Official Hong Kong Observatory measurements recorded a 27°C daily maximum on May 15 amid persistent overcast skies, 83–89% relative humidity, and widespread thundery showers driven by a low-pressure trough. These conditions sharply curtailed solar insolation and surface heating during the spring-to-summer transition, keeping temperatures below climatological expectations for mid-May when median highs typically climb toward 29–31°C. The market’s near-certain 27°C resolution reflects this verified observation rather than forecast uncertainty. Clearer skies or reduced moisture could have allowed greater radiative warming and pushed readings higher, but the entrenched trough and high humidity made such shifts unlikely once the day began.