Hong Kong nears three-decade July rainfall high

Hong Kong is heading towards its wettest July since 1997 after weeks of persistent downpours, tropical cyclone activity and unstable low-pressure systems pushed rainfall far above the long-term average.

Rain measured at the Hong Kong Observatory’s headquarters had already exceeded the normal July total before the final third of the month. Further heavy showers linked to Typhoon Noul and moist south-westerly air lifted the cumulative figure closer to levels last recorded nearly three decades ago.

The territory received 474.7 millimetres of rain during the first 20 days of July, 23 per cent above the monthly average of 385.8 millimetres. Additional downpours later in the month strengthened the prospect of July becoming one of the wettest on record since observations began in 1884.

Forecasters expect unsettled conditions to extend into early August. A broad area of low pressure over the northern South China Sea and the western Pacific is likely to maintain a flow of moisture towards the Guangdong coast, bringing showers, thunderstorms and periods of heavier rain.

Rainfall this month followed an unusually wet June, when about 600.5 millimetres was recorded, roughly 22 per cent above normal. The back-to-back wet months have saturated hillsides, raised the risk of landslides and placed greater pressure on drainage systems across densely built districts.

Typhoon Noul added to the disruption after approaching the southern China coast and bringing squally showers, powerful winds and rough seas. Hundreds of flights were cancelled in Hong Kong, while transport services were adjusted and residents were warned to avoid shorelines and exposed areas.

At least 21 people were injured as the storm affected the city. Fallen trees, damaged scaffolding and blocked roads were reported, although Hong Kong avoided the larger-scale destruction seen in parts of Guangdong, where hundreds of thousands of people were evacuated before landfall.

Noul became the third tropical cyclone to affect southern China during July, underlining an active phase in the western North Pacific. Cyclones Bavi and Maysak had already contributed to difficult weather conditions across parts of the region, with heavy rain extending beyond coastal areas.

Meteorologists say the extraordinary rainfall has resulted from repeated interactions between tropical systems, low-pressure troughs and moisture-rich monsoon winds. Such patterns can produce long periods of rain even when the centre of a cyclone remains some distance from Hong Kong.

The city’s steep terrain and dense urban development increase its exposure to sudden flooding. Intense rainfall can overwhelm drains within minutes, particularly in low-lying districts and older neighbourhoods where water has fewer routes to escape.

Officials have repeatedly reminded residents to monitor rainstorm warnings, avoid watercourses and report signs of slope instability. Schools, businesses and public transport operators are required to follow warning arrangements when Amber, Red or Black rainstorm signals are issued.

The Observatory has issued dozens of rainstorm warnings since June, far more than during the same period last year. The frequency reflects not only the amount of rain but the number of intense bursts capable of disrupting travel and daily activity.

Hong Kong has experienced several severe rainfall episodes over the past few years. A record-breaking storm in September 2023 produced 158.1 millimetres of rain in a single hour, the highest hourly total since records began, flooding roads, underground stations and shopping areas.

A cluster of four Black rainstorm episodes between late July and early August 2025 further demonstrated the difficulty of forecasting small, fast-developing systems. Some storms formed rapidly near the Pearl River estuary, leaving limited time for highly localised warnings.

Climate researchers expect warmer air and seas to increase the moisture available to tropical weather systems. A warmer atmosphere can hold more water vapour, raising the potential for heavier short-duration rainfall when atmospheric conditions trigger thunderstorms.

Natural year-to-year variations remain important, and no single wet month can be attributed solely to climate change. Long-term observations, however, show growing concern over the intensity of extreme rainfall events across subtropical coastal cities.

The financial impact extends beyond damaged property. Persistent bad weather can affect aviation, shipping, construction, retail activity and outdoor tourism. Insurance claims may rise when flooding damages vehicles, homes and commercial premises.