Why has Hanoi been battered by so much rain?

Hanoi did not lie directly in the path of the storm, yet heavy rains have persisted for the last two days.

The rainstorms, spaced just 15-30 minutes apart, caused flooding at 82 points across the city throughout Tuesday and into Wednesday, disrupting traffic and severely affecting daily life.

According to data from the Hanoi Drainage Company, there was 320 mm of rainfall in O Cho Dua between 7 a.m. and 1 p.m. on Tuesday, 310 mm in Vinh Thanh, 200 mm in Hai Ba Trung, 190 mm in Tay Ho, 165 mm in Thanh Xuan, and 150 mm in Van Mieu and Tu Liem.

Truong Ba Kien, deputy director of the Center for Meteorological and Climate Research at the Vietnam Institute of Meteorology, Hydrology, and Climate Change, listed three reasons for the non-stop rain in the capital.

Typhoon Ragasa last week and Bualoi recently left the air in the northern delta, including Hanoi, "highly humid and unstable," he said.

The expansion to the southeast of a subtropical high-pressure system (a dry, stable, high-pressure zone at a latitude of 23–25 degrees north) has intensified the weather's impact on the region, he said.

"Under these conditions, an easterly to southeasterly wind was formed, blowing from the sea (the Gulf of Tonkin) into the land, bringing moisture and directly impacting the weather in various areas."

Finally, he said, at 5,000 meters above sea level, there has been a strong convergence of southwestern winds, combining with eastern winds to form a continuous wind belt from Hung Yen, Hanoi, Thai Nguyen, and up to Cao Bang, stretching 200-300 km wide.

"This time these two factors coincidentally created a convergence right over Hanoi, causing non-stop rain throughout the day."

A high-pressure system over China prevented the wind convergence from moving and kept it in place, he added.

Prof. Dr. Nguyen Minh Truong, vice chair of the department of oceanology, meteorology and hydrology at the University of Natural Sciences, Vietnam National University, said Typhoon Bualoi, having moved to Laos after weakening into a tropical depression on Monday afternoon, merged with a low-pressure area, trapping moisture and causing widespread rains across northern Vietnam.

"Meanwhile, a weak cold air mass moving from the north to south interacted with the eastern and southeastern edges of the low-pressure area, causing widespread convergence of winds and moisture. The result is heavy downpours across the north and north-central regions."

Mai Van Khiem, director of the National Center for Hydro-Meteorological Forecasting, said while the residual effects of Bualoi weakened in Laos, they continued to cause heavy rains in northern and north-central Vietnam.

Recent rainfall data shows that the soil in the northern mountains and midlands is now saturated with moisture, increasing the risk of landslides in sloped areas, he warned.