The Changing Face of Shanghai's Plum Rain

The rain stops, the sun comes out, and before long another downpour arrives. This increasingly erratic pattern has become the new face of Shanghai's plum rain season.

Plum rain is a hallmark of the Jiangnan region (the area south of the lower reaches of the Yangtze River), named for the season when green plums ripen. It is also commonly known as the "mold rain" because humidity often exceeds 80 percent, leaving the air hot and clammy.

While rainfall is generally less intense than the heavy downpours of midsummer, plum rain lasts much longer and can bring substantial cumulative precipitation, increasing the risk of urban flooding and river flooding.

A changing plum rain season

In the past, the plum rain season was marked by steady rainfall and long stretches of gray, overcast weather. Today, that familiar pattern is changing. Prolonged periods of continuous rain have become less common, replaced by more frequent swings between sunshine and showers.

At the same time, rainfall has become more extreme. Instead of days of light, lingering rain, the season now brings more short-lived but intense downpours and thunderstorms, with heavier rain falling in shorter periods.

Rainfall is also becoming more unevenly distributed. While some areas may experience torrential downpours, others may receive only light rain or none at all.

The science behind the shift

The formation of plum rain is driven by a combination of large-scale atmospheric patterns.

During summer, the East Asian summer monsoon carries warm mosit airflow from the ocean northward. When it meets lingering cold air over the Jianghuai region (the plain between the lower reaches of the Yangtze River and the Huai River Basin), a persistent rain belt forms.

A key factor shaping the plum rain season is the Western Pacific Subtropical High. Its position and movement influence where the rain belt forms and how it shifts.

So why is plum rain becoming more unpredictable? Scientists point to global warming and its impact on atmospheric circulation. Changes in mid- and high-latitude weather patterns can cause the Western Pacific Subtropical High to shift more frequently, pushing the plum rain belt back and forth.

At the same time, rising temperatures are increasing atmospheric instability, making convective rainfall more frequent. As a result, the traditional pattern of prolonged drizzle and overcast skies is becoming less common, while more intense rainfall events are emerging.

Nowadays, meteorological departments must meet certain criteria before declaring plum rain season.

After the South China Sea summer monsoon breaks out, daily average temperature, subtropical high ridge position, consecutive rainy days, and rainfall intensity must be met simultaneously. Only then can the dates for the start and end of the plum rain season be determined.

These days, Fengyun satellites, meteorological radar, and artificial intelligence forecasting collaborate to anticipate downpours and changes between rain and sunshine during the plum rain season.

Plum rain season tips

During the plum rain season, checking real-time weather updates before going out, keeping indoor spaces dry, and taking care with food storage can help reduce the impact of persistent humidity and sudden downpours.