Rain Clouds Refuse to Retreat Even in September... Late-Summer Heat and Torrential Rain Last Longer

September rain-cloud corridor 'front' shifts 213 km north

Joint research team from the APEC Climate Center and KAIST

Analysis of East Asian fronts from 1979 to 2025

A study drawing attention has found that the rain-cloud corridor, which should retreat southward by September, is staying farther north than in the past and driving late-summer heat and weather changes.

An analysis of the position of East Asia's summer front over the past 20 years found that its central axis moved about 213 km northward in mid-to-late September compared with the past. The findings shed light on why late-summer heat and torrential rain continue into early autumn.

A joint research team from the APEC Climate Center (APCC, President Ha Kyeong-ja) and the Korea Advanced Institute of Science and Technology (KAIST, President Bae Choong-sik) analyzed 47 years of climate data from 1979 to 2025 and confirmed the northward shift of the East Asian summer front, they announced on October 23. The findings were published in the international journal npj Climate and Atmospheric Science.

A front is the boundary where cold air meets warm, humid air with different properties. Air rises along the boundary, allowing rain clouds to develop and potentially bringing torrential rain. When hot, humid air lingers, it also becomes harder for the body to regulate its temperature, increasing heat stress.

Comparison of Changes in Summer Front Locations Between Past and Recent Climates.

An analysis of reanalysis data from the European Centre for Medium-Range Weather Forecasts found that the front was about 146 km farther north during the rainy season in the most recent 20-year period (2006-2025) than in the previous 20-year period (1979-1998). From September 10 to 30, the northward shift was even greater, at about 213 km. The change in the front's position was more pronounced at the start of autumn than during the rainy season.

The researchers also analyzed wet-bulb globe temperature (WBGT), which reflects the effects of humidity and sunlight as well as temperature. The results showed that the northward shift of the front in September increases the risk of late-summer heat and torrential rain occurring together.

Dr. Moon Su-yeon of APCC said, "The significance of this study is that it identified changes in the position of atmospheric fronts as the reason why unusually late-season heat and heavy rain have become more frequent even in September in recent years."

Dr. Moon added, "As summer-related disasters continue later into the year, an integrated disaster-prevention strategy that prepares for both heat waves and torrential rain is needed."

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