The Long Summer May Finally Be Ending: Autumn Nears the Gangwon Mountains First, Nationwide by Late September

The long South Korean summer may finally be nearing its end, with the first signs of autumn appearing in the Gangwon mountains and some inland areas.

Average daily temperatures have begun falling below 20°C in those regions. Under the standard used to define the start of autumn by average temperature, some mountain areas could cross the threshold around the middle of September. A nationwide transition, including Seoul, is more likely in late September.

The calendar says autumn has already begun because the Korea Meteorological Administration classifies September through November as autumn. But its climate analysis uses a separate measure for the start of each season. Autumn begins when the nine-day moving average of daily temperatures first drops below 20°C and does not rise above it again.

That means one or two cool days do not immediately mark the start of autumn. Recent observations showed temperatures falling below the 20°C line first in northern and mountainous areas. Daegwallyeong’s average daily temperature was 21.9°C on Aug. 30, then fell to 20.1°C on Sept. 1, 19.0°C on Sept. 3 and 17.5°C on Sept. 4.