In Korea, summer heat waves and tropical nights have been starting earlier and retreating much later than in the past, a clear trend shows.
On the 24th, the Korea Meteorological Administration analyzed the start and end dates of heat waves and tropical nights over the past 10 years (2016–2025) and the initial 10 years of observations (1973–1982), finding that the onset of severe heat (heat waves), when daytime highs exceed 33 degrees, has moved up nationwide by as little as 10 days to as much as about 30 days. By contrast, the end of these scorching conditions has been delayed by around 10 days.
In the Yeongnam region, including Gangneung, Daegu, Hapcheon, and Miryang, where heat arrives earlier than in other areas due to geographic characteristics, heat waves have already begun to rage from early June over the past 10 years. Compared with the past, when the real heat did not begin until mid to late June, the clock has moved up by more than 10 days.
In fact, the average start dates of heat in the past 10 years were found to be June 1 for Daegu and June 9 for Gangneung, moved up by about 10 days and 20 days, respectively, compared with the past. These areas are hit head-on by early heat because warm, humid winds flowing in along the edge of the North Pacific high become further heated as they cross rugged mountain ranges.
Regions such as the central inland, including Seoul, as well as Gwangju and Gumi, where heat waves used to begin in July, have also recently seen sweltering conditions arrive in June. In particular, in Seoul, the extreme heat that used to begin on average on July 19 now starts on June 24, nearly a month earlier. Even southern coastal cities such as Yeosu, Tongyeong, and Geoje, where heat waves were rare in the past, have recently seen a sharp rise in the number of heat-wave days.
The point at which the heat recedes has also been delayed. In the past, the force of heat waves weakened by mid-August, but over the past 10 years they have often continued through late August. In Daegu, the end date of the heat has been pushed back to Sept. 2, clearly lengthening compared with the past, and in Seoul, the scorching heat that used to subside around Liberation Day now does not end until after Aug. 21.
Tropical nights, when temperatures do not fall below 25 degrees after sunset and make it hard to sleep, show the same pattern. Nighttime heat that used to appear from mid to late July has arrived 5 to 15 days earlier, in early to mid-July, over the past decade. Conversely, the time when nighttime heat fully disappears has been delayed by up to 20 days, from late August in the past to early September recently. In particular, on Jeju Island and along the southern coast, tropical nights did not end until September.
As the start and end of heat have stretched out, both heat-wave days and tropical-night days have increased. Over the past 53 years, the number of heat-wave and tropical-night days in Korea has risen by 1.8 days and 1.6 days per decade, respectively. In the past 10 years, the annual average number of heat-wave days reached 18.3 days, 2.2 times the past figure, and tropical-night days rose to 12.2 days, three times higher than before. Statistically, the hottest year on record was 2018, when heat waves lasted 31 days, and the worst year for tropical nights was 2024, at 24.5 days.
The Korea Meteorological Administration pointed to changes in the North Pacific high as the cause of this phenomenon. An analysis of the 500 hPa upper atmosphere (about 5.5 kilometers in altitude) geopotential height distribution for the past 10 years versus the previous 10 years found that over the past decade this high expanded northwest from early June, taking up a position south of Korea. The tendency for this high to stay for an extended period through late August into September has also solidified.
In addition, the warming of seawater temperatures in waters near Korea has contributed to lengthening the period of tropical nights. When sea surface temperatures rise, more water vapor flows onto land, and this warm, humid air tightly blocks radiative cooling, the process by which ground heat escapes to space overnight, preventing nighttime temperatures from dropping.