Heatwaves and Tropical Nights Starting Earlier and Ending Later: 'June–September Heat' Becoming the Norm

It has been confirmed that heatwaves and tropical nights are starting earlier and ending later than in the past.

This is because the North Pacific High expands its influence earlier and persists longer.

According to an analysis released today (the 24th) by the Korea Meteorological Administration comparing the start and end dates of heatwaves and tropical nights between the recent decade (2016–2025) and a past decade (1973–1982), the onset of heatwaves has advanced by 10 to 30 days nationwide, while their end dates have been delayed by around 10 days.

A heatwave day is defined as a day when the daily maximum temperature reaches 33°C (91°F) or higher.

In regions where heatwaves begin earlier than in other areas—such as Gangneung, Daegu, Hapcheon, Miryang, Yeongdeok, and Pohang in the Yeongnam region—heatwaves over the past decade have started in early June, advancing by more than 10 days compared to the past decade when they began in mid-to-late June.

Notably, the average start dates for heatwaves in Gangneung and Daegu over the past decade were June 9 and June 1, respectively, which were about 20 days earlier for Gangneung and 10 days earlier for Daegu compared to the past decade (June 28 and June 11).

These regions experience earlier heat because hot and humid winds blowing along the edge of the North Pacific High become even hotter as they cross over the mountains.

In inland central areas such as Seoul, Icheon, Chuncheon, and Cheongju, as well as Gwangju, Gumi, Uiseong, and Uljin, heatwaves have also begun in June during the recent decade, arriving earlier than in the past decade when they started in early-to-mid July.

In Seoul, heatwaves that used to start on average on July 19 during the past decade began on June 24 in the recent decade.

In Busan, Yeosu, Namhae, Tongyeong, and Geoje, where heatwaves rarely occurred in the past decade, years with heatwaves have become more frequent in the recent decade.

Nationwide, the end dates of heatwaves in the recent decade fell in 'mid-to-late August,' delayed by around 10 days compared to the past decade when they ended in 'mid-August.'

In Daegu, the average heatwave end date over the past decade extended beyond August into September 2, showing a significant delay compared to the past decade (August 23).

In Seoul, heatwaves used to end by Liberation Day (August 15) in the past decade, but in the recent decade, they did not end until August 21.

Tropical nights—defined as nights (from 6:01 PM to 9:00 AM the following day) where the lowest temperature remains at 25°C (77°F) or higher—historically began in 'mid-to-late July' nationwide during the past decade, but now start in 'early-to-mid July' in the recent decade, arriving about 5 to 15 days earlier.

The end dates for tropical nights have been delayed by 10 to 20 days, shifting from 'mid-to-late August' in the past decade to 'mid-August to early September' in the recent decade.

In particular, tropical nights along the southern coast and Jeju Island did not end until early September in the recent decade.

During the past decade, tropical nights no longer occurred along the southern coast and Jeju Island once late August arrived.

In Gangneung, tropical nights that used to start on July 17 in the past decade began 26 days earlier on June 21 in the recent decade, and ended on August 28 instead of August 12 (16 days later), resulting in a 40-day increase in the number of tropical nights.

As they start earlier and end later, the number of both heatwave days and tropical night days has increased.

Over the 53-year period from 1973 until last year, nationwide heatwave days and tropical night days increased by 1.8 days and 1.6 days per decade, respectively.

In the recent decade, the average number of heatwave days reached 18.3 days, which is 2.2 times the past decade (8.3 days), while tropical night days surged 3.0 times to 12.2 days compared to the past (4.1 days).

Looking at heatwave and tropical night days by year, the record highs were set in 2018 for heatwaves (31.0 days) and 2024 for tropical nights (24.5 days).

Along with these, 4 of the past 10 years ranked in the top 10 for heatwave days, and 7 of the past 10 years ranked in the top 10 for tropical night days.

The Korea Meteorological Administration pointed to changes in the North Pacific High as the cause of these shifting patterns in heatwaves and tropical nights.

The North Pacific High is the single biggest factor determining South Korea's summer weather. When it expands toward the Korean Peninsula, hot and humid air flows in along its edges, and clear skies with intense sunlight trigger heatwaves.

However, an analysis of the 500 hPa upper atmospheric (at an altitude of about 5.5 km) geopotential height distribution between the past and recent decades confirmed that the North Pacific High has been expanding northwestward starting from June and positioning itself to the south of the Korean Peninsula in the recent decade.

Furthermore, the North Pacific High has been shown to linger in the southern part of the country until late August in the recent decade.

This is the reason why the weather remains hot from June through September.

The warming of the seas surrounding the Korean Peninsula was also analyzed as a contributing factor to the prolonged duration of tropical nights.
※ Please note: This article was translated by AI and may contain errors.