Cheoseo's cooling promise fades as Korea's summers grow longer and hotter

After a sweltering, humid summer, a popular Korean proverb says that come the time for Cheoseo, even the mosquitoes' mouths will twist because the weather gets too cold for them. 

Literally translating to “the end of heat,” Cheoseo is the 14th of the 24 solar terms used in East Asia to mark seasonal changes, falling around late August each year. That seasonal marker is increasingly becoming irrelevant, however, as heat waves arrive earlier each year and tropical nights extend into September. 

The Korea Meteorological Administration (KMA) on Friday released a report analyzing heat wave and tropical night records from 66 weather stations between 2016 and 2025, comparing them with data from 1973 to 1982.

It found that heat waves are beginning 10 to 30 days earlier and ending later, pushed back by about 10 days, indicating that Korea's summers are growing longer rather than simply hotter.

Tropical nights are beginning five to 15 days earlier and ending 10 to 20 days later than they did in the past decade. A tropical night occurs when the overnight low remains at or above 25 degrees Celsius.

Along the country's southern coast and on Jeju Island, tropical nights have increasingly continued into early September even after daytime heat waves subside.

Last year, 21 weather stations including those in Gangneung, Gwangju and Daejeon recorded their earliest tropical nights since observations began. Seogwipo, located at the southern part of Jeju, recorded its latest tropical night of 2025 on Oct. 13.

The intensity of summer heat has also increased.

The nationwide average number of heat wave days over the recent decade reached 18.3, up from 8.3 in 1973 to 1982. The average number of tropical nights nearly tripled to 12.2 days from 4.1 days over the same period.

Last year alone, the nationwide average reached a record of 24.5 tropical nights, the highest since observations began. 

The KMA attributed the trend to climate change.

Rising temperatures have allowed the North Pacific High — a high-pressure system over the North Pacific that brings hot and humid summer air to the Korean Peninsula — to expand earlier and remain over the Korean Peninsula longer than in the past, bringing earlier heat waves, according to the agency. Higher sea surface temperatures around the peninsula have also prevented nighttime temperatures from cooling, extending tropical nights later into the season.

The agency said longer summers require a broader approach to heat preparedness, including longer-term measures to prevent heat-related illnesses, protect vulnerable populations, ensure stable electricity supplies and reduce damage to the agriculture, livestock and fisheries sectors.

“Heat waves and tropical nights have been starting earlier and lasting later in recent years, extending the period during which people experience summer heat,” KMA administrator Lee Mi-seon said. “We will continue providing scientific analyses of climate change to support effective responses to the climate crisis.”

BY RHEE ESTHER [[email protected]]