No relief, even after dark: Record tropical nights add to Korea's heat crisis

The greater Seoul area is sweltering under extreme heat nearing 40 degrees Celsius (104 degrees Fahrenheit) during the day, while tropical nights approaching 30 degrees Celsius persist after dark.

With little relief day or night, the risk of heat-related illness also remains high.

Temperatures nearing 40 degrees Celsius were recorded across the greater Seoul area on Wednesday, with the Korea Meteorological Administration issuing its highest-level heat wave warning. Nowon District in northern Seoul and Hanam in Gyeonggi recorded 39.2 degrees Celsius and 39.3 degrees Celsius, respectively.

The heat lingered even long after the sun went down.

Temperatures in Seoul stayed above 29 degrees Celsius overnight, well above the 25-degree threshold for a tropical night. Dongjak District in southern Seoul recorded a nighttime low of 30.1 degrees Celsius on Tuesday, meeting the standard for a super tropical night, defined as a nightly low of at least 30 degrees Celsius.

Hottest nights on record

This summer is on track to become the hottest on record, both during the day and at night. Regions nationwide saw an average of 12.1 tropical nights between June through Tuesday, the highest figure since nationwide weather observations began in 1973.

The average number of heat wave days reached 13.1, the fourth-highest on record.

Temperatures also remained high overnight, with the nationwide average daily minimum reaching 23.4 degrees Celsius in June, 2.2 degrees above normal and the highest on record.

July recorded an average of 9.7 tropical nights, surpassing the previous record of 8.8 days set in July 2024.

“Warm, humid air has increased the amount of water vapor in the atmosphere,” Im Bo-kyung, an official at the Korea Meteorological Administration's climate change monitoring division said. “That water vapor and cloud cover weakened radiational cooling overnight, preventing daytime heat from dissipating and leading to widespread tropical nights.”

Morning heat illness cases surge

Tropical nights have been increasing faster than heat waves in recent decades. There was an average of 16.9 heat wave days between 2020 and 2024, a 2.2-fold increase from the average of 7.7 days in the 1910s.

Over the same period, the average number of tropical nights increased more than fourfold, from 6.7 to 28 days.

The morning hours are particularly dangerous. A total of 2,441 people were treated for heat-related illnesses between May 15 and Tuesday, with 317 visiting emergency rooms between 6 a.m. and 10 a.m. The share of heat-related illness patients during those hours rose from 8 percent in 2022 to 13 percent this year.

The World Health Organization says cumulative stress on the human body increases when daytime and nighttime temperatures remain high for extended periods, raising the risk of heat-related illness and death. The organization also recommends keeping indoor temperatures below 24 degrees Celsius at night.

“Climate change is raising temperatures while also increasing atmospheric water vapor, making tropical nights more likely,” Ban Ki-sung, head of weather analysis agency K Climate, said. “Seoul's urban heat island effect is particularly severe, preventing heat from dissipating overnight and contributing to more heat-related illness cases in the morning.”

BY CHON KWON-PIL [lee.taehee2@joongang.co.kr]