'Cooler After Rain? An Illusion': The Terror of 'Humid Heatwaves' Following Showers

With maximum temperatures around 39°C (102°F) and maximum sensible temperatures of 38°C (100°F) or higher expected, a "Severe Heatwave Warning" was issued for Gyeongsan and Pohang in North Gyeongsang Province at 10:00 AM on July 12, 2026, for the first time in South Korea's meteorological history. This is the highest level of the heatwave warning system, which was reorganized for the first time in 18 years, becoming a reality just one month after its introduction. The administrator of the Korea Meteorological Administration (KMA) personally held a briefing, urging the immediate suspension of outdoor activities, warning that even healthy individuals face a significantly high risk of serious harm, including heat-related illnesses or death. But it is strange. Why is it said to be more dangerous when rain is forecast? Why specifically the southern part of North Gyeongsang Province? And will this heatwave ever end?

1. "Isn't 39°C (102°F) Just Hot?" This Illusion Threatens Lives

This warning does not simply mean "it is very hot." The World Health Organization (WHO) warns that extreme heat disrupts the body's temperature regulation system, causing heat stroke and heat exhaustion, and worsening cardiovascular, respiratory, and kidney diseases. Crucially, even healthy adults can be at risk. According to an analysis by the Korea Disease Control and Prevention Agency (KDCA) based on data from the domestic heat-related illness emergency room surveillance system over recent years, when the sensible temperature reaches 38°C (100°F) or higher, the risk of death for those under 65 increases by 4% compared to usual, and the risk of death from cardiovascular disease increases by 7%. For those aged 65 and older, these risks jump to 19% and 14%, respectively. The 2024 report of the international medical journal The Lancet Countdown is even more shocking. Heat-related deaths among the population aged 65 and older increased by 167% compared to the 1990s, and exposure to heat stress during outdoor physical activity in 2023 increased by 27.7%.

2. Why Gyeongsan and Pohang? The 'Oven Effect' Created by Mountains

The southern part of North Gyeongsang Province is particularly vulnerable due to its topography. The "Foehn effect" occurs as hot southwesterly winds cross the mountains, compressing and further heating the air. Hayang-eup in Gyeongsan recorded 39.9°C (104°F) on the afternoon of July 11, while Gigye-myeon in Pohang soared to 37.2°C (99°F). A study on South Korean heatwaves published in Climate Dynamics, a world-renowned journal in climatology, clearly points this out: "South Korean heatwaves can be intensified by topographic effects, particularly the Foehn effect." Furthermore, the basin topography surrounded by mountains traps the heat once it enters. A World Bank report explains that the urban heat island effect in East Asian cities is on average 1.6 to 2.0°C (2.9 to 3.6°F) higher than surrounding areas, becoming particularly stronger at night. While the term "Daefrica" (a portmanteau of Daegu and Africa) was used in the past, the entire North Gyeongsang Province, including Gyeongsan and Pohang, is now becoming the new epicenter of extreme heat.

3. "Will It Get Cooler Once It Rains?" It Is Actually More Dangerous

Why does the KMA say the "sweltering heat will continue" even though rain is forecast starting July 14? A 2025 study in the global scientific journal Nature Communications provides the answer. When surface moisture is replenished after light rain and the sun shines again, active evaporation can actually increase the Wet Bulb Globe Temperature (WBGT). The researchers warned, "More than half of extreme humid-heat events can occur accompanied by or immediately after light rain." The KMA expressed the same concern: "If rain falls heavily but briefly in a narrow area, and strong sunlight returns under high humidity conditions, the sensible temperature will rise rapidly." In particular, the Gyeongsang region is highly likely to see the heat persist, as the rain will start late and the amount will be small.

4. Why Tropical Nights Are Scarier: The Body That Cannot Cool Down Even at Night

Seoul recorded its first tropical night of the year between the night of July 11 and the early morning of July 12. Although this is 12 days later than last year, the real problem lies elsewhere. A study on South Korean tropical nights published in JGR Atmospheres, an international journal published by the American Geophysical Union (AGU), revealed that nighttime heat is not simply a remnant of daytime heat. It states, "Warm and humid air flowing in on southwesterly winds, high water vapor, and increased longwave radiation combine to intensify it through a separate mechanism." The WHO and the U.S. Centers for Disease Control and Prevention (CDC) warn that failing to lower body temperature at night accumulates strain on the cardiovascular, respiratory, and renal systems, increasing vulnerability to heat stress the following day. Tropical nights have occurred across the country, including Incheon, Gangneung, Cheongju, Gwangju, Pohang, Busan, and Jeju. It is no longer enough to be careful only during the day; nights are dangerous too.

5. Korean Peninsula Trapped in a 'Double Heat Dome': A Collaboration of Desert Heat and Ocean Moisture

What is the true nature of this heatwave? The KMA explained, "The Tibetan High in the upper atmosphere and the North Pacific High in the middle and lower atmosphere have covered the Korean Peninsula simultaneously." This is referred to as a "double heat dome" or a "double high-pressure blanket phenomenon." A paper in Climate Dynamics analyzes this as a typical pattern of South Korean heatwaves: "The combination of the abnormal expansion of the Western Pacific Subtropical High, upper-level anticyclonic circulation, and stagnant patterns intensifies the East Asian anticyclonic anomaly." The North Pacific High traps heat from the ground up to 5 kilometers in the air, while the Tibetan High traps it at an altitude of 10 to 15 kilometers. On top of this, hot and humid winds blowing from the south have further intensified the heatwave. This is no coincidence; it is a result organized within larger intraseasonal and planetary-scale circulation changes.

6. Sensible Temperature of 38°C (100°F) vs. Air Temperature of 39°C (102°F): Why Both Are Criteria

Why does the new Severe Heatwave Warning standard use both a maximum temperature of 39°C (102°F) "or" a maximum sensible temperature of 38°C (100°F)? A study on Seoul and Busan published in the Journal of Exposure Science & Environmental Epidemiology, which covers exposure science and epidemiology, provides the answer: "Composite indices such as heat index, sensible temperature, and WBGT capture health impacts better than simple air temperature. In Busan, in particular, WBGT showed the strongest correlation with mortality." WBGT is an international standard heat stress index that reflects not only temperature but also humidity, radiation, and wind. This means that the presence of a sea breeze does not automatically make it safe. The heat strain on the body, including humidity, must be examined. That is why the KMA presented both criteria simultaneously.

7. Not a One-Time Anomaly: Asia Is Already Warming Up

Viewing this heatwave as a "one-off extreme weather event unique to this year" misses the bigger picture. The State of the Climate in Asia 2024 report officially released by the World Meteorological Organization (WMO) is shocking: "Asia is warming at nearly twice the rate of the global average, and 2024 was either the hottest or the second hottest year on record in Asia." The report specified that long-term heatwaves continued in East Asia from April to November 2024, and South Korea also broke monthly average temperature records in April, June, August, and September. An even bigger problem is the ocean. The WMO stated that sea surface temperatures in Asia in 2024 were record-high, and the area of marine heatwaves was also the largest in history. In particular, strong marine heatwaves were reported in waters near Japan and around the Yellow Sea and the East China Sea. Warmer surrounding seas enhance the supply of water vapor to the atmosphere. This is the background against which sticky heat does not easily dissipate even after rain.

8. "Wet-Bulb Temperature of 35°C (95°F)" Is Theoretical; Real Danger Begins at Much Lower Temperatures

The "wet-bulb temperature of 35°C (95°F)" that scientists talk about is a theoretical limit for survival. However, a study in Science Advances, a top-tier multidisciplinary scientific journal, and explanatory materials from the National Aeronautics and Space Administration (NASA) warn: "Actual deaths and health damage occur at much lower wet-bulb temperatures." In other words, thinking "It's not even 40°C (104°F) yet" underestimates the danger. A 2023 paper in Nature is more specific: "In humid climate zones, cities may not just be hotter, but can create more dangerous humid heat, resulting in urban residents experiencing an additional 2 to 6 days of hazardous heat stress each summer." This finding is particularly important in environments where the monsoon season, high temperature and humidity, and high-density urbanization overlap, such as Seoul, Busan, Daegu, and Gwangju. Heatwave responses must use humidity management and nighttime cooling resilience as key indicators, in addition to trees, shades, and ventilation.

9. Who Collapses First: Those Without Air Conditioning, Outdoor Workers, and Elderly Living Alone

This warning is also an issue of inequality. The International Labour Organization (ILO), a UN agency, states that excessive heat has a significant impact on workers' safety and health, causing heat exhaustion, heat stroke, death, and long-term chronic diseases. Construction, delivery, logistics, and outdoor service work are the most concerning sectors. According to statistics from the Ministry of the Interior and Safety and local governments, Gyeongsan is home to a total of 34,399 people vulnerable to heatwaves, including 15,778 elderly living alone, 16,475 basic livelihood security recipients, and 2,146 people with disabilities. Pohang is in a similar situation. The World Bank reports that urban heat islands and extreme heat in East Asia take a toll on productivity, health, and habitability. Those who lack access to air conditioning, residential insulation, shade, the right to rest, and work control for mobile workers are hit first. Heatwaves are not just meteorological disasters, but disasters of labor, welfare, and urban policy.

10. What Lies Ahead: More Frequent, Longer, and Humid

Although this heatwave peaks on July 13 and rain falls on July 14, the KMA states clearly: "In the future, temperatures are on a trend of being maintained or strengthened rather than decreasing, and heatwave warnings are also likely to be maintained or expanded. The Severe Heatwave Warning may also be expanded or additional regions may be added depending on the circumstances." According to the KMA's analysis of data over the 10-year period from 2016 to 2025, if the Severe Heatwave Warning had existed at that time, it is estimated that it would have been issued in Gyeongsan for an annual average of 3.1 days. This is the highest level among all meteorological warning zones nationwide. During the "worst heatwave of the 21st century" in 2018, it is analyzed that the warning would have been maintained for eight consecutive days in Gyeongsan. The Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6) views that the risk of heat stress in Asia's hot and humid environments is already at a medium or higher level, and urban heat islands can worsen this by more than 2°C (3.6°F). Due to its location in the East Asian monsoon and coastal urban zone, South Korea is a typical example of the hot, humid coastal and urban risk zones warned of by the IPCC.

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The warning of 39°C (102°F)—this was no simple heat. Double heat domes, the Foehn effect, humid heat, tropical nights, and the sweltering heat after rain. South Korea must now simultaneously upgrade its warning systems, labor standards, urban design, and public health responses for the era of humid heatwaves. We must remember the words of the KMA administrator. The Severe Heatwave Warning does not simply mean the weather is very hot; it represents a situation where even healthy individuals face a significantly high risk of serious harm, such as heat-related illnesses or death.

Stop. Move. Check. These three rules determine survival.


Deep Dive Q&A
Q1. How does the "Severe Heatwave Warning" differ from the existing heatwave warning, and why was it introduced now?

A1. The existing heatwave warning was issued when the maximum sensible temperature was expected to be 35°C (95°F) or higher for two or more consecutive days. However, as ultra-extreme heatwaves with sensible temperatures nearing 38°C (100°F) became more frequent due to climate change, it became difficult to warn of actual fatal risks with the existing warning alone. The newly issued "Severe Heatwave Warning" is triggered even if a maximum temperature of 39°C (102°F) or a maximum sensible temperature of 38°C (100°F) or higher is forecast for just one day. It signifies a top-level disaster state where not only the elderly and vulnerable but even healthy adults can suffer heat-related illnesses or death during outdoor activities.

Q2. It is common for it to cool down temporarily when it rains in summer. Why does the KMA warn that it is more dangerous after it rains?

A2. This is due to the "humid heat" phenomenon, which has also been warned about in international academic journals such as Nature Communications. When the sun shines again immediately after a shower or short rain, the moisture absorbed by the ground evaporates rapidly, driving atmospheric humidity to extremes. When humidity is high, the human body loses its ability to regulate temperature because sweat does not evaporate. In other words, even if the air temperature itself drops slightly, the composite indicators that determine sensible temperature skyrocket, doubling the heat stress on the human body.

Q3. What are the structural reasons why Gyeongsan and Pohang in North Gyeongsang Province became the targets of this first Severe Heatwave Warning?

A3. It is due to the combination of topographic factors (the Foehn effect) and basin structures. The "Foehn effect," where hot and humid winds blowing from the southwest cross the Daegwallyeong Pass or nearby mountain ranges, compressing the air and making it hotter and drier, is concentrated in this region. In addition, the southern parts of North Gyeongsang Province, including Gyeongsan and Pohang, have a basin shape that traps heat, creating a vicious cycle where the oven effect generated during the day continues into the night without cooling down.
※ Please note: This article was translated by AI and may contain errors.