What Temperatures Trigger a Tropical Night and a Heat Advisory, and Where Are They Written Down?

The short answer: 25°C at night and a feels-like 33°C or 35°C

A tropical night is a night when the minimum temperature stays at 25°C or higher. For heat alerts, an advisory is issued when the daily maximum feels-like temperature is expected to reach 33°C or higher for two days or more, and a warning when it is expected to reach 35°C or higher for two days or more. These figures come from the Korea Meteorological Administration's explanation of tropical night counts and from its criteria for issuing alerts. Heat wave thresholds are judged not by air temperature but by the feels-like temperature, which factors in humidity.

The night window is defined as well. The KMA Open MET Data Portal sets night as running from 18:01 on one day to 09:00 the next. This statistical definition must be distinguished from the criteria for a tropical night advisory. The nighttime minimum thresholds used for that advisory vary by region. A KMA press release states that large cities with populations of 500,000 or more and coastal and island areas use 26°C, while Jeju Island uses 27°C.

How the Open MET Data Portal defines a tropical night and the 18:01–09:00 night window

The KMA Open MET Data Portal's explanation of tropical night counts contains this sentence: "The number of tropical nights is defined as days on which the nighttime minimum temperature is 25 °C or higher." The same page gives the reasoning: when the temperature does not drop to 25°C or below even at night, it is too hot for people to sleep, so tropical nights serve as an indicator of heat. Averages for the country as a whole and for each major region draw on 62 stations, and four stations on Jeju Island are used only for the Jeju figures. The original text can be read directly on the KMA Open MET Data Portal.

International bodies describe heat waves somewhat differently. The World Health Organization (WHO) calls a heat wave a period of unusually hot days and nights during which excessive heat builds up locally. The World Meteorological Organization (WMO) notes that heat waves are generally defined by regional thresholds rather than by a single global temperature value. Korea's statistical definition of a tropical night at 25°C and its regional advisory thresholds of 25°C, 26°C and 27°C serve different purposes and have different conditions. The bigger patterns behind nighttime temperatures are covered separately in our article on El Niño and La Niña.

KMA criteria: 33°C for a heat wave advisory, 35°C for a warning

Weather alerts cover 11 phenomena: strong wind, high seas, heavy rain, heavy snow, dryness, storm surge, typhoon, yellow dust, heat wave, tropical night and cold wave. The tiers vary by phenomenon: heat waves have advisory, warning and critical warning levels, while tropical nights have an advisory only. A heat wave advisory is issued when the daily maximum feels-like temperature is expected to stay at 33°C or higher for two days or more. A heat wave warning uses the same conditions with a threshold of 35°C. The same figures appear in the ehtis guide to heat waves.

Both thresholds turn on the same condition: a span of two days or more. The WHO explains that prolonged high temperatures by day and night build up cumulative stress on the body, raising the risk of illness and death. The WMO notes that unusually warm nights interfere with the recovery process by which the body sheds the heat it absorbed during the day. The US National Weather Service (NWS) explains that high relative humidity slows the evaporation of moisture from the body. High humidity slows the evaporation of sweat, so the same air temperature can feel hotter.

May 15, 2023: the switch to feels-like-temperature heat alerts

The KMA announced that it would factor humidity into heat alerts and add a "super strong" category to its typhoon scale. Korea.kr reported that heat alerts based on feels-like temperature would enter full operation on May 15, 2023. Previously the alerts considered only air temperature; the improved system is based on a feels-like temperature that takes humidity into account. The same material notes that new climate normals were also taken into account, reflecting environmental shifts brought by climate change. The feels-like temperature is a value representing the heat or cold a person actually senses, depending on humidity and wind.

Explanations from international bodies point in the same direction. The WHO states that the heat accumulating in the body depends on environmental heat stress such as high temperature, high humidity, weak wind and strong thermal radiation. The WMO explains that heat stress is determined by a combination of temperature, humidity, solar radiation and wind speed, which is why scientists use heat indices that combine temperature with humidity, solar radiation and wind speed. The NWS states that the values in its heat index table apply to shaded conditions. For wind and ocean conditions set side by side, see our comparison of winter in Seoul and winter in London.

The 2026 overhaul and Japan's heatstroke alert, 38°C, 39°C and WBGT 33

A KMA press release states that a top tier, the critical heat wave warning, and a tropical night advisory have been added to the heat alert system introduced in 2008. The changes take effect from 2026. A critical heat wave warning is issued in areas where a daily maximum feels-like temperature of 35°C or higher has been observed for two days or more. In such an area, the alert is issued even if a feels-like temperature of 38°C or higher, or a maximum temperature of 39°C or higher, is expected for just one day. The tropical night advisory exists only at the advisory tier. The KMA criteria require a daily maximum feels-like temperature of 33°C or higher to be forecast for two days or more, together with the relevant nighttime minimum threshold. That threshold is 25°C in general, 26°C in cities with populations of 500,000 or more and coastal and island areas, and 27°C on Jeju Island.

Setting Korea and Japan side by side item by item makes the differences clear. Korea's names are heat wave advisory, heat wave warning, critical heat wave warning and tropical night advisory. Japan has the heatstroke alert (熱中症警戒アラート). Korea's heat alerts combine daily maximum feels-like thresholds of 33°C, 35°C and 38°C, and an air-temperature threshold of 39°C, with duration and observation conditions. Its tropical night advisory adds regional nighttime thresholds of 25°C, 26°C and 27°C to the heat conditions. Japan's is a forecast daily maximum wet-bulb globe temperature (WBGT) of 33 for the next day or the same day. The issuing body is the KMA in Korea and the Ministry of the Environment and Japan Meteorological Agency jointly.

Japan's Ministry of the Environment describes WBGT as a heatstroke prevention index calculated from temperature, humidity, solar radiation and other factors. Values for 841 stations nationwide are available on the ministry's heatstroke prevention information site. In Korea, the Korea Disease Control and Prevention Agency has run an emergency room surveillance system for heat-related illness since 2011, covering some 500 medical institutions with emergency rooms nationwide. In 2023, 2,818 heat-related illness cases were reported, up 80.2% from the previous year, and estimated deaths reached 32, an increase of 255.6%. The single busiest day during the surveillance period was August 3, 2023, with 133 patients.

Putting it together

The ehtis guide defines a heat wave as a disaster in which abnormally high temperatures persist for several days to several tens of days, causing harm to people and property. The same material notes that heat waves in Korea cluster in June through August and tend to become more frequent after the monsoon ends in mid-July. It also explains that because the urban heat island effect occurs frequently, city residents tend to suffer more than those in rural areas. Impact-based heat wave forecasts sort risk into four levels: attention, caution, alert and danger.

The numbers make the meaning of the thresholds clearer. A KMA press release notes that the national average number of heat wave days rose from 8 in the 1970s to 19 over the past five years, and tropical nights from 4 to 14. The same material states that for an identical daily maximum feels-like temperature, heat-related illness cases rise by up to about 90% if the previous night was a tropical night. Korea Disease Control and Prevention Agency data put the number of heat wave days in 2023 at 14.2, which is 3.6 more than the 10.6 recorded in 2022. The national average summer temperature in 2023 was 24.7°C, 1.0°C above the normal of 23.7°C. Records like these help explain why nighttime temperatures were written into the criteria separately.