tropical night


August 14, 2022

A tropical night is a term used by the Japan Meteorological Agency and refers to a nighttime (from the evening to the next morning) when the lowest temperature is 25 degrees or higher.


The word was originally coined by weather essayist Atsushi Kurashima. The Japan Meteorological Agency defines "nighttime" in tropical nights as "from the evening to the next morning" and does not define the time from what time to what time, and there are no official statistics on "tropical nights" according to temperature changes. Since the statistics of ``days with a minimum temperature of 25 degrees Celsius or more in a day (from 0:01 to 24:00)'' are published, newspapers and broadcasters use this as the basis for reporting for convenience. There is no special term used by the Japan Meteorological Agency to refer to "a day with a minimum temperature of 25 degrees Celsius or higher." In haiku, it is a season word for summer, but because it is a word based on modern meteorology, there is no example in traditional haikai or early modern haiku such as the Meiji period.

Super tropical night

"A night with a minimum temperature of 30 degrees Celsius or more" is sometimes described as a super tropical night. Although this is not an officially defined term by the Japan Meteorological Agency, the Japan Weather Association has decided to call it this way from August 2, 2022. This was decided by taking a questionnaire from 130 weather forecasters belonging to the Japan Weather Association, and among them, there were answers such as "burning night" and "scorching night". Toyama on July 31, 2000, Tokyo on August 11, 2013, Kuchinotsu (Nagasaki Prefecture) on August 5, 2017, Fukuoka on August 22, 2018, and August 2020. It was recorded in Osaka on the 11th and Hamamatsu on August 16th of the same year. Kanazawa on August 22, 1990 and Tokyo on July 21, 2004 were about to become ultra-tropical nights, but there were times when the lowest temperature per unit time was below 30 degrees Celsius, so they did not become ultra-tropical nights. rice field.


In summer, the sunshine hours increase, heat is transferred to the ground and buildings and accumulates, heat radiation is generated from materials with high heat capacity such as concrete and asphalt, and the night continues after sunset, so the temperature does not drop even at night. It is also considered to be one of the causes of the heat island phenomenon. Inland seas such as Tokyo Bay, Ise Bay, Osaka Bay, Seto Inland Sea, Hakata Bay, and Ariake Sea can have surface water temperatures above 30 degrees Celsius in midsummer. Nagoya City, Osaka City, Kobe City, Okayama City, Hiroshima City, Fukuoka City, Saga City, Nagasaki City, etc., in addition to the heat island effect, also maintains temperatures of 30 degrees or more until the end of the day due to the effect of bathing in the inner bay. can be seen.


The number of tropical nights in Tokyo is increasing, and was 56 days in 2010. In the past, most of the observations on Honshu were from mid-July to mid-August, but in recent years there have been occasional records from late-June to mid-October, so the trend is longer. In June 2011, the daily minimum temperature of 25°C or higher was recorded four times in Tokyo, 25.2°C and 25.4°C in Hiroshima and Koshino on October 3, 2016, and 25.0°C in Fukuoka on October 4, 2018. Tamano recorded 25.1℃ on October 6 and October 2, 2019, setting new records for the lowest temperature in October at both locations (Tamano tied for first place with the same number on both days).

Changes in the number of days over 30°C with minimum daily temperature

Until then, AMeDAS had been observing temperatures every 10 minutes, but since the new AMeDAS started operating on March 25, 2008, it has been calculated from observations every 10 seconds, just like the meteorological observatory and other meteorological offices. is delivered every 10 minutes. Due to unusually high temperatures recorded the day before, or due to the Foehn phenomenon caused by passing typhoons, the minimum temperature rarely rises above 30 degrees Celsius between midnight and 9:00 the next morning.