Tropical night (nettaiya)

Definition: the temperature that makes a tropical night

JMA forecast terminology defines it as "a night on which the minimum temperature is 25 degrees or higher" (source: JMA, Terms used in weather forecasts: temperature and humidity, in Japanese, accessed August 17, 2026).

JMA does not use this term in its statistics. This site counts days with a daily minimum of 25°C or higher, not only at night.

Tropical nights in Tokyo

Days per year (1996–2025)

Number of tropical nights per year, counted from observations in Tokyo (Kitanomaru Park, Chiyoda City). The station moved from Otemachi in December 2014, so the series may not be continuous across the move. Most on record: 2023 (57 days), 2010 (56 days), 2025 (55 days) (records 1875–2025; days with observations only).

More about the tropical night

A tropical night means that the minimum temperature during the night is 25°C or higher. The Japan Meteorological Agency (JMA) lists it with this meaning among the terms it uses in weather forecasts. In the same place, however, it adds a note that it is not one of JMA's statistical categories (as of August 17, 2026). It is a term with a slightly unusual standing: widely known as a way to describe hot summer nights, yet not used as the name of a statistic.

Counting nighttime temperatures on their own is harder than you might think. The value changes depending on which hours you define as night, and there is also the question of which day to count the hours that cross midnight. So when the number of tropical nights is shown side by side, a common approach is to count days on which the daily minimum temperature over the whole day was 25°C or higher. The daily minimum usually occurs around dawn, so it can serve as a gauge of nighttime heat.

When the temperature does not fall below 25°C even at night, the breeze coming through an open window is lukewarm, and the heat built up during the day does not leave the room. These are the nights when it is hard to sleep, showing that heat is not only a daytime thing. Public agencies such as JMA and the Ministry of the Environment publish guidance on preparing for summer heat, so please check it when deciding what to actually do. This page lists temperature records and does not give health advice.

This page shows how the number of days in Tokyo with a daily minimum temperature of 25°C or higher has changed over time, alongside this year's situation across Japan. Compared with the pages on midsummer days (30°C or higher) and extremely hot days (35°C or higher), it shows that daytime heat and nighttime heat do not always move in step.

Super tropical night (minimum of 30°C or higher)

A night on which the daily minimum temperature is 30°C or higher is sometimes called a super tropical night. The term describes a state in which the temperature hardly drops during the night and the heat does not ease even by morning.

This term does not appear among the terms the Japan Meteorological Agency (JMA) uses in weather forecasts (as of August 17, 2026). Because it is often mentioned together with tropical nights, we do not give it a separate page here and only add its meaning within the page on tropical nights.