Summer day (natsubi)

Definition: the temperature that makes a summer day

JMA forecast terminology defines it as "a day with a daily maximum temperature of 25 degrees or higher" (source: JMA, Terms used in weather forecasts: temperature and humidity, in Japanese, accessed August 17, 2026).

Summer days in Tokyo

Days per year (1996–2025)

Number of summer days 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: 2024 (153 days), 2025 (146 days), 2023 (143 days) (records 1875–2025; days with observations only).

More about the summer day

A summer day is a day on which the maximum temperature reaches 25°C. Only the daily maximum temperature counts; humidity, wind strength and the amount of sunshine are not part of the condition. Even if the morning is cool, the day counts as a summer day once the temperature reaches 25°C during the day; conversely, even if it stays muggy all day, it is not a summer day unless the maximum temperature reaches 25°C.

Besides summer days, the terms that distinguish degrees of heat include midsummer days (30°C or higher) and extremely hot days (35°C or higher). All three look at the same daily maximum temperature, so they are not side by side but nested. A day that counts as an extremely hot day is also a midsummer day and a summer day. The number of summer days is therefore close to the total number of days that leaned toward heat, while the numbers of midsummer days and extremely hot days show how many of them brought strong heat. Viewed together, they let you read the quantity and the quality of heat separately.

Summer days also serve as a marker of the start of a season. As the spring sunshine grows stronger, days appear on which the temperature reaches 25°C in the daytime only, even while the mornings and evenings stay cold. The first such day has long been taken as a sign that summer is approaching in that area. Autumn works the same way: after the coolness has settled in, a summer day sometimes returns just once.

The name suggests summer, but summer days are not only a summer phenomenon. In the Nansei Islands, the daily maximum temperature sometimes reaches 25°C on warm winter days. This page shows how the number of summer days in Tokyo has changed over time, alongside this year's situation across Japan. If there is an area you are curious about, you can trace its records from each station's page.