Japan records its first ‘cruelly hot day’ as cities swelter in 40-degree heat

Japan officially recorded its first kokushobi, or ‘cruelly hot day’, on Tuesday. This new term, meaning temperatures above 40 degrees Celsius, was introduced in April. The Japan Meteorological Agency created this term to warn people about the dangers of extreme heat, which is becoming more common.

Cities in central Japan, including Gujo, Tajimi in Gifu prefecture, and Toyota in Aichi prefecture, all reached 40C. Gujo’s temperature of 40C and Toyota’s 40.1C were the highest ever recorded at their local weather stations.

Across the country, 278 out of 914 weather stations reported temperatures above 35C. These are called moshobi, or extremely hot days, which was the highest warning level before the new term was created. Days over 25C are known as natsubi, or summer days, and days over 30C are manatsubi, meaning true summer days.

Tragically, the heat has already led to deaths. On Monday, a man in his 40s was found dead on a street in Sanjo, Niigata prefecture. Another man in his 70s died in Kanagawa, southwest of Tokyo. Both are thought to have died from heatstroke.

In 2024, deaths from heatstroke exceeded 2,000 for the first time. The fire and disaster management agency also reported that over 100,000 people were taken to hospital last year between May and September.

The increasing heat is impacting many parts of Japanese life, including rice production, which is a main food source. Farmers are starting to plant varieties of rice that can better handle the heat. Some are also beginning to grow crops like avocados, which have not traditionally been farmed in large quantities in Japan.

The effects of the heat are also noticeable in sports. The popular koshien national high school baseball tournament now splits its games into morning and evening sessions. This change was made in 2024 to help prevent heatstroke among players and spectators during the hot August weather.

Companies are also encouraging office workers to wear shorts on hot days as part of the Cool Biz initiative. This campaign aims to reduce electricity use by lowering air conditioning. It’s a continuation of efforts to relax Japan’s traditional formal dress code for businesses. Previous advice included removing jackets and ties and wearing short-sleeved shirts. This year, with very high temperatures expected, the government is suggesting even more casual clothing.

Tokyo’s governor, Yuriko Koike, announced that office wear can now include polo shirts, T-shirts, and trainers if appropriate. This decision was made due to the expected high temperatures and concerns about the stability of the electricity supply.

A study from 2025 by the National Institute for Environmental Studies and Waseda University predicted that without significant reductions in greenhouse gas emissions, outdoor school sports could become impossible by the 2060s.

According to the Japan Meteorological Agency, average summer temperatures in Japan have increased by about 1.3C over the last 100 years. Both 2023 and 2024 were the hottest years since records began in 1898. Last year, these levels were surpassed again. The summer of 2025 was 2.4C warmer than the average of the previous 30 years.