Japan's Hottest Temperature Day

What Is Japan's Hottest Temperature Day?

Every July 25th is "Japan's Hottest Temperature Day" (最高気温記念日, Saikō Kion Kinenbi) in Japan. On July 25, 1933, Yamagata City in Yamagata Prefecture recorded a temperature of 40.8°C — Japan's highest officially recorded temperature at the time, setting a national heat record that stood for 74 years. The heat was driven by the foehn effect — hot, dry wind descending after crossing the mountains. (The record was broken on August 16, 2007, when Kumagaya in Saitama and Tajimi in Gifu recorded 40.9°C. The current national record is 41.8°C, observed in Isesaki, Gunma Prefecture, on August 5, 2025.) The day serves as a reminder of Japan's summer heat extremes and the importance of heatstroke prevention.

Japan's Summer Heat

Japan's summers are characterized by intense heat and humidity, particularly in its Pacific coast regions. The combination of high temperatures and high humidity creates heat index conditions that can be extremely dangerous, particularly for elderly people and those engaged in outdoor activities. Japan's maritime climate — bringing warm, moist air from the Pacific — produces "mushiatsui" (蒸し暑い, "steaming hot") conditions that are subjectively more oppressive than the same temperature in drier climates.

Heat Island Effect in Urban Japan

Japan's densely built cities — particularly Tokyo, Osaka, and Nagoya — experience severe "heat island" effects where urban surfaces absorb and re-radiate heat, creating temperatures several degrees higher than surrounding rural areas. Tokyo's summer temperatures have risen significantly over the past century due to the combined effects of climate change and urban expansion — the city now regularly experiences temperatures above 35°C that were once rare.

Heatstroke Prevention

Heat-related illness (熱中症, nettchūshō) is a serious public health concern in Japan. Each summer, tens of thousands of people are taken to hospital with heatstroke, and in some recent years more than a thousand have died — primarily elderly people and outdoor workers. Public health campaigns around July 25th and throughout summer emphasize hydration, rest during peak heat hours, use of air conditioning, and checking on elderly neighbors. Japan has also developed cooling facilities (クーリングシェルター, kūringu sherutā) — designated public spaces with air conditioning available to anyone who needs cooling during heat waves.

Fun Facts

• Records keep falling: In the summer of 2025, Japan's national record was broken twice within a week — 41.2°C in Tamba, Hyogo, on July 30, then 41.8°C in Isesaki, Gunma, on August 5. Japan's Meteorological Agency has documented a long-term warming trend in Japanese summer temperatures that climate scientists attribute to global climate change.
• Cooling fashion: Japan has developed distinctive "cooling fashion" (クールビズ, Kūru Bizu, "Cool Biz") — a government-promoted campaign encouraging lighter summer business dress without ties and jackets, allowing office air conditioning to be set at 28°C rather than lower, reducing energy consumption.