As September rolls in, most of us expect cooler autumn breezes, but Japan, particularly Kyushu, is grappling with heat that feels more like peak summer. According to the Japan Meteorological Agency (JMA), the summer of 2025 (June to August) in Kyushu was the hottest on record since 1946, driven by an unusual weather phenomenon dubbed the “double high-pressure system.” This combination of the Pacific High and the Tibetan High has created relentless heatwaves, intense sunlight, and record-breaking temperatures across the region. Even as September begins, the JMA warns that this scorching weather is far from over. Let’s dive into why this is happening, its impacts, and what you can do to stay safe in this prolonged heat.
Understanding the Double High-Pressure System
The unprecedented heat in Kyushu stems from a rare alignment of two powerful high-pressure systems: the Pacific High and the Tibetan High. These atmospheric giants have converged over Japan, creating a perfect storm of sweltering conditions. But what exactly does this mean?
The Pacific High
The Pacific High is a high-pressure system originating over the Pacific Ocean, typically influencing Japan’s summer weather. This year, warmer-than-average sea surface temperatures in the southern oceans have intensified this system. Warmer waters fuel stronger updrafts, causing the Pacific High to expand rapidly. This pushed the rainy season’s front northward earlier than usual, leading to an exceptionally early end to the rainy season in Kyushu. Northern Kyushu saw the fastest-recorded end to the rainy season in history, while southern Kyushu followed as the second fastest. This early transition ushered in summer conditions much sooner than expected.
The Tibetan High
Meanwhile, warmer sea surface temperatures in the Indian Ocean have strengthened the Tibetan High, a high-pressure system located at higher altitudes over the Asian continent. When high-pressure systems dominate, they create sinking air (downward airflow), which compresses and heats up as it descends. This process inhibits cloud formation, allowing intense sunlight to bake the land below.
The Perfect Storm
When these two systems overlap, as they have over Japan this year, the result is a “double high-pressure” effect. The absence of clouds, combined with intense heat and sunlight, has caused temperatures to soar, creating prolonged heatwaves across Kyushu and beyond. Even August’s record-breaking rainfall offered only temporary relief, as the double high-pressure system quickly regained its grip.
Record-Breaking Heat in Kyushu
The numbers tell a staggering story. From June to August, northern Kyushu’s average temperature was 1.7°C above normal, surpassing last year’s record as the hottest summer since observations began in 1946. Southern Kyushu wasn’t far behind, with average temperatures 1.3°C above normal, also setting a new record.
The heat was particularly brutal in July, with Dazaifu City in Fukuoka Prefecture and Hita City in Oita Prefecture hitting a blistering 39.0°C on July 28 and 29, respectively. Hita City endured an astonishing 55 days of extreme heat (temperatures above 35°C), the highest in Japan. Other cities, like Kurume and Dazaifu, recorded 47 days of extreme heat, while Fukuoka City saw 26 such days.
Several locations also set new all-time high temperature records, including:
- Minamata City, Kumamoto Prefecture: 38.1°C
- Kihoku City, Oita Prefecture: 38.0°C
- Nakatsu City, Oita Prefecture: 38.0°C
- Karatsu City, Saga Prefecture: 37.5°C
- Saiki City, Oita Prefecture: 37.1°C
- Kokuraminami Ward, Kitakyushu City: 36.4°C
These figures highlight the intensity of this summer’s heat and its widespread impact across Kyushu.
Why September Feels Like Mid-Summer
While September typically marks the transition to autumn in Japan, the JMA warns that Kyushu’s heat is lingering, feeling more like mid-summer than early fall. The persistent double high-pressure system continues to dominate, keeping temperatures high and cloud cover minimal. This prolonged heatwave poses challenges for residents and visitors alike, as the usual cooling trends of September remain elusive.
Staying Safe in Japan’s Extreme Heat
For those in Japan or planning a visit, the ongoing heatwave requires proactive measures to stay safe and comfortable. Here are some culturally sensitive and practical tips to beat the heat:
- Stay Hydrated: Carry a reusable water bottle and sip frequently, especially during outdoor activities. In Japan, convenience stores like 7-Eleven and FamilyMart offer a variety of electrolyte drinks, such as Pocari Sweat, to replenish lost minerals.
- Dress Appropriately: Opt for lightweight, breathable clothing. Many Japanese locals use UV-protective parasols or wide-brimmed hats to shield themselves from the sun.
- Use Cooling Products: Japan is famous for innovative cooling products, such as cooling towels, portable fans, and gel pads. These are widely available at stores like Don Quijote or online.
- Plan Outdoor Activities Wisely: Schedule sightseeing or outdoor activities for early morning or late afternoon to avoid peak heat. Many attractions, like Fukuoka’s Dazaifu Tenmangu Shrine, offer shaded areas to rest.
- Follow Local Advice: Pay attention to JMA heat advisories and warnings, often broadcast on NHK or local news. These alerts provide critical information about heatstroke risks and safety precautions.
- Respect Local Customs: While staying cool, be mindful of cultural norms. For example, avoid overly revealing clothing in traditional or religious sites, even in hot weather.
The Bigger Picture: Climate and Weather Patterns
This extreme heat in Kyushu is part of a broader trend of rising temperatures globally, driven by climate change. Warmer sea surface temperatures, as seen in the Pacific and Indian Oceans, are becoming more common, intensifying weather systems like the double high-pressure phenomenon. For travelers and residents, understanding these patterns can help with planning and preparedness.
For those visiting Japan, this heatwave underscores the importance of flexibility in travel plans. Checking weather forecasts and packing appropriately can make a significant difference in your experience. Additionally, exploring indoor attractions like museums, shopping arcades, or cultural centers can provide a respite from the heat while immersing you in Japan’s rich heritage.
Conclusion: Preparing for a Hot September in Japan
The double high-pressure system has turned Kyushu into a hotspot this summer, with September offering little relief. By understanding the science behind this phenomenon and taking practical steps to stay cool, you can navigate Japan’s scorching weather safely and enjoyably. Whether you’re a local or a traveler, staying informed and prepared is key to thriving in this unusually hot season.
For the latest updates on Japan’s weather, visit the Japan Meteorological Agency or check local news sources. Stay cool, stay safe, and enjoy all that Japan has to offer, even in the heat!
