The Lingering Summer Heat of 2025, as Seen Through Weather Data — How Does It Differ from Last Year?

“Autumn According to the Calendar”… But the Lingering Summer Heat Continues

September is usually the time when we start to feel the arrival of fall, but in 2025, forecasts continue to predict midsummer-like days with temperatures exceeding 30°C nationwide.
We’ve analyzed meteorological data to see how this compares to “last year’s record-breaking lingering summer heat.”

We’ll cover:

• Temperature trends for September 2025
• Differences in the lingering summer heat compared to 2024
• Practical tips for staying cool

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Weather Trends for September 2025

• According to the Japan Weather Association’s forecast, temperatures are expected to remain above average, with midsummer days (30–34°C) continuing across the country throughout September.
• From the Tokai region to Okinawa, heat is expected to reach the “Extreme Alert Level,” and a “Warning Level” has been issued for the Kanto and Hokuriku regions as well.
• The highest temperature in Tokyo is not expected to fall below 30°C until mid-September, so the lingering summer heat is expected to persist.

The actual forecast data is shown in the graph below.

Temperatures in 2025 are expected to be higher than average, and while not as extreme as last year, the trend clearly shows that “the heat will continue for some time.”

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September 2024 Was “Unprecedented”

Last year (September 2024) saw truly historic late-summer heat.

• Osaka and Sakai recorded temperatures exceeding 37°C, setting new all-time highs for September.
• The number of locations experiencing extreme heat days (35℃ or higher) also reached a record high.
• There were also many emergency transport cases due to heatstroke, making it a summer that could be described as “abnormal weather.”

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Comparison with 2025: The Difference in the “Nature” of the Heat

• 2024: An unusually intense heatwave. September saw not just “midsummer days,” but a string of “extreme heat days.”
• 2025: Warmer than average, but with fewer extreme readings than last year, showing signs of a gradual return to autumn-like conditions.

In other words, the situation is “better than last year, but we still can’t let our guard down.”

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3 Tips for Beating the Heat

1. Stay Hydrated and Replenish Electrolytes → Make a point of doing this during the cooler hours of the morning and evening.

2. Use Air Conditioning Wisely → Use timers and circulators together to prevent the room from getting too cold.

3. Optimize Your Sleeping Environment → Use breathable bedding and cooling items to ensure a good night’s sleep even on sweltering nights.

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In Closing

Even if we don’t experience a record-breaking late-summer heatwave like the one in 2024, September 2025 will still see “heat requiring caution.”
Understanding the data is the “first step toward taking action.” To protect your health, be sure to put the tips above into practice.

➡ Recommended related article: What’s Behind the “Fatigue” You Feel in September?