A Comprehensive Explanation of the Dangerous Heat Spreading Across Japan: Differences from Overseas Heatwaves and Countermeasures

Table of Contents

1. Weather Overview in Japan on June 16

2. Mechanism of the Humid "Japanese-style Heatwave"

3. Comparison with Overseas Heatwaves (Europe, North America, China)

4. Basic Knowledge of Heat Index/WBGT

5. Symptoms of Heatstroke and Self-check by Severity

6. Action Plan for Travelers and Residents: What You Can Do Starting Today

7. City-specific Danger Ranking Based on Temperature, Humidity, and Wind Speed Data

8. Common Misunderstandings and Q&A

9. Latest Reviews of Heat Protection Goods (Portable Fans, Cooling Towels, etc.)

10. Summary: Five Rules to Protect Your Life

11. List of Reference Articles
    
    

1. Weather Overview in Japan on June 16

According to announcements by the Japan Meteorological Association and Weathernews, as of this morning, the maximum temperature is expected to exceed 30°C at over 350 locations from western to eastern Japan. In Toyooka City, Hyogo Prefecture, and Tottori City, Tottori Prefecture, a scorching day of 35°C is forecasted, and there is a possibility of even higher temperatures inland in the Kanto region due to the Foehn phenomenon. Humidity is expected to reach around 70% by 9 a.m., with the discomfort index likely reaching the "dangerous" level in the afternoon. weathernews.jptenki.jp

Key Points for the Next 72 Hours

• Peak on the 17th to 18th: Consecutive scorching days in Kanto-Koshin, with 35°C even in the city center.

• Tropical Nights: Minimum temperature exceeding 25°C, caution for heatstroke even during sleep.

• Concurrent Localized Heavy Rain: Risk of heavy rain in northern Kyushu, check weather radar when going out. tenki.jp

2. Mechanism of the Humid "Japanese-style Heatwave"

Even at the same 35°C, when relative humidity exceeds 70%, heat dissipation through sweating is hindered, causing body temperature to rise rapidly. Due to the geography of the Japanese archipelago, surrounded by seasonal winds and the sea, high temperature and humidity become the norm in summer, resulting in extremely high values for the Heat Index and WBGT (Wet Bulb Globe Temperature) which evaluate "temperature + humidity".

• Example: Temperature 35 ℃, Relative Humidity 70% → Heat Index = Approximately 57 ℃ (Danger)

• Example: Same temperature with 20% humidity (Arizona, USA) → Heat Index = Approximately 37 ℃ (Caution) noaa.govnoaa.gov

3. Comparison with the Latest Trends in Overseas Heatwaves

Europe

Since early June, temperatures have exceeded 40 ℃ in Spain and Portugal, and reached 47 ℃ in the Xinjiang Uygur Autonomous Region of China. "Extreme high-temperature nights," where temperatures do not fall below 30 ℃ even at night, are occurring frequently, prompting meteorological agencies in various countries to issue red alerts. theguardian.com

North America

According to long-term statistics from NOAA, the average temperature in June across the United States is the second highest on record. Particularly in the Mid-Atlantic region, the heat index has reached 48 ℃, and heat-related deaths are on the rise. ncei.noaa.gov

Decisive Differences with Japan

Conclusion: Japan's climate tends to enter the "danger zone" in terms of perceived temperature even if the actual temperature is slightly lower.

4. Risk Assessment Using Heat Index / WBGT

• Heat Index: A perceived temperature combining air temperature and relative humidity.

• WBGT: Encompasses temperature, humidity, radiant heat, and wind speed, and is used internationally as a safety standard for outdoor work and sports.

• Japan Sports Association Standards: "Strict Warning" at WBGT 28°C, "Stop Exercise" at 31°C.

App Examples: Ministry of the Environment's "Heat Stroke Alert" LINE Notification / JMA Official Website.

5. Symptoms of Heat Stroke and Self-Check

Important Emergency Numbers in Japan

• 119: Call for Ambulance (English Support Available)

• #7119: Emergency Consultation Center (Available in Some Areas)

6. For Travelers and Residents: Action Plan You Can Start Today

1. Clothing: Highly breathable linen or moisture-wicking T-shirts + UV-cut lightweight jacket.

2. Hydration: Oral rehydration solution with 0.1-0.2% salt, 200 mL each before feeling thirsty.

3. Activity Timing: Shift sightseeing to 7-10 AM, after 5 PM.

4. Accommodation: Check air conditioning performance in advance (beware of old private lodgings).

5. Digital Tools: Use Meteorological Agency Radar, Google Maps congestion layer to identify indoor evacuation spots.

6. Signs from Surroundings: Consider it a danger signal when Japanese people start using parasols and cooling towels.

7. City-by-City Danger Ranking (Forecast for June 16, 2 PM)

8. Common Misunderstandings and Q&A

Q1: Is Japan's humidity really dangerous? Even though temperatures are lower than in dry areas
A: The Heat Index is a safety indicator for the human body. The Japanese type of heat impedes heat dissipation due to humidity.

Q2: Is it okay to just drink lots of water?
A: Risk of hyponatremia. Replenish salt and sugar simultaneously.

9. Latest Reviews on Heat Protection Goods

• USB Rechargeable Neck Fan: Three wind speeds, continuous 8 hours.

• High Absorption Cooling Towel: Wet and shake for 2 minutes to lower surface temperature by 17 ℃.

• Instant Cooling Spray: Spray over clothing for an approximate -20 ℃ sensation.
  
  
  
  

10. Summary ─ Five Rules to Protect Your Life

1. Check the Heat Index every morning

2. Ensure water + salt + rest as a set

3. Prioritize air conditioning for the elderly, children, and those with chronic illnesses

4. Avoid long outdoor activities

5. Call 119 without hesitation if you feel any abnormality
   
   

11. List of Reference Articles

• Japan Meteorological Association "Today 16th Temperature Rise Widely Above 30°C, Over 35°C Heatwave in Western Japan" 2025/06/16 tenki.jp

• Weather News "Heatwave Expected in Western Japan, Be Cautious of Heatstroke" 2025/06/16 weathernews.jp

• Guardian “Europe and China in midst of record-breaking heatwaves” 2025/06/13 theguardian.com

• NOAA Jetstream “Heat Index” Technical Explanation (2023) noaa.gov

• NOAA National Climate Report “U.S. Climate in June 2024” ncei.noaa.gov