Is 2025 About More Than Just Temperature? The Truth Behind “A Summer with Excessively High Humidity”

A Humid Summer in 2025

“Doesn’t this summer feel a little stifling?”

Of course, the temperatures are high. But more than that, “I can’t seem to stop sweating,” “even when there’s a breeze, it doesn’t feel cool,” and “it’s still muggy even at night”—
I’m sure many of you are feeling this sense of unease, aren’t you?

The cause lies in the “abnormally high humidity” of 2025.

We’ll explain in simple terms from a scientific perspective why the summer of 2025 feels “more uncomfortable than the temperature alone” would suggest.

How We “Feel” the Heat Isn’t Determined by Temperature Alone

The “heat” we feel isn’t determined by temperature alone.
When humidity is high, sweat doesn’t evaporate, so your body temperature doesn’t drop.

This leads to the following effects:

Your body doesn’t cool down despite the heat
Sweating doesn’t stop, making you prone to dehydration
The risk of heatstroke skyrockets

In other words, compared to “35°C of dry heat,”
“31°C with 80% humidity” puts a greater strain on the body.

2025: A Double Whammy of “Temperature + Humidity”

A defining feature of this year is that both temperatures and humidity are high.

For example, looking back at trends in Osaka in July 2025:

Average temperature: 31.6°C (+1.3°C compared to last year)
Average humidity: 80–85% for consecutive days
70% of days had a Discomfort Index (DI) of 80 or higher

🔸 When the Discomfort Index exceeds “80,” it reaches a level where “most people feel uncomfortable.”

What Is the “Discomfort Index”? A Simple Explanation

🌡 Discomfort Index (DI) Guidelines and How It Feels

Up to 70: Comfortable
- Temperatures are on the lower side and humidity is moderate, making it feel pleasant
- You hardly break a sweat and can stay comfortable even without air conditioning
- This range is common on pleasant days in spring or early summer
71–75: Slightly Hot
- Common when the temperature is around 25°C and humidity is on the high side
- You’ll break a light sweat when moving, but a fan or natural breeze is still enough to keep you comfortable
- A level where some people begin to feel “muggy”
76–80: Hot and sweaty
- Sweat begins to trickle even when sitting still indoors
- The risk of heatstroke begins to rise when going out during the day
- It becomes more difficult to sleep at night, and sleep quality may decline
- The use of air conditioning or dehumidifiers is strongly recommended
81 and above: Extremely hot and uncomfortable
- This stage can be described as “scorching heat”, and most people experience severe discomfort
- The body struggles to dissipate heat, making it easier for body temperature to rise
- A dangerous environment where failing to replenish fluids and electrolytes can directly lead to heatstroke
- Heat builds up even at night, making sleep and daily life difficult without air conditioning
- People who have difficulty regulating their body temperature, such as the elderly and children, are at particularly high risk

📌 Characteristics of 2025

“DI 81–84” will be observed for consecutive days in many regions
A summer that goes beyond the usual “heat,” making air conditioning essential to save lives
These figures indicate direct risks to health and daily life, not just mere discomfort

“3 Reasons” Why Humidity Doesn’t Drop

Rising sea surface temperatures
→ High sea temperatures around Japan supply large amounts of water vapor to the air
The urban heat island effect
→ Concrete and asphalt store heat, making it difficult for moisture to evaporate even at night
Changes in atmospheric pressure patterns (shift in the Pacific High)
→ Humid air stagnates, leading to an increase in days with little wind

Countermeasures: Focus on “Humidity,” Not “Temperature”

The following information is gaining attention as a key factor in preventing heatstroke:

🔷 WBGT (Wet Bulb Globe Temperature)

This is an index that represents “actual heat stress,” incorporating “air temperature + humidity + solar radiation (radiant heat).”

Below 25: Caution
25–28: Alert
28–31: High Alert
31 or higher: Dangerous (Level where you should avoid going outside)

→ Even if the “air temperature is still just over 30°C,” it’s dangerous if the WBGT exceeds 31.

Conclusion: The “Dangers of Summer” That Can’t Be Measured by Numbers Alone

If you feel that “this summer is unusual,” you’re right.
There’s a “humidity trap” that you’ll overlook if you only look at the temperature.

💡 Get into the habit of trusting how you feel and checking the numbers.

In the summer of 2025, when the risk of heatstroke will rise,
let’s protect ourselves by keeping an eye on the three key factors: “temperature, humidity, and sunlight.”