日本はより高湿化する!夏の湿度対策のススメ

The dreaded rainy season is fast approaching. As we're approaching this season, I'd like to share with you a commentary my mentor, Professor Matsuo, recently published in Shinken Housing, a newspaper aimed at professionals. I felt it was important to know, so I'll add my own annotations. Today's theme is, "Will Japan become even more humid in the future?" It's a recommendation for taking measures to address the summer humidity problem. As always, I hope you'll listen while looking at my notes on the board. I'm a man from the Showa era, but I really do think that Japanese summers have gotten hotter. I really feel that temperatures have been rising. And with that, humidity has risen, too. Rising temperatures are understandable because of global warming. But why does humidity also rise? I've had customers ask me this question, and I've explained it to them before. The bottom line is that Japan is surrounded by the sea on all sides. As temperatures rise, the amount of water vapor rising from the sea also increases. As a result, humidity increases inland across the entire Japanese archipelago. So, in Japan, rising temperatures inevitably mean rising humidity. The situation is different from that of landlocked countries like China or Russia. This is precisely because Japan is a maritime nation. Now, I'd like you to take a look at the data that Professor Matsuo introduced. It's not just about the intuitive feeling that "it's gotten hotter," but also about the actual data. He provided statistics on the trends in average temperature, average relative humidity, and average absolute humidity in Tokyo in August over the 20-year period from 2005 to 2024. Looking at this data, we can see that temperatures certainly rise and fall. 27.5°C drops to 28°C, 28.5°C drops to 27.8°C, then rises again. But on average, the average temperature in Tokyo in August over these 20 years is 28.3°C. It's incredibly hot, isn't it? And the relative humidity is a whopping 75.2%. Just hearing 75% makes you feel a little depressed, doesn't it? You might be thinking, "What kind of humidity is that?" But here's where things get confusing: the "relative humidity trick." Relative humidity is literally "relative." What it means is that it expresses the amount of moisture contained in one cubic meter of air (roughly 1 kg) at a given time, expressed as a percentage, with the amount of moisture contained at that time (that is, the saturated moisture content) as the denominator. So, for example, if the temperature is 25°C and the relative humidity is 60%, and the moisture content remains the same when it drops to 30°C, the relative humidity will decrease. Even if the moisture content doesn't decrease, the percentage will decrease. That's why you can't make a proper judgment unless you look at absolute humidity, which is measured in terms of the actual "weight of moisture." Looking at absolute humidity, in 2005, 1 kg of air contained 17.3 g of moisture, but this has averaged 18.2 g over the 20-year period. This is also difficult to understand at first glance, but Professor Matsuo has graphed it, and while the values ​​fluctuate, if you take the median, you can see that temperature and humidity are generally trending upward. Every year, the temperature increases by about 0.07°C, and humidity by 0.08g. That's quite a pace, isn't it? Ten years from now, when a 35-year-old today turns 45, Japan may be even hotter and more humid than it is today. By the way, globally, the absolute humidity level at which humans feel "humid" is 12g. Perhaps because Japanese people have more pores, many can tolerate up to about 13g, but there are regional differences. I have a friend from Hokkaido, and when he comes to Kansai, he asks, "Why is it so hot?" The temperature doesn't change much, but he feels sluggish. That's why I often joke, "You guys must have very few pores." Osaka, in particular, is a city of water, so the humidity is unbearable. With that in mind, people in western Japan may be able to tolerate temperatures above the 13g barrier, but it might be a bit tougher for people in eastern Japan. In that sense, what Professor Matsuo is saying is that room air conditioners are generally set to 26°C, maintaining an absolute humidity of around 12.8g. However, if this exceeds 13g, it becomes uncomfortable even when using the air conditioner. This is why the air conditioner seems ineffective. When I visited Hong Kong and Singapore a long time ago, I thought, "It feels so humid." That is exactly the condition, making it difficult to adapt to the climate. Japan will face the risk of this happening in the future, he says. This means that we will have to rely even more on air conditioners. However, the highly energy-efficient air conditioners that are mainstream in Japan today are good at lowering the temperature, but they stop when the temperature reaches 26°C. This is because they prioritize energy efficiency. When this happens, the dehumidification stops. Wh...