What is the Rainy Season? A Season Where 'Spring' and 'Summer' Push Against Each Other in the Sky
When June arrives, it rains in the morning. It rains on the way home, too.
My bangs won't behave, and it's always uncertain whether we can have P.E. class outside. I had always thought of the rainy season as just a period where rainy days continue for a long time.
But when I looked it up, I found that the rainy season wasn't just a long spell of rain.
It was a time when spring and summer were pushing against each other in the sky, slowly swapping seasons.
First, let's look at an actual weather map.
Honestly, weather maps look like code at first.
But there is only one place to look in this image: the line crossing over Japan with alternating red semicircles and blue triangles.
This is the symbol for a 'stationary front.' The stationary front that appears around Japan and the Chinese mainland during this time is called the Baiu front (rainy season front).
Even though it's called 'stationary,' it doesn't mean it's completely stopped. It sways back and forth, moving south and north, and lingers near Japan for a long time.
So, what exactly is the rainy season?
The Japan Meteorological Agency defines the rainy season as a phenomenon where there are many rainy or cloudy days from late spring to summer, or the period during which this occurs.
In other words, the rainy season is closer to the name of a 'seasonal state' than a type of rain.
From the end of spring to the beginning of summer, there is warm, moist air to the south of Japan and cooler air to the north. The rainy season front forms where these air masses with different properties meet.
Warm air is lighter than cold air, so when they collide, it is pushed upward. The air that goes up into the sky cools down, and the water vapor it contains turns into tiny water droplets, forming clouds. When those droplets get big enough, they fall as rain.
Simply put, rain during the rainy season is created by the combination of 'moisture arriving from the south' and 'the force that lifts air upward.'
A single line on a weather map is a band of clouds when seen from space.
This is a satellite image from the same time as the weather map earlier.
You can see that the rainy season front, which was just a thin line on the weather map, is a long band of white clouds stretching near Japan in the satellite image.
So, it's not that there are literally red and blue lines floating in the sky.
Fronts on a weather map are a way to clearly represent the places where air masses with different characteristics meet as a single line. In reality, they are areas with width where clouds and rain spread out.
Why does it last for weeks?
With normal rain clouds, it clears up after they pass by.
However, during the rainy season, the strength of the summer-like air from the south and the cool air from the north remains about the same for a while. Since neither side can win decisively, the rainy season front at the boundary tends to remain near Japan.
When the front approaches, it becomes rainy or cloudy, and when it moves away slightly, it clears up. That is why it is not necessarily raining all day every day during the rainy season.
When the summer Pacific high-pressure system gradually strengthens and pushes the rainy season front to the north, the rainy season finally ends. The main character of the sky changes from spring to summer.
Can it be clearly decided that 'today is the start of the rainy season'?
I used to think that the rainy season started automatically after a few days of rain.
But there is no switch-like standard such as 'the rainy season starts when X millimeters of rain fall'.
There is an average transition period of about five days for the start and end of the rainy season. Therefore, the Japan Meteorological Agency announces it as something like 'around June 7th'.
Moreover, the initial announcement is a preliminary report, and after the rainy season ends, the dates may be changed upon reviewing the actual weather.
Seasons do not switch on at midnight. Like the sky changing color little by little, the boundaries are blurry.
Are the rainy season front and linear precipitation zones the same thing?
This is the part where I was most confused while watching the news.
To conclude, they are not the same thing.
The rainy season front is a large stage where rain clouds easily form over a wide area.
A linear precipitation zone is a phenomenon where cumulonimbus clouds form one after another in parts of that stage, line up in a narrow strip, and continue to dump heavy rain on almost the same location.
The red and purple areas represent places where a lot of rain fell in a short period of time. In this example, the rainfall in Kumamoto Prefecture exceeded about 200 to 300 millimeters in 3 hours.
Rather than 'a single rain cloud staying put,' it is closer to the image of new rain clouds flowing in one after another in the same direction, creating a line of rain.
The Japan Meteorological Agency's diagram explains the process roughly as follows.
1. Warm, moist air continues to flow into the lower atmosphere.
2. Due to the influence of fronts or mountains, that air is lifted upward.
3. If the atmosphere is unstable, the clouds develop into cumulonimbus clouds.
4. The cumulonimbus clouds are lined up by high-altitude winds, creating a linear zone of intense rain.
The gentle rain of the rainy season and the torrential rain of a linear precipitation zone have completely different levels of danger, even though both are 'rain'.
On days when heavy rain forecasts or warnings are issued, it is better to check the movement of rain clouds and evacuation information from your local government rather than dismissing it as 'just the usual rainy season'.
The rainy season is not just a troublesome time of year.
The rainy season is damp. Laundry is hard to dry. It is difficult to make plans to go out.
However, Japan's water resources rely heavily on the rain during the rainy season and typhoon season. If there is too little rain, dam water levels can drop, leading to water shortages.
Of course, if it rains too much at once, it can cause floods and landslides.
It is necessary, yet dangerous if it is too intense.
The rainy season may be a time that shows us just how difficult it is for nature to find the perfect balance.
Summary
The rainy season is a period during the transition from spring to summer when there are many rainy and cloudy days.
A rainy season front forms at the boundary between warm, moist air from the south and cold air from the north, and clouds and rain are created as water vapor is transported there.
On a weather map, it is a single line. But seen from space, it is a band of white clouds stretching for hundreds of kilometers.
The rainy season, where it seems like the season has stopped because of all the rain.
In truth, above those gray skies, spring is quietly leaving its seat, and summer is gradually drawing near.
When you think of it that way, the raindrops streaming down the window also start to look like signs that the seasons are shifting.
Reference Materials
Japan Meteorological Agency: 'About the Rainy Season'
https://www.jma.go.jp/jma/kishou/know/faq/faq33.html
Japan Meteorological Agency: 'Past Rainy Season Start and End Dates'
https://www.data.jma.go.jp/cpd/baiu/index.html
Nagano Local Meteorological Office: 'Summer Weather Maps'
https://www.data.jma.go.jp/nagano/shosai/tenkizu/tenkizu.html
Japan Meteorological Agency: 'Fronts'
https://www.data.jma.go.jp/sat_info/himawari/obsimg/image_front.html
Japan Meteorological Agency: 'About Heavy Rain Caused by Linear Precipitation Zones'
https://www.jma.go.jp/jma/kishou/know/yohokaisetu/senjoukousuitai_ooame.html
Ministry of Land, Infrastructure, Transport and Tourism, Kanto Regional Development Bureau: 'Causes of Water Shortages'
https://www.ktr.mlit.go.jp/river/bousai/river_bousai00000061.html