The Mechanics and Joys of the Rainy Season / The 24 Solar Terms: "Shosho" (Lesser Heat)

Today, July 7th, is the 24th solar term Shosho (Lesser Heat).

And today is also Tanabata. I hope we can see the Milky Way, but it's cloudy again in the Kanto region...

What is Shosho?

Shosho is the solar term that follows the Summer Solstice, and it is written as "small heat." As the name suggests, it is the period just before the full-blown heat of summer begins.

It is a time when the end of the rainy season approaches and the temperature gradually rises.
It is also the time to start sending "summer greeting cards" (shochu-mimai), though I have never written one lol.
It has a certain charm, though... Like New Year's cards, perhaps these things will disappear with the times.

And in the 72 micro-seasons, the first micro-season of Shosho is Atsukaze Itaru (Warm Winds Arrive).
You can tell just by looking at the characters, right? That feeling of being hit by a wave of heat the moment you step outside. A humid, hot southern wind is likely blowing.

The Mechanics of the Baiu Front

Since there were no particular events this time lol,
I'd like to talk about the Baiu Front (Rainy Season Front) as a way to review it for myself.

In the first place, a "front" is the boundary where air masses with different properties collide. There are several types, but the one that appears during the rainy season is the stationary front type. As the name suggests, its characteristic is that it doesn't move much.

From spring to summer, the warm and humid "Ogasawara air mass" exerts its influence to the south of the Japanese archipelago, while the cold and humid "Okhotsk air mass" does the same to the north.
(We learn about this around middle school, don't we? How nostalgic!)

These two collide right around Japan, and because they are pushing against each other with almost equal force, the front at the boundary cannot easily move.
This is the Baiu front.

Near the front, the collision of air creates an updraft, which generates clouds and makes it easy for rain to fall. Because the front stays in the same area, it leads to the humid, rainy weather of the rainy season that lasts for days.
We are currently right in the middle of that "push-of-war."

So, why does the rainy season end?

Toward summer, the southern Ogasawara air mass gradually strengthens and pushes the northern air mass away. When the Baiu front is pushed to the north of the Japanese archipelago, Japan is covered by the Ogasawara air mass, and finally, the blue skies of summer arrive. This is the signal that the rainy season has ended.

However, the definition of this is not actually clearly determined.
Sometimes, the rainy season continues with unsettled weather until around August. When this happens, it becomes indistinguishable from the autumn rains, and it is recorded as "the end of the rainy season is not specified". In fact, this occurred in the northern Tohoku region in 2020.

Even as weather forecasting technology advances, we are still swayed by these whims of nature. That is what makes it so interesting, though.

Enjoying the Sky After the Rain

Although the days are humid, there is one joy in the rainy season.
That is, a rainbow after the rain!

A rainbow is a phenomenon where sunlight enters many raindrops floating in the air, and the light is refracted and reflected inside the raindrops, making it visible as seven colors.
The raindrops act like a prism.

You can always see a rainbow in the direction opposite the sun.

When the rain stops in the evening, the sun is in the west, so the point is to look at the eastern sky. Rainbows are easier to see as large when the sun's altitude is low in the evening, so the timing of a sudden evening shower or just after the rain is a prime target.

Rainbows are also a "summer kigo" (seasonal word).
It has long been known that rainbows are likely to appear after a summer evening shower, and it makes me feel deeply moved to think that people in the past were looking up at the sky after the rain in the same way.

On a clear break in the rainy season or after the rain in the evening, let's look up at the eastern sky.

Conclusion

Once you understand the mechanics of the Baiu front, the daily weather forecast and weather maps start to look a little different.

It is quite fun to look at them while thinking, "The front is stationary again today" or "The high pressure in the south is working hard."

Just a little more until the end of the rainy season.
Even on rainy days, let's enjoy the changes in the sky.