It seems the rainy season has begun in earnest. Heavy rain has continued mainly in Kyushu, and there were gusts in northern Kanto. Our sincere sympathies to those affected.

Speak of the rainy season and one thinks of hydrangeas. The photo is a snapshot just before the season started, on a walk in the East Gardens of the Imperial Palace. They were blooming on the west slope of the former Honmaru plaza. For hydrangeas, Hakone Tozan Railway’s hydrangea trains are famous. From tomorrow, the 20th, the “Night Hydrangea” trains are to run as usual. Patronage on the mountain railway has fallen about 30 percent after the eruption alert around Owakudani rose to Level 2, but riding to see the 8,000 hydrangeas along the line is not considered dangerous; staff hope as many people as possible will enjoy the fantastical lighted night landscape.

Why, then, does the rainy-season front stall for a full month? Murase Tetsushi, a geography colleague of Hayashi Osamu of “Now, right?” Tojin High School fame, explained on a TV quiz show. I half-listened, but as I recall: (1) the rainy season exists only in part of East Asia, from Japan to China; (2) that is because of the Himalayas.

I more or less knew that the cold continental high (the Okhotsk High) meeting the warm Pacific High destabilizes the air and forms the front—but asked for the detailed mechanism, I am stuck.

Enter Mr. Murase. The jet stream aloft, blowing west to east, gradually shifts north as spring turns toward summer, then splits north and south when it reaches the Himalayas. Other climate sites say the main flow is around 5,500 meters; the 8,000-meter Himalayas and, farther east, the Tibetan Plateau above 4,000 meters keep the split streams from rejoining until around the Sea of Okhotsk. High-altitude air is cold, so cold air sinks over the Okhotsk and builds a high; it then shoves against the Pacific High like a crowded oshikura-manju scrum, and the rainy-season front forms and stalls.

Toward high summer the jet is pushed farther north and, once it crosses the Himalayas as one stream, the Pacific High surges north and the rains end.

This year’s onset in the Kanto-Koshin region was June 8, about average. “Nyubai,” a miscellaneous seasonal day after the twenty-four solar terms, is June 11, so it feels about right. The average end for Kanto-Koshin is July 21. A little over a month remains when we cannot let down our guard; we will face it in full readiness, aiming for a safe building.