Q: It’s my first summer in South Korea, and everyone’s telling me not to leave my room without an umbrella. What’s up with that?

A: It’s good advice — it’s almost rainy season in South Korea, and when it rains, it pours. Or drizzles. Or mists. We’re not talking monsoons here, but we are talking a four-to-six-week span of rain, some days a little and some days a lot.

Mainland Japan and Okinawa have their own versions of rainy season, and they all have different usual start dates — but the root cause is the same.

Air Force Tech. Sgt. Dionne M. Tirschel with Kadena Air Base, Okinawa’s weather flight explains that the culprit is a slow-moving weather boundary that separates two semi-permanent high-pressure areas, and their corresponding air masses, in the Western Pacific.

One system, which camps out north of Japan this time of year, carries cold, dry air. The other keeps things warm and wet over the Pacific Ocean. The line along which these systems meet is called the Bai-u front, and that’s where the clouds and the rain happen.

That’s also where the thunderstorms happen, caused by the instability of the two competing weather systems, Tirschel said. The Bai-u front moves slowly northward starting in late spring, first parking over Okinawa, then sweeping over mainland Japan and, eventually, the Korean peninsula.

Okinawa is the first to get drenched, typically starting in early May. This year, the island’s rainy season — tsuyu in Japanese — started May 16, according to the Japan Meteorological Agency. On mainland Japan, the rainy season usually starts around June 9, but this year it looks to be running about a week late.

In South Korea the fun begins in late July and lasts through the middle of August — most of the time. This year, however, the Korea Meteorological Administration has predicted an early changma — so things may start getting wet right about … now.

Got a question about goings-on in the Pacific? E-mail Stacy Chandler at:

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