Every summer, Korea's jangma rainy season is the result of a stationary front formed as two air masses push against each other. Here's how the jangma front forms, why its start and end dates shift so much year to year, and how the pattern has been changing recently.
In early summer, the cold, moist Okhotsk air mass meets the hot, moist North Pacific High from the south, and neither side gives way easily — they collide at a boundary. This boundary is a stationary front, called the jangma front in Korea. When the two air masses are evenly matched, the front lingers over one area, pouring heavy, concentrated rain there.
The jangma front's position is determined by how fast and how strongly the North Pacific High expands northward. On average, Jeju's jangma starts in mid-June and central Korea's in late June, ending around late July, but depending on the high's expansion speed and strength, the timing shifts significantly year to year. Some years bring a short, intense jangma; others a long, scattered one.
While the jangma front lingers, moving up and down, a narrow, elongated rain band forms where strong updrafts occur along the front. When this narrow band sits over one area for a long time, hourly rainfall spikes sharply into a concentrated downpour — which is why Seoul can be clear while another region gets drenched on the same day.
When the jangma front retreats north or dissipates entirely, it means the North Pacific High has fully taken over the Korean peninsula. The hot, humid air this high brings often kicks off intense heat waves and tropical nights right after jangma ends.
Recently, rather than the front lingering for long stretches in the classic pattern, short, intense local downpours have become more common. In some years, a stationary front re-forms in late summer or early autumn even after jangma has officially ended, bringing heavy rain in what's sometimes called "autumn jangma." For general season-by-season tips, see the seasonal outdoor guide.
During jangma, whether it's raining can change sharply hour to hour even in the same area. Saving the places you check often in sanCheck lets you compare hourly rain probability and outdoor-activity scores at a glance to time your outings.