Definition
A stationary front is the boundary between two huge air masses of different kinds that meet and push on each other, so the boundary stays in one place for a long time. It forms when cold air and warm air are evenly matched, and it can sit over one region for days, bringing cloudy skies and heavy rain.
Two air masses in a tug-of-war
Picture a tight tug-of-war match at a field day. If both teams are about equally strong, the red ribbon tied to the middle of the rope doesn't get pulled to either side. It just stays where it is.
The same kind of contest happens in the sky. Sometimes a cold air mass coming down from the north crashes head-on into a warm, moist air mass pushing up from the south. If the two are about equal in strength, neither can easily shove the other away, and they hold their ground.
When two air masses keep pushing and pulling like this, and the boundary between them stays in place, we call it a stationary front. The East Asian summer rainy season front, which Koreans call jangma, is the best-known example. In early summer near the Korean Peninsula, the North Pacific air mass to the south and a cool, moist air mass to the north play a giant game of tug-of-war. The Okhotsk air mass is often named as that northern air, but some studies suggest its role is smaller than people thought, so views differ.
Where do the clouds and rain form?
Just because the two air masses are standing still doesn't mean the weather is calm and quiet. Not at all. The front looks like it isn't moving, but along the boundary where the two air masses touch, the air is moving very violently.
Warm air is less dense, so it is light. Cold air is denser, so it is heavy. When they meet, the warm air slides up the gentle slope of the cold air, getting pushed high into the sky.
As air rises, the pressure drops and the temperature falls, and water vapor clumps into droplets that build huge clouds. The rain clouds form in a long stretch high above the cold air side, where the warm air has climbed up. As a result, a narrow band of rain clouds stretching east to west forms north of the front, and heavy rain pours down for days.
To be more precise: it's not completely still
To be more precise, a stationary front isn't nailed to one spot on the map and totally frozen.
When the sun heats the ground during the day, or winds from a nearby low-pressure trough arrive, the front can wiggle north and south by tens to hundreds of kilometers in a single day. That's why a TV weather presenter might say, "The rainy-season front will dip south for a while, then move back north starting tomorrow."
This long standoff ends only when one air mass becomes overwhelmingly stronger. In midsummer, the sun's heat builds up the North Pacific high pressure to the south, and when it pushes north with overwhelming force, the front is driven up past the Korean Peninsula toward Manchuria. The long rainy season finally ends, and the real summer heat arrives.
On weather maps, this evenly matched standoff is drawn by alternating a pointed blue triangle (for a cold front) and a rounded red semicircle (for a warm front) on opposite sides of the same line.
Common misconceptions
A stationary front is a calm state where the air doesn't move at all and no wind blows.
It's only the boundary of the front that moves very slowly. Inside, warm air actively rises over the cold air, constantly producing huge rain clouds and heavy downpours.
Where you meet it
In early summer, the rainy-season front that soaks Korea for several weeks is a classic stationary front.
The autumn rainy-season front, which forms in early fall when cold northern air regains strength and meets southern air, is also a stationary front.
In one sentence
A stationary front is a front where cold air and warm air of similar strength face off, stay in one place, and drop long stretches of rain.