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01 Four seasons that look nothing alike
Rainfall follows an even more extreme pattern. The annual total is 1,417.9 mm, but its distribution is wildly uneven: July alone accumulates 414.4 mm and August 348.2 mm. Adding June, July and August gives 892.1 mm — that is 63% of the year's entire rainfall in three months. Winter, by contrast, contributes around 5%.
And a third layer: snow. Seoul records an average of 23.9 snow days a year, concentrated in December, January and February, with seven days on average in January alone. This is not a city where snow is exceptional; it is a city where it snows every winter.
02 The jangma, or why July doesn't pay off
July gets almost eight times more rain than October, and with fifteen degrees more humidity on top.
The culprit has a name: the jangma, the rainy season associated with the East Asian monsoon front. The front reaches the whole peninsula around mid-June and usually shifts north between mid and late July. In practice that means one long month — typically late June to late July — of persistent rain and very high humidity.
August does not solve the problem: it still rains heavily (348.2 mm) and it is also the hottest month of the year. The combination of a thirty-degree high with high humidity is, in practice, the worst condition in the calendar for walking the city.
03 October, the clear window
If you can only pick one month, pick October. The figures say so unambiguously: 52.2 mm of rain for the whole month — an eighth of what falls in July — average highs of 20.2 °C and lows of 10.6 °C. That is stable weather, light-jacket temperatures and none of the summer humidity.
On top of that comes the autumn foliage. In Seoul the colour usually starts around mid-October and peaks between the last week of the month and the first days of November. On Bukhansan, because of altitude, the mountain turns before the city below, so a single trip can catch both phases if planned well.
An honest warning: in recent years the peak has been running later than the historical average because of warmer Septembers and Octobers. Treat foliage dates as a moving window and check the official forecasts before locking in the trip, rather than as a fixed calendar date.
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04 Spring: blossom and dust
April and May are the second good window. Cherry blossom in Seoul usually begins around 3 April and reaches full bloom around the 10th, roughly a week later. Temperatures cooperate: an average high of 17.9 °C in April and 23.6 °C in May, with still-moderate rainfall (72.9 and 103.6 mm).
Spring's trade-off is air quality. Seoul recorded an average of 8.9 yellow-dust days a year over 1991-2020, and more than half are concentrated in just two months: 2.2 days in March and 3.1 in April. During the most intense episodes, the daily average for fine particles has reached 129 micrograms per cubic metre.
That is no reason to write off spring, but it is a reason to pack a filtering mask and check the air quality index in the morning, exactly as you would check the rain forecast. If you have respiratory problems, March is the month most worth avoiding.
05 Winter: dry cold and empty palaces
Seoul's winter is harsh but very dry: January accumulates barely 16.8 mm of precipitation, the lowest of any month. That means frequently clear skies and excellent light, in exchange for highs of 2.1 °C and lows of −5.5 °C.
If you come properly equipped, it has real advantages: palaces without queues, snow-covered mountains half an hour away by metro, and a city whose interiors are better prepared for cold than almost anywhere else, with underfloor heating in practically every building. Seoul's cold is spent indoors, and indoors is very comfortable.
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