The year in Jeju, month by month
The figures come from two Korean weather service stations, 1991–2020 series: Jeju City on the north coast (1,502.3 millimetres a year over 127.8 days) and Seogwipo on the south (1,989.6 over 125.3). The scoring crosses rainy days, wind, humidity and typhoon risk.
Jeju is sold as a subtropical island and that creates the wrong expectation of the calendar. Summer is not the good season, and neither is winter, however little it rains. The window is somewhere else.
01 Six and a half days of rain
The Jeju City station records 1,502.3 millimetres a year over 127.8 days. That averages a little over ten rainy days a month, and indeed almost every month is close to that figure: between 9.4 and 13.2.
All but one. In October it rains on 6.7 days, with 95.1 millimetres. That is almost a third fewer days than the next driest month on the calendar, and half as many as in August.
On the south coast the pattern repeats even more sharply: Seogwipo drops to 5.8 rainy days in October, against 14.3 in August. On both coasts, the same month and the same fall.
To those days are added comfortable temperatures, average highs of 21.6 degrees in the north and 23.5 in the south, and humidity of 66.4 per cent, twenty points below July's peak.
02 What comes right before
What makes October remarkable is not only its figure, but the contrast with what precedes it. In September 227.8 millimetres fall in the north and 208.8 in the south, and it is the month of highest typhoon risk in this part of the sea.
Typhoon risk matters more than rainfall totals, because a raincoat does not solve it. A typhoon closes the airport and stops the ferries, and an island without a connection is an island you cannot leave for days.
August is worse still for water: 272.3 millimetres in the north and 315.7 in the south, the year's maximum at both stations, with highs of 30.1 degrees and lows of 24.8. And it coincides with the Korean holidays, when the island receives the most people.
From September to October rainy days fall from 11.2 to 6.7 in the north and from 10.9 to 5.8 in the south: there is no other transition like it in the year.
03 Two coasts, two climates
There is a correction worth making before booking, and it concerns which side of the island you mean. Hallasan splits Jeju into two flanks and the two have different regimes, despite being some thirty kilometres apart.
Seogwipo, in the south, receives 1,989.6 millimetres a year; Jeju City, in the north, 1,502.3. That is 487 millimetres of difference, almost a third more water on the southern side.
The curious part is that it rains on practically the same number of days: 125.3 in the south and 127.8 in the north. In other words, it does not rain more often in the south, it rains harder. And in April and May the difference in volume shoots up: 187.0 against 89.7 millimetres, and 223.6 against 95.6.
In winter the relation reverses. In January the north receives 67.5 millimetres against the south's 60.7, and above all it accumulates 7.2 snow days against 3.8. The north-westerly wind hits that coast and the south stays sheltered.
04 Winter, which here means wind
Winter figures mislead if you look only at volume. December is the month with the least rain of the year, 55.6 millimetres, and yet it is a bad month to come: those millimetres are spread over 11.5 days, with 5.3 of snow.
January adds the wind. The island is exposed to the cold north-westerly flow, and that is why the north coast accumulates snow despite average highs of 8.6 degrees. The station's cold record is −6.0.
For a walker that means something concrete: it is not a question of clothing but of exposure. On the exposed coast, eight degrees with sustained wind are more uncomfortable than zero without it, and on Hallasan the summit is coated in ice formed by the wind itself.
05 The other window, in spring
If October does not fit, the alternative is April and May. On the north coast those are months of 89.7 and 95.6 millimetres over 9.4 and 9.8 days, with highs climbing from 18.0 to 22.1 degrees and humidity still low.
Here the warning about which flank matters. Those same months, in Seogwipo, give 187.0 and 223.6 millimetres. That is more than twice the north's, concentrated over practically the same number of days.
In exchange, spring brings what October does not have: the countryside in flower and barley growing between volcanic stone walls. It is the island's farming picture at its best.
06 How to choose the month
If you can choose without constraints, October, and by a clear margin. Six and a half rainy days a month in the north and five point eight in the south are not repeated anywhere else in the calendar.
If summer is unavoidable, the strategy is twofold. First, sleep in the north, which gets less water in the wet months. And second, leave slack on the return flights, because a typhoon in September can close the island for two or three days.
And if you come in winter, choose the southern flank. Not for the rain, which is similar, but for the wind and the snow: 3.8 snow days in Seogwipo against 7.2 on the north coast, and about two degrees more of average high every month.
The year's four stretches
From 55.6 to 67.5 millimetres a month in the north, but over 10.2 to 12.2 days, with up to 7.2 of snow in January. Highs of 8.6 to 11.0 degrees. Little rain falls and yet there is almost never a whole clear day.
The north coast stays between 89.7 and 95.6 millimetres over 9.4 to 10.3 days, with highs climbing from 13.3 to 22.1 degrees. In the south it rains between 40 and 130 per cent more, but over the same number of days.
881.5 of the north's 1,502.3 annual millimetres fall in these four months, 59 per cent. August reaches 272.3 in the north and 315.7 in the south, and September is the month of highest typhoon risk.
October drops to 6.7 rainy days in the north and 5.8 in the south, the year's minimums, with highs of 21.6 and 23.5 degrees. November holds 69.5 and 86.2 millimetres and remains comfortable.
