Map from Google - Jeju

The climate of Jeju (Jeju-do), an island located to the south of Korea, is temperate humid, with fairly mild winters, and hot, muggy and rainy summers. Like the rest of Korea, the island is affected by the monsoon circulation: in winter the northwest cold currents prevail (which are initially dry, but before they get to the island, they must pass above a stretch of sea quite large), while in summer, hot and humid currents of tropical origin prevail. Spring and autumn are transitional periods, mild and pleasant, especially the month of October.
The rains are plentiful, given that they amount to 1,500 millimetres (60 inches) per year on the north coast (the most sheltered, at least in the warm season, dominated by southern currents), and reach 1,900 mm (75 in) on the south and east coast.
Due to the southern location and the influence of the sea, winter in Jeju-do is milder than in the Korean peninsula, but it is also more rainy, and besides the wind can heighten the feeling of cold. There are quite mild periods, with highs above 10 °C (50 °F), which alternate with cold periods, with wind and rain, highs around 5/7 °C (41/45 °F) or less, and possible falls of sleet or snow. However, snowfalls are typically light, as well as night frosts. In January 2016 there has been an unusual snowfall of 12 centimetres (4.7 in), the most abundant since 1984, which led to the closure of the airport, and at night the temperature dropped to -5.8 °C (21.6 °F).
Summer is hot and humid, though fortunately the sea breeze blows, relieving the heat; there can be periods of good weather, with highs about 32/34 °C (90/93 °F), tropical nights and high humidity, but also periods of bad weather, due to the summer monsoon.
Here are the temperatures of Jeju City, located on the northern coast, where there is also the airport.
Average temperatures - Jeju City
Jeju CityJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)34610141923242015105
Max (°C)8913182225293026211611
Min (°F)373943505766737568595041
Max (°F)464855647277848679706152

Here is the average rainfall in Jeju City.
Average precipitation - Jeju City
Jeju CityJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)65659090951802402652208060501500
Prec. (in)

Here are the temperatures of Seogwipo, located on the southern coast: as you can see, the temperatures are slightly milder.
Average temperatures - Seogwipo
Min (°C)44711151924252116116
Max (°C)111214182225283027231813
Min (°F)393945525966757770615243
Max (°F)525457647277828681736455

And here is the average rainfall in Seogwipo.
Average precipitation - Seogwipo
Prec. (mm)60751301752052753102901958070451925
Prec. (in)2.435.

From July to early October (and occasionally also in the second half of June), Jeju can be affected by typhoons, the tropical cyclones of south-east Asia, which can bring strong winds and torrential rains, as happened for example with typhoon Maemi in September 2003, typhoon Sanba in September 2012 and typhoon Chaba in early October 2016.

The amount of sunshine in Jeju is never excellent, however, there are two peaks, one in spring (April-May), and the other in autumn (October). The cloudiest period is the first part of the rainy season (June-July), as you can see in the table concerning the hours of sunshine per day in Seogwipo.
Sunshine - Seogwipo
Sun (hours)556665566765

However, in the north of the island, directly exposed to the winter monsoon, the sun shows up even more rarely in winter.
Sunshine - Jeju City
Jeju CityJanFebMarAprMayJunJulAugSepOctNovDec
Sun (hours)245676665643

In the island of Jeju, the sea becomes warm enough for swimming from July to September, with a maximum in August.
Sea temperature - Jeju City
Jeju CityJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)141313141620242725221916
Sea (°F)575555576168758177726661

Jeju in spring

The interior of the island is occupied by a mountain, Mount Halla, an extinct volcano of 1,950 metres (6,398 feet), which is also the highest mountain of Korea. The area is protected in the Hallasan National Park.
Here are the average temperatures of Witse Oreum, 1,672 metres (5,486 ft) above sea level, where the roads at the base of the volcano end.
Average temperatures - Witse Oreum
Witse OreumJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)-9-8-516101514114-1-7
Max (°C)-2131015182021181370
Min (°F)161823344350595752393019
Max (°F)283437505964687064554532

Mount Halla is very rainy: even 4,700 mm (185 in) of rain per year fall; rainfall exceeds 300 mm (12 in) per month from May to October, with a maximum of as high as 835 mm (33 in) in August. Although winter is the driest season, there is still room for heavy snowfalls, so that the top can remain snow-capped also in spring. Tourists on the mountain slopes go skiing (though there are no lifts and skiing is actually forbidden), and sledding. Here is the average precipitation.
Average precipitation - Witse Oreum
Witse OreumJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)45130300425655650740835525125165654670
Prec. (in)

On the island, the characteristic Dol hareubang ("Stone grandfathers"), basalt statues, up to 3 metres (10 feet) high, are found.

Stone grandfathers in the Jeju Stone Park