Map from Google - Jeju

The climate of Jeju (Jeju-do), 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 (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 per year on the north coast (the most sheltered, at least in the warm season, dominated by southern currents), and reach 1,900 mm 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, which alternate with cold periods, with wind and rain, highs around 5/7 degrees or less, and possible falls of sleet or snow. Snowfalls are still typically light, as well as night frosts. In January 2016 there has been an unusual snowfall of 12 centimetres, the most abundant since 1984, which led to the closure of the airport, and at night the temperature dropped to -5.8 °C.
Summer is hot and humid, though fortunately the sea breeze relieves the heat; there may be periods of good weather, with highs about 32/34 °C, 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
Jeju CityJFMAMJJASOND
Min (°C)34610141923242015105
Max (°C)8913182225293026211611

Here is the average rainfall in Jeju City.
Average precipitation - Jeju City
Jeju CityJanFebMarAprMayJunJulAugSepOctNovDecYear
Jeju CityJFMAMJJASONDY
Prec. (mm)65659090951802402652208060501500
Days1310111010121214117911130

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

Here is the average rainfall in Seogwipo.
Average precipitation - Seogwipo
SeogwipoJanFebMarAprMayJunJulAugSepOctNovDecYear
SeogwipoJFMAMJJASONDY
Prec. (mm)60751301752052753102901958070451925
Days101011101113141410678125

In August, September and early October (more rarely in July), Jeju can be affected by typhoons, 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), but in the north of the island, directly exposed to the winter monsoon, the sun shows up even more rarely in winter.

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
Jeju CityJFMAMJJASOND
Sea (°C)141313141620242725221916

Jeju in spring

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

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

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

Stone grandfathers in Jeju Stone Park