The climate of Jeju (Jeju-do), an island located to the south of Korea, is temperate humid, with quite 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 reach the island, they have to 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, in fact they amount to 1,500 millimeters (60 inches) per year on the northern coast (the most sheltered, at least in the warm season, which is dominated by southern currents), while they reach 1,900 mm (75 in) on the southern and eastern coasts.
Owing to both the southern location and the influence of the sea, winter in Jeju-do is milder than in the Korean peninsula, but it is also rainier, in addition, 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 centimeters (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 the airport is also located.
Jeju City - Average temperatures
Min (°C)34610141923242015105
Max (°C)8913182225293026211611
Min (°F)373943505766737568595041
Max (°F)464855647277848679706152

Here is the average rainfall in Jeju City.
Jeju City - Average precipitation

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

And here is the average rainfall in Seogwipo.
Seogwipo - Average precipitation

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 not 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.
Seogwipo - Sunshine

However, in the north of the island, directly exposed to the winter monsoon, the sun shows up even more rarely in winter.
Jeju City - Sunshine

On the island of Jeju, the sea is warm enough for swimming from July to September, with a maximum in August. Here are the average sea temperatures.
Jeju City - Sea temperature
Temp (°C)141313141620242725221916
Temp (°F)575555576168758177726661

Jeju in spring

The interior of the island is occupied by a mountain, Mount Halla, an extinct volcano of 1,950 meters (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 meters (5,486 ft) above sea level, where the roads at the base of the volcano end.
Witse Oreum - Average temperatures
Min (°C)-9-8-516101514114-1-7
Max (°C)-2131015182021181370
Min (°F)161823344350595752393019
Max (°F)283437505964687064554532

Mount Halla is rain-soaked: up to 4,700 mm (185 in) of rain fall per year; 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 the winter is the driest season, there is still room for heavy snowfalls, so 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.
Witse Oreum - Average precipitation

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

Stone grandfathers in the Jeju Stone Park