Jeju surrounded by sea on all sides, is categorized as a volcanic island in subtropical zone. Jeju displays a temperate oceanic climate because the Kuroshio Current and the Tshushima current have a direct effect on the island.
The annual average temperature reaches 11.7℃, with Seogwipo boasting the highest and the peak of mount Halla the lowest. The annual average temperature of Jeju is relatively low, compared with those of other places at the same latitude of the earth. This is due to the cold winter seasons, which is influence of continental climate, and also by summers that do not become too hot due to the cool oceanic currents. The annual temperatures of Jeju City and Seogwipo City for the last 30 years are reported to be 15.2 ℃ and 15.9℃ respectively. Winters in Jeju are rather short and warm, compared with those of the mainland. One cannot experience a monthly average temperature of Jeju slipping down below 0℃. From a climatological perspective, there is no winter on the island, if winter is measured as a time period when the lowest monthly temperature is below 0℃. The time period of temperatures below 0℃ is over 100 days at the middle and northern areas of the mainland and about 50-100 at the southern areas and only 17 at Jeju.
The annual precipitation of Jeju for the last 30 years is reported to be 2,044mm for the island as a whole, and 1,560mm for the coastal areas.
The annual precipitation of Jeju for the last 30 years is reported to be 2,044mm for the island as a whole, and 1,560mm for the coastal areas. The annual average amount of rainfall on Jeju is relatively high, compared with those of other places on the same latitude of the earth. The reason is that Hall Mountain, at the center of the island, serves to block the effect of air masses of high humidity that move from the coastal areas. The amount of rainfall is the highest at the southeast areas of mount Halla. In addition, the precipitation on mid-slopes and ridges of the mountain reaches 1,780mm and 2,766mm respectively. About 43% of the annual rainfall is reported in June, July, August in Jeju compared with 60% for Seoul and 51% for Daegu. The evenly distributed rainfall in Jeju is attributed to high amount of precipitation in the spring and fall. The island is unique in that the so-called ''Bracken Rainy Spell,'' prior to the monsoon season, sweeps through the island in the spring. In the spring rainy season, a series of drizzle serves to make spring the second highest rainy season, providing 23% of the annual precipitation. The "Bracken Rainy Spell" is equivalent to the "zzu you(梅雨)" in Japan.
Jeju is called "The Island of 3 abundance," which are stones, winds, and women. Wind is one of the three major factors affecting climate that includes temperature and rainfall, and plays a decisive role in the shaping of it.
The average wind velocity across coastal areas of the island reaches 3.5m/s. Western areas of the island, centering on Moseulpo, are categorized as areas with strong winds, reaching 4.1m/s. Generally, areas with strong winds are densely located in the northeast coastal areas of the lsland, which are heavily affected by the winter seasonal winds, whereas areas with mild winds lie in the southeast coastal areas. Effective temperature, triggered by wind, would have a cooling effect to the nominal temperature by 1.3℃ in summer and 2.6℃ in winter. Jeju has the second most frequent rainstorms, over 10m per second with 117 days a year, following Ulreung Island, which records 179 days. Annual average wind velocity is 4.7 meters per second on Jeju, compared with 2.5meters in Seoul and 1.3 meters in Junggangjin.
Rainstorm days throughout the 4 seasons reveal that winter is the season with the windiest days of 36.7%, followed by spring with 27.3%, and summer and fall with 17.9% respectively. The reason that winter has the most windy days lies in the fact that barometric pressure in winter, a gap between air pressure of land and that of sea, is much greater than that in the summer. A report, tracing typhoons of the North Pacific for 43 years (1940~1982), indicates that a total of 110 typhoons had passed Jeju, which would mark Jeju as the area with the most typhoons in Korea. Thus, it is safe to say that the winter in Jeju is the season of rainstorms whereas the summer is the season of typhoons.