The annual average temperature reaches 11.7°, with Seogwipo boasting the highest and the peak of Mt. 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 winters, which are influenced by the continental climate, and also by the 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°C. From a climatological perspective, there is no winter in the Island, if winter is measured as a time period when the lowest monthly temperature is below 0°C. The time period of temperatures below 0°C are over 100 days in the middle and northern areas of the mainland and about 50-100 in the southern areas and only 17 at Jeju.

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 Mt. Halla, at the center of the island, serves to block the effect of air masses of high humidity that move from the coastal areas. The amounts of rainfalls are the highest at the southeast areas of Mt. 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 rainfalls are reported in June, July, and August on Jeju compared with 60% for Seoul and 51% for Daegu. The evenly distributed rainfalls on Jeju are attributable to high amounts of precipitation in the spring and fall seasons. 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 drizzles 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' in Japan.

The average wind velocity across coastal areas of the island reaches 3.5 m/s. Western areas of the island, centering on Moseulpo, are categorized as areas with strong winds, reaching 4.1 m/s. Generally, areas with strong winds are densely located in the northeast coastal areas of the Island, which are heavily affected by the winter seasonal winds, whereas areas with mild winds lie in the southeast coastal area. Effective temperature, triggered by wind, would have a cooling effect to the nominal temperature by 1.3 m/s in summer and 2.6 m/s in winter. Jeju has the second most frequent rainstorms, over winds at 10m/s for 117 days a year, following Ulreung Island, which records 179 days. Annual average wind velocity is 4.7 m/s on Jeju Island, compared with 2.5m/s in Seoul and 1.3 m/s in Junggangjin.

Rainstorm days throughout the 4 seasons reveal that winter is the windiest season with 36.7% windy days, 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, a gap between air pressure of land and that of sea, is much greater in winter 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 on Jeju is the season of rainstorms whereas the summer is the season of typhoons.