Covering about 70% of the Earth's surface, the oceans are a fundamental component of the climatic and seasonal variations in the weather. In East Asia, interactions between the rapidly mixing atmosphere and the slowly changing oceans are largely responsible for the monsoon season, particularly as they affect Korea, China and Japan. In order to better understand these patterns and to better prepare for their outcome, joint collaborative projects among these countries' top meteorologists have been launched.

In its geographical features Korea is a transitional zone between the continental landmass of northeastern Asia and the island arc rimming the western Pacific Ocean. The west coast, which is open to continental Asia, is vulnerable to the influence of the winter monsoon. The east coast, on the other hand, is sheltered from the winter monsoon by the Taebaeksan Range, the backbone mountains of the Korean Peninsula. Although Korea has general characteristics of a temperate monsoonal climate, there are geographic diversities, particularly during the cold winter season.

The climate of Korea is characterized by four distinct seasons: spring, summer, autumn and winter. The contrast between winter and summer is striking. Winter is bitterly cold and is influenced primarily by the Siberian air mass. Summer is hot and humid due to the maritime Pacific high. The transitional seasons, spring and autumn are sunny and generally dry. Temperatures of all seasons are somewhat lower than those at the corresponding latitudes in other continents, such as North America or Western Europe. The temperatures in Seoul, which is in the latitude of Richmond, Virginia, are closer to those in New York, which is located 500 kilometers (300 miles) farther north from the latitude of Seoul. The variation of annual mean temperature ranges from 10oC to 16oC except for the mountainous areas. August is the hottest with the mean temperature ranging from 20oC to 26oC. January is the coldest with the mean temperature ranging from -5oC to 5oC. Annual precipitation is about 1,500mm in the central region. More than a half of the total rainfall amount is concentrated in summer, while precipitation in the winter is less than 10% of the total.

The prevailing winds are the southeasterly in summer, and the northwesterly in winter. The winds are stronger in winter, from December to February, than those of any other season. The land-sea breeze becomes dominant with weakened monsoon wind in the transitional months, September and October.

The relative humidity is the highest in July with 80% to 90% nationwide, and is the lowest in January and April with 30% to 50%. It has a moderate value of about 70% in September and October. The monsoon front approaches the Korean Peninsula from the south in late June, migrating gradually to the north. Significant rainfall occurs when a stationary front lies over the Korean Peninsula.

The rainy season over Korea, the so-called jangma season, continues for a month from late June until late July. A short period of rainfall comes in early September when the monsoon front retreats back from the north. This rain occurs over a period of 30-40 days in June through July at all points of South Korea, with only some lag in time at different stations, and accounts for more than 50% of annual precipitation at most stations.

Annually, about 28 typhoons occur in the western Pacific. Generally speaking, only two or three among them approach the Korean Peninsula, from June through September.

Precipitation distribution on the Korean Peninsula is mainly affected by topography. The southern coastal and its adjacent mountain regions have the largest amount of annual precipitation which is over 1,500mm (60 inches). The sheltered upper Amnokgang (Yalu) river basin in the northern region, on the other hand, experiences less than 600mm (24 inches). Since most of the precipitation is concentrated in the crop growing areas in the south, the water supply for agriculture is normally well met. Even though the annual mean precipitation is more than 1,200mm (48 inches), however, Korea often experiences drought due to the large fluctuation and variation of precipitation, making the management of water resources difficult.

Four Seasons

Spring: Spring begins during the middle of April in the central region of the country, and toward the end of April in the northern region. Spring is rather short in the north. As the Siberian high pressure weakens, temperatures rise gradually. "Yellow sand" dust known as hwangsa blows into Korea from the Gobi Desert in Mongolia, early every spring. The hwangsa usually causes low visibility and eye irritation.

Summer: Summer can be divided into two periods; jangma, a rainy period which occurs during the early summer months and hannyeoreum, a hot and humid period during late summer. The weather during the jangma period is characterized by a marked concentration of rainfalls. More than 60 percent of the annual precipitation is concentrated between June and July. In particular, July sees many rainy days, which are followed by short dry spells and clear skies. Much of the rainfall is due to summer monsoons, which originate in the Pacific Ocean.

Rainfall during the summer is characterized by heavy showers. Daily precipitation often exceeds 200mm (8 inches), with extremes topping 300mm (12 inches). Occasional torrential storms by typhoons that pass through the peninsula from China may sometimes cause a great deal of damage, although the loss of the life is rare in these instances.

Regional temperature contrasts are not striking during the summer season although the northern interior and the littoral are cooler than the inland south. When hannyeoreum begins in August, the temperature rises abruptly as the jangma front moves north toward Manchuria. During this period, the weather becomes extremely hot and humid, particularly in the western plains and the Nakdonggang river basin area. The daily high temperature often rises to over 38ºC (100ºF). Nights are also hot and humid.

Autunm: Autumn enjoys crisp weather, much sunlight and changing autumn leaves. This is the transitional season between the hot and humid summer and the cold and dry winter months. Beginning in October, the continental air mass brings dry, clear weather. Traditionally, Koreans enjoy the season of harvest and chuseok, one of the most important holidays in Korea. It is a national holiday, celebrated as a harvest festival, and occasionally referred to as the Korean version of American Thanksgiving. Autumn weather is nicely expressed in the simple words of old Korean saying "The sky is high and the horses get fat."

Winter: The monsoonal arctic air from the interior of the Asian continent brings bitter cold and dry weather and occasional snowfall, adding warmth to the cold and dry winter weather periodically. Significant regional climate variations are caused by differences in elevation and proximity to the seas as well as by differences in latitudinal location. Regional differences in the monthly mean temperature during the month of January between the northern and southern part of peninsula is about 26ºC (80ºF). Snow remains longer on the ground in the north. The frost-free period varies from about 130 days in the northern interior to about 180 days in the central region. Along the southern coast, the frost-free period is roughly 225 days of the year.