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In East Asia, interaction between the rapidly mixing atmosphere and the slowly changing oceans are largely responsible for the monsoon, which affects Korea, China and Japan as elsewhere in the region. In order to better understand these patterns and to better prepare for their outcome, joint collaborative projects among these countries have been launched by their top meteorologists. Temperature and Precipitation
Temperature and Precipitation
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 cold Siberian fronts. Summer is hot and humid due to the maritime pacific high. The transitional seasons, spring and autumn are sunny and generally dry. Temperatures in 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, Va., 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 month with the mean temperature ranging from 19oC to 27oC. January is the coldest month with the mean temperature ranging from -8oC to 7oC. Annual precipitation is about 1,300mm in the central region. More than a half of the total rainfall is concentrated in summer, while precipitation in winter is less than 10 percent of the annual total.
The prevailing winds are southeasterly to southwesterly in summer, and northwesterly in winter. The winds are stronger in winter, from December to February, than those in other seasons. The land-sea breeze becomes dominant with weakened monsoon winds in the transitional months, September and October.

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

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

Annually, about 28 typhoons occur in the western Pacific, and only two or three among them approach the Korean Peninsula between July and October.

Precipitation distribution on the Korean Peninsula is mainly affected by orography. 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 north, on the other hand, experiences less than 600mm (24 inches). Since the precipitation is larger in the crop growing areas in the south, water needs for agriculture are normally well met. Even though the annual mean precipitation is more than 1,200mm (48 inches), Korea often experiences drought due to the large fluctuation and variation of rainfall, making the management of water resources difficult.
Spring begins during the middle of March in the central part 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 front weakens, the temperature rises gradually. Yellow sand which originates in the desert or arid areas of Mongolia and China, known as hwangsa, occasionally blows into Korea during early spring. The hwangsa often causes low visibility and eye irritation.
The summer can be divided into two periods; jangma, a rainy period which occurs during the early summer months, and a hot and humid period which occurs in August.

Rainfall during the summer time is characterized by heavy showers. Daily precipitation often exceeds 100mm (4 inches), with extremes topping 300mm (12 inches). Occasional storms caused by typhoons that pass through the peninsula sometimes cause a great deal of damage, although the loss of life is rare.

Regional temperature contrasts are not very striking during the summer season although the northern interior and the littoral are cooler than the southern region. 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 37 C (100 F). Nights are also hot and humid.
A field of rape growing
on Jejudo island
  A ravine in summer   Fall foliage on
Mt. Seoraksan
  Snow on Mt. Deogyusan
Autumn is known for crisp weather, much sunlight and the changing colors of tree leaves. Beginning in October, the continental air mass brings dry, clear weather. Traditionally, Koreans enjoy the season of harvest with festivities of chuseok which is one of the most important national holidays in Korea. It is often referred to as the Korean version of the American Thanksgiving. Autumn in Korea can be summed up with the simple words of an old Korean saying "The sky is high and the horses get fat."
The arctic air from the interior of the Asian continent brings bitter cold and dry weather and occasional snowfall, while also 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. The monthly mean temperature during the month of January differs by about 20 degrees centigrade between the northern and the southern peninsula. 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. On the southern coast, it lasts roughly 225 days of the year.

Korea has four distinct seasons. Spring and autumn are rather short, summer is hot and humid, and winter is cold and dry with abundant snowfall, especially in the mountainous regions, but not along the southern coast.

Temperatures differ widely from region to region within Korea, with the average being between 6oC (43oF) and 16oC (61oF). The average temperature in August, the hottest period of the year, ranges from 19oC (66oF) to 27oC (81oF), while in January, the coldest month, temperatures range from -8oC (17oF) to 7oC (43oF).
Spring on
Anmyeondo Island
  Summer on
Baengnyeongdo Island
  Fall on Mt. Juwangsan   Winter on Mt. Deokyusan
In early spring the Korean Peninsula experiences "yellow sand/dust" carried by wind from the deserts in northern China. But in mid-April, the country enjoys balmy weather with the mountains and fields garbed in brilliant wild flowers. Farmers prepare seedbeds for the annual rice crop at this time.

Autumn, with its crisp air and crystal blue sky, is the season most widely loved by Koreans. The countryside is particularly beautiful, colored in a diversity of rustic hues. Autumn, the harvest season, features various folk festivals rooted in ancient agrarian customs.
Source: www.Korea.net