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South Korea tends to have a humid continental climate and a humid subtropical climate, and is affected by the East Asian monsoon, with precipitation heavier in summer during a short rainy season called jangma (장마), which begins end of June through the end of July.
Winters can be extremely cold with the minimum temperature dropping below −20 °C in the inland region of the country: in Sŏul, the average January temperature range is −7 °C to 1 °C (19 °F to 33 °F), and the average August temperature range is 22 °C to 30 °C (71 °F to 86 °F). Winter temperatures are higher along the southern coast and considerably lower in the mountainous interior.
Summer can be uncomfortably hot and humid, with temperatures exceeding 30 °C (86 °F) in most parts of the country. South Korea has four distinct seasons; spring, summer, autumn and winter. Spring usually lasts from late-March to early- May, summer from mid-May to early-September, autumn from mid-September to early-November, and winter from mid-November to mid-March. Rainfall is concentrated in the summer months of June through September. The southern coast is subject to late summer typhoons that bring strong winds and heavy rains. The average annual precipitation varies from 1,370 millimeters (54 inches) in Sŏul to 1,470 millimeters (58 inches) in Pusan. There are occasional typhoons that bring high winds and floods.
The nature of Korea's climate is defined by its mid-latitudinal location and peninsular configuration, as well as its position as an appendage to the world's largest continent, Asia, and is influenced more by the continent than the ocean. Korea has a humid, East Asian monsoonal climate. The mean temperature during winter is generally below freezing. The mean temperature in January in Sŏul is 3.5 degrees C. below freezing, while that of London is 4.6 degrees C. above freezing, and that of San Francisco is 10.1 degrees C. Summer in Korea is hot. In most of the country, the mean temperature of hottest month is above 25 degrees C. except in the northern interior. The mean temperature of the hottest month for Sŏul is 25.3 degrees C. The annual temperature range between the coldest and hottest months for Sŏul is about 28.3 degrees C. The range of temperature is much greater in the north and in the interior than in the south and along the coasts.
Korea is located in the East-Asian monsoon belt. During the winter months, continental high pressure air masses develop over inland Siberia, from which strong northwesterly winds bring dry, cold air into Korea. The winter monsoon, usually stronger than the summer one, causes much hardship.
The summer monsoon brings abundant moisture from the ocean, and produces heavy rainfall. About 70 percent of the annual rainfall comes during June through September. Heavy showers with thunder and lightning are common. In addition, passing cyclonic storms add still more rainfall. Annual precipitation varies from 500 millimeters in the northeastern inland areas to 1,400 millimeters along the southern coast. The amount of rainfall decreases from the south to the north. The middle and upper parts of the river basins of the Sŏmjin-gang, South Korea, Han-gang River, and Ch'ŏngch'ŏn'gang, North Korea are areas of comparatively heavy rainfall, partly because of the convergence of wet airflows along the river valleys and the orographic uplift of the air flows. The southern coastal belt and the islands of Jeju-do and Ulleung-do also are heavy rainfall areas which receive about 1,400 to 1,500 millimeters annually.
The winter monsoon, which originates in the interior of the Asian continent, is dry and low in temperature. It produces little precipitation except for a few winter snowfalls. The winter months normally receive less than 10 percent of the total annual precipitation. Sŏul receives about 126 millimeters of precipitation during the winter months, December to March, which is less than one third of the amount of rainfall for July -- 383 millimeters. There are great variations in precipitation from year to year. According to 178 years of precipitation records, the maximum annual precipitation in Sŏul was 2,135, recorded in 1940; the minimum was 633.7 millimeters, in 1949. The statistics show that about once every eight years, annual precipitation falls below the 1,000 millimeters mark, which is usually considered the minimum for rice cultivation where no irrigation is available. This explains why in the days of an insufficient irrigation system the Sŏul area would have a rice crop failure once every eight years.
There are two types of storms which affect the climate of Korea. One originates in the Yangtze Valley and usually passes Korea in March and April and in the early part of summer, bringing abundant rainfall which is important for transplanting rice. The other type is the typhoon. Typhoons usually originate in the east Philippines, move toward the north and around Taiwan and shift direction mostly northeastward. The most common period for typhoons in Korea is July through August. The southern coast generally gets one or two mild typhoons a year, and a strong one every two or three years.
Spring in the Korean Peninsula comes with the arrival of the swallow, a migratory bird from the south, and the blooming of cherry blossoms in the last part of March or early April along the southern coast and nearby islands. It begins in the middle of April in the central part of the country, and in the last part of April in the north. By April, a thaw sets in, and streams swell as ice breaks up, and spring rain comes in drizzles. Rainfall increases gradually during March, April and May. This rainfall is slight, but it is adequate for preparing seedbeds for rice, sowing spring vegetables, and planting summer crops such as barley and soybeans. Spring winds are characterized by varied direction. The early spring has more gusty and dusty winds from the northwest, and as summer approaches, moist airstreams start blowing from the south. Spring is rather short, usually little more than two months, April and May, especially in the northern part of Korea.
Summer in Korea is hot and rainy. The season usually begins in June, when the temperature rises above 20 degrees C, in most of Korea except the northern interior, and lasts about four months. The monthly rainfall is generally more than 100 millimeters. Summer is the season of cyclonic storms and monsoonal rains. Especially in July there are many rainy days mingled with short clear spells and less cloudy days. Summer rainfall is characterized by heavy showers, often exceeding 200 millimeters of rainfall in a day, or sometimes more than 300 millimeters. This heavy concentration of rainfall occurs frequently in inland basins where airflows converge. During the summer rainy season, rivers and streams throughout Korea are filled with runoff water from the upper reaches which often causes floods. Abundant rainfall and hot summers are necessary for growing rice. Inadequate rainfall or the late arrival of the rainy season is liable to cause failure of rice crops. There is very little difference between north and south in summer temperature. Temperature differences are greater between the coast and the inland areas than between north and south. The highest temperature, 40 degrees C, was recorded at Taegu and the inland basin on August 1, 1942.
Autumn is rather short, lasting about two months from October to November. This is the season of transition from the hot and wet summer monsoon to the cold and dry winter monsoon. October brings a decided shift in air mass movements with a dry, continental airstream resulting in clear days. Nights are cool, particularly in the north, where frost normally occurs at this time. The clear autumn days are the most pleasant of the year. Dry and sunny weather is indispensable for rice to ripen and for farmers to reap it. In the central and southern parts of Korea, this is the season for planting winter crops such as barley and wheat.
During the winter the climate contrast between the northern and southern regions is most apparent. A January isothermal map shows that the minus 20 degrees C. line passes the northern inland region while the 4 degrees C. line passes the southern coast. Junggangjin, North Korea, the coldest place on the Korean Peninsula, has a mean temperature of minus 20.8 degrees C. and Jeju-do Island has one of 5.2 degrees C. during January, the coldest month. Thus, the north-south difference in January temperatures is about 26 degree C. In the winter of 1933, Junggangjin had the lowest temperature ever recorded in Korea, minus 43.6 degrees C. Winter is characterized as cold and dry with the cold spell generally caused by the influence of the Siberian high pressure cell, a cold and dry air mass. However, due to fluctuations in the high-pressure cell, the northwest winter monsoon is not continuous, allowing calm periods when the weather is milder. Winter in Korea is long, lasting four months in the central and southern regions. Most rivers in North Korea are frozen over for several months. The winter temperature has an important bearing on agriculture. Where the cold is not overly severe, two crops, usually rice and barley, may be grown.