Seoul is in the northwest of South Korea. Seoul proper comprises 605.25 km2 (233.69 sq mi), with a radius of approximately 15 km (9 mi), roughly bisected into northern and southern halves by the Han River. The Han River and its surrounding area played an important role in Korean history. The Three Kingdoms of Korea strove to take control of this land, where the river was used as a trade route to China (via the Yellow Sea). The river is no longer actively used for navigation, because its estuary is located at the borders of the two Koreas, with civilian entry barred. Historically, the city was during the Joseon dynasty bounded by the Seoul Fortress Wall, which stretched between the four main mountains in central Seoul: Namsan, Naksan, Bukhansan and Inwangsan. The city is bordered by eight mountains, as well as the more level lands of the Han River plain and western areas. Due to its geography and to economic development policies, Seoul is a very polycentric city. The area that was the old capital in the Joseon dynasty, and mostly comprises Jongno District and Jung District, constitutes the historical and political center of the city. However, for example, the city's financial capital is widely considered to be in Yeouido, while its economic capital is Gangnam District.
Seoul has a humid subtropical climate influenced by the monsoons (Köppen: Cwa), being in the extreme east Asia the climate can be described as humid continental with great variation of the precipitation throughout the year and warm to hot summer (Dwa, by 0 °C isoterm). The suburbs of Seoul are generally cooler than the center of Seoul because of the urban heat island effect. Summers are generally hot and humid, with the East Asian monsoon taking place from June until September. August, the hottest month, has average high and low temperatures of 32.6 and 23.4 °C (91 and 74 °F) with higher temperatures possible. Winters are usually cold to freezing with average January high and low temperatures of 1.5 and −5.9 °C (34.7 and 21.4 °F) and are generally much drier than summers, with an average of 24.9 days of snow annually. Sometimes, temperatures drop dramatically to below −10 °C (14 °F), and on some occasions as low as −15 °C (5 °F) in the mid winter period of January and February. Temperatures below −20 °C (−4 °F) have been recorded.
Climate data for Seoul (normals 1981–2010, extremes 1907–present)
Source: Korea Meteorological Administration (percent sunshine and snowy days)
Unhealthy for sensitive groups
According to the Environmental Performance Index 2016, South Korea ranked 173rd out of 180 countries in terms of air quality. More than 50 percent of the populations in South Korea exposed to dangerous levels of fine dust.
Since the early 1960s, the Ministry of Environment has implemented a range of policies and air pollutant standards to improve and manage air quality for its people. The "Special Act on the Improvement of Air Quality in the Seoul Metropolitan Area" was passed in December 2003. Its 1st Seoul Metropolitan Air Quality Improvement Plan (2005–2014) focused on improving the concentrations of PM10 and nitrogen dioxide by reducing emissions. As a result, the annual average PM10 concentrations decreased from 70.0 μg/m3 in 2001 to 44.4 μg/m3 in 2011 and 46 μg/m3 in 2014. As of 2014, the annual average PM10 concentration was still at least twice than that recommended by the WHO Air Quality Guidelines. The 2nd Seoul Metropolitan Air Quality Improvement Plan (2015–2024) added PM2.5 and ozone to its list of managed pollutants.
Asian dust, emissions from Seoul and in general from the rest of South Korea, as well as emissions from China, all contribute to Seoul's air quality. A partnership between researchers in South Korea and the United States is conducting an international air quality field study in Korea (KORUS-AQ) to determine how much each source contributes.