Climate of Seoul
The climate of Seoul features a humid continental climate with dry winter, called "Dwa" in the Köppen climate classification. Seoul is classed as having a temperate climate with four distinct seasons, but temperature differences between the hottest part of summer and the depths of winter are extreme. In summer the influence of the North Pacific high-pressure system brings hot, humid weather with temperatures soaring as high as 35 °C (95 °F) on occasion. In winter the city is topographically influenced by expanding Siberian High-pressure zones and prevailing west winds, temperatures dropping almost as  low as -20 °C (-4 °F) in severe cold waves. The bitterly cold days tend to come in three-day cycles regulated by rising and falling pressure systems. The most pleasant seasons for most people in the city are spring and autumn, when azure skies and comfortable temperatures are typical. Most of Seoul's precipitation falls in the summer monsoon period between June and September, as a part of East Asian monsoon season. Factors. Seoul has a wet and very humid climate during the summer season, with cold and dry weather during the winter season. Spring (although windy ) and autumn are mild but are short in duration. These seasons are considered the best time to visit Seoul. Seasonal climate. As a temperate climate, Seoul has winter, summer and autumn seasons. The monsoon season and typhoons also occurs in the summer season. Winter. The winter in Seoul is controlled by the large Siberian high pressure system (the Asiatic high), which results in predominantly cold, dry north-westerly winds. The influence of the Siberian high leads to significantly colder winter temperatures than would be expected at this latitude. About every 4 to 6 days a low-pressure trough will move  through South Korea, bringing with it cloudiness and light precipitation. The amount of precipitation locally depends mostly on the elevation of the station and the length of time that the air has been over the Yellow Sea. Maximum snowfall occurs over the northwest coast, which is the most exposed to the northwesterly flow, and in the mountain areas. Normally less than 10 percent of the annual precipitation falls (falls very much) during the winter. Frequently the weather is cloudless, clear, and dry, except for the southwestern region of the peninsula. The mean January temperature in Seoul is -2.4 °C  (27 °F). January is the coldest month in Seoul, often with the lowest temperatures, almost always dropping below -10 °C (14 °F), and sometimes below -15 °C (5 °F). Summer. During the summer, East Asian monsoon winds engulf the city and the warm, moisture laden air moving off the oceans clashes with the drier air to the north. These fronts oscillate back and forth across Korea during the summer months. The interior highlands disturb the winds, forcing them into a westerly/southwesterly direction. The majority of the annual precipitation falls between late June and the  middle of September, with rains fully developing along the entire peninsula by mid-June. Seoul receives approximately 126 mm (5 in) of precipitation during the winter (December–March), but in July alone receives approximately 383 mm (14.3 in). July is the wettest month in the metropolis. Thunderstorms usually occur about 2 to 5 days per month during this period. Summer precipitation in Seoul is as likely to occur at 0200 as at 1400. Humidity is very high and fog will develop whenever a cold air mass confronts this moisture laden air, often forming on cloudless days. The typhoon season occurs  from July through September. About once each year, a typhoon will pass very close to or move over Korea, causing heavy showers. Strong winds are usually confined to islands and exposed coastal areas. Although winds might not pose a problem, the associated rainfall can cause significant flash flooding, a very real threat during the rainy season, especially in rough terrain. The mean temperature for Seoul in August is 25.7 °C (78 °F) Autumn. October is the transition month between the summer rainy season and the cold, dry winter. The predominantly tropical cloudy weather of the summer is replaced  by cooler, drier, and less cloudy conditions. The primary weather producers during October are cold frontal systems from the Asian mainland. On the average, one frontal passage per week can be expected during the month. A typical frontal passage is proceeded by increasing middle and high cloudiness with light rain. Following the frontal passage, mostly clear skies can be expected for 3 or 4 days. During this clear period it is very likely for fog to form. Fog is especially prevalent in river valleys and in low-lying areas.