Because geography affects climate it's important to understand where Korea is and what the lay of the land is. South Korea borders North Korea to the North, faces China west across the Yellow Sea (which the Koreans call the West Sea), Japan to the east and south across the Sea of Japan (which Koreans call the East Sea). 
Weather in South Korea
Korea, as you will hear over and over again during your sojourn in South Korea, has four seasons. Why, you may ask, do Koreans insist on telling you about the virtues of their four seasons, when chances are your country of origin also has a spring, summer, winter and fall- as do most countries on either side of the Tropics of Cancer and Capricorn? This mystery has yet to be solved- I suspect that Korean school children are taught that the phenomenon of four seasons is unique to Korea. When you are told this amazing fact, the best response is to nod knowingly and express your amazement that Korea has four distinct seasons.
Check out the follow articles about the various season in Korea, what to expect and what you can get up to.
Korean Geography
Weather Synopsis
South Korea is temperate with rainfall heavier in summer than in winter. There is a wet monsoon season in the middle of the year and a dry, cold winter from November to March. During the winter, winds pick up speed and are carried from the northwest. The coldest month is January. The average winter temperature is around -5C, and begins to warm up in March.
Spring in Korea
Spring is one of the most beautiful seasons in Korea and several cities, including Masan, near Pusan, hold annual Cherry Blossom Festivals. 
In March and April Korea experiences the ‘Yellow Dust’ phenomenon. Deforestation in central China and Mongolia causes massive amounts of particulate matter to be swept up during windstorms. Yellow Dust is usually simply an irritation- clothes and cars have to be washed more frequently than usual, however in recent years the amount of dust has become more severe, causing eye infections and respiratory ailments. The further east you are on the penninsula, the less effect this has on you. Those on the west coast take the full brunt of the Yellow Dust.
Winter in Korea
Despite the relatively small geographic footprint of Korea, they do have diverse weather around the penninsula and even on Jeju Island - the island off the southern tip of Korea.
In winter, in Seoul, the temperature hovers around freezing, and although snow occasionally falls it rarely accumulates- winter weather in Korea is similar to weather in the mid-Atlantic United States - think Washington D.C., or the Pacific Northwest minus the rain.
Fall in Korea
Autumn is generally considered to be the most pleasant season in Korea - the leaves turn color and the weather is usually cool and sunny which makes for absolutely lovely days to check out a temple or to go for a stroll in the local park.
The fall is beautiful t-shirt weather. The huge colourful leaves falling on the sidewalk add lots of colour to the city. Fall is also the perfect time to go hiking. The temperature is just right, not too hot but still warm and the scenery is amazing.
Places of particular interest in the fall are Gyeonggju, Soraksan, Chirisan, Cheuncheon and Mokpo.
Summer in Korea
Summers in Korea tend to be hot and humid. The monsoon season usually starts in July and runs through August- expect brief, driving rains on an almost daily basis during these months. Cheju (Jeju) Island, off the South Coast of Korea, is the warmest and also the wettest area in South Korea.
August is the hottest month in South Korea, with average temperatures ranging from 20 to 26 degrees Celsius (68 to 78 degrees Fahrenheit). On average Pusan (Busan), the southern most major city in Korea, tends to be five degrees warmer Celsius than Seoul.
