South Korea Weather, climate and geography
South Korea's weather is temperate but can be extreme -- the summer can be as hot as 45 C with humidity in the high 70s. Nights in November can drop suddenly and be sub-zero with Siberian winds. It's best to bring layers or (even better) take advantage of the great shopping malls and buy clothes that suit the season while you're there. It is important to note too that weather in Seoul (in the north) can be quite different from that of Busan or Jeju.
Moderate climate with four seasons, making any season a fine time to visit as long as you wear appropriate clothing. The hottest part of the year is during the rainy season between July and August, and the coldest is December to February. Occasional typhoons bring high winds and floods. Spring and autumn are mild and mainly dry and are generally considered the best times to visit.