The average January temperature ranges from –5° C (23° F ) at Seoul to –2° C (28° F ) at Pusan and 4° C (39° F ) on Cheju Island. In the hottest part of the summer, however, the regional variation in temperature is not nearly so marked, with average temperatures ranging from 25° C to 27° C (77–81° F ) in most lowland areas. Average rainfall is 100 to 150 cm (40 and 50 in). Nearly all the rainfall occurs in the April–September period, especially during the rainy season, late June to early August. From one to three mild typhoons normally strike the south in the early fall, with a severe one occurring every two or three years. Days free of frost number about 240 in the southern regions.