If you’re hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in Siheung: The hottest months are usually August and July with an average temp of 74°F, while the coldest months are January and December with an average of 34°F. The rainiest months in Siheung are July, August, September, and June, with each month seeing an average of 9 inches of rainfall.