December in Seoul, South Korea, brings average daily highs of 4°C and overnight lows of -5°C in the 2016–2025 historical record. Pack for cold conditions, with warm layers for time spent outside. Overnight averages fall below freezing; check the forecast and local conditions before committing to an exposed walk or a long outdoor day.
In the record, at least 1 mm of rain or snow falls on 5 of 31 days on average, a daily chance of 15%. Leave some freedom to swap an outdoor day with a sheltered one. There is no need to read the average as a fixed number of lost days; it says nothing about how long rain lasts on each occasion.
The calculated day length, sunrise to sunset, averages about 10 hours. Sunshine averages 8 hours a day. Use the daylight window to plan the length of excursions, and check local sunrise and sunset times for the dates of your stay. Sunshine is a separate measure, and those hours need not arrive in one uninterrupted stretch.
For nature trips or walks, keep the longest outings movable and check route conditions locally; a monthly climate average cannot establish whether a particular path is accessible. For a culture-focused visit, group the places you want to see into manageable areas, with time between stops. Check each venue's opening dates before fixing the order.
January has an average daily high of 3°C, 1°C lower than December. Its historical rain-day chance is 10%, compared with 15% this month. If your dates are flexible, compare those trade-offs with the kind of days you want. Neither set of averages predicts the conditions of one particular holiday.