Will I be cold?
Will I be cold in Busan in January?
Verdict
Yes — Yes, cold.
Lows drop to 33°F on average — noticeably cold, especially after sundown.
Day vs. night
Day
45°F
Cold
Direct sun feels cool at best; shade is jacket weather.
Night
33°F
Cold
Cold enough that a hat and gloves become mandatory.
Why
Busan in January averages highs of 45°F, lows of 33°F, 6 rainy days.
Plan for real winter kit — an insulated coat, warm hat, and gloves rather than a light jacket.