Though May usually signals the start of warmer weather in Seoul, it’s not uncommon for chilly days to linger. This can be attributed to a few key factors:

1. Cold Air from the North: Occasionally, cold air masses from Siberia or northern China move down, bringing a dip in temperatures, especially at night.

2. Delayed Seasonal Transition: Spring in Korea can be brief and unpredictable. A warm spell may be followed by a sudden cold snap as the atmosphere adjusts between seasons.

3. East Sea Influence: Coastal winds from the East Sea (Sea of Japan) can also bring cooler air inland, especially during early spring.

While the days are getting longer and sunnier, it's wise to keep a jacket handy until late May or early June. Warmer days are just around the corner!