Why are autumn mornings chilly but afternoons warm?

According to a Korea Meteorological Administration analysis of 1991–2020 observations in the Seoul metropolitan area, the average daily temperature range was 7.7°C in August, 9.4°C in September, and 11.1°C in October, growing wider as autumn deepens. Days with a range of 10°C or more averaged 19.4 in October, or about two days out of every three.

The gap varies a lot even within the same region. From September to November, inland Yangpyeong and Icheon each averaged more than 60 days with a range of 10°C or more, while coastal Incheon had only 18.8.

The ground warms up in the sunshine during the day, and after sunset it cools by giving that heat off as invisible infrared light. This is called radiative cooling. Clouds and water vapor catch this heat and send some of it back toward the ground, so on cloudy or humid nights the temperature drops less.

In autumn, migrating high-pressure systems bring many cloudless days, and the air is drier than in summer. After the autumn equinox, nights also grow longer, giving the heat more time to escape. So, contrary to a common belief, a clear night full of bright stars tends to be colder than a cloudy one.

When the wind blows, it mixes the cold air near the ground with the air above it, which keeps the temperature from plunging overnight. That is why the calmer the night, the lower the dawn temperature tends to be.

By the sea, the air holds more water vapor, and seawater warms and cools slowly, so temperatures change gently. In contrast, in inland basins surrounded by mountains, air that has cooled and grown heavy overnight collects in low ground, making mornings even chillier.

Midnight might seem like the coldest time, but the ground keeps losing heat until the sun shines on it again. So the day's low usually comes just before or right around sunrise, and the warmest time is not noon but around 2 to 3 p.m.

A jacket you put on in the morning can feel too warm by afternoon, so thin layers that are easy to take off and carry work best. Check both the low and the high in the forecast, and take extra care on days when the gap is close to 10°C.