By Ian Werther in News — 19 Mar 2026

Vernal equinox brings rapid warming in South Korea, with fog and weekend rain.

Tomorrow marks the vernal equinox, the moment when day and night are roughly equal. In South Korea, forecasters say the transition will bring more pronounced daytime warming as a high-pressure system continues to dominate the weather pattern.

Daytime temperatures are expected to rise quickly, bringing largely sunny skies for the near term. However, as the sun goes down, temperatures will drop rapidly, with diurnal swings of up to about 20 degrees Celsius possible.

Clear, calm conditions mean fog can form after sunset and linger into the morning. In particular, dense fog with visibility under 200 meters is forecast for Gyeonggi Province and northern South Chungcheong Province through tomorrow morning, with fog also affecting other inland areas around the capital region.

There are two times each year, in March and September, when the amount of daylight and darkness is “nearly” equal at all latitudes. This phenomenon, commonly referred to as an equinox, happens when the Earth’s axis is tilted neither toward nor away from the sun. During an equinox, the sun is directly over the equator at noon.
“The ‘nearly’ equal hours of day and night is due to refraction of sunlight, or a bending of the light's rays that causes the sun to appear above the horizon when the actual position of the sun is below the horizon,” according to the National Weather Service.
On Wednesday, March 20, 2019, at 5:58 p.m. ET the Earth will be at its equinox, marking the first astronomical day of spring. This differs from meteorological spring, which began on March 1.
So what does all of this mean for you? Well, if you live in the Northern Hemisphere, this means it’s time to soak up the sun because the days will be longer at higher latitudes!
(Note: This GOES East image was captured on March 20, 2019, at 8 a.m. ET prior to the equinox)

Terms of use: Please credit NOAA — Representative image for context; not directly related to the specific event in this article. License: Public domain. Source: Wikimedia Commons.

Satellite imagery suggests the sky will stay mostly clear, as high-altitude clouds thin and shift with the prevailing high-pressure air mass. The air quality is expected to be satisfactory across the country.

Morning temperatures will be chilly in parts of the country: Seoul around 2°C and Chuncheon around -3°C, with many locations staying below freezing. By daytime, highs should reach about 14°C in Seoul and 17°C in Daegu, offering a welcome warmth for spring activities.

The hiker stands as a back figure in the center of the composition. He looks down on an almost impenetrable sea of fog in the midst of a rocky landscape - a metaphor for life as an ominous journey into the unknown. — Representative image for context; not directly related to the specific event in this article. License: Public domain. Source: Wikimedia Commons.

Looking ahead to the weekend, Saturday is forecast to be mostly sunny, while Sunday will bring more cloud cover and rain is expected to reach Jeju Island in the afternoon.

Why this matters beyond Korea: the transitional weather pattern can influence energy demand, as warmer days reduce heating needs but cooler nights raise it again, affecting electricity usage and potentially grid operations. Fog and low visibility pose travel disruptions that ripple through air and road transport, with implications for regional logistics and supply chains tied to South Korea’s electronics, automotive, and consumer goods sectors. Jeju’s rain on Sunday could impact tourism and regional shipping and travel. For U.S. readers, these patterns reflect broader East Asian climate dynamics that shape global markets, manufacturing schedules, and cross-border trade disruptions.

Context for non-Korean readers: the forecast comes from South Korea’s weather services, with local forecasters such as Nam Yu-jin providing daily updates. Major cities cited include Seoul, Chuncheon in Gangwon Province, Daegu in the southeast, and Jeju Island off the southern coast. The vernal equinox is a traditional marker of spring in the region, often followed by rapid changes in temperature as cold air yields to increasingly sunny days.

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