A heat wave warning was issued for all parts of Seoul during the Chuseok holiday Wednesday, marking the latest-ever red alert in September, weather authorities said.

Seoul was put under a heat warning, after a heat advisory was upgraded to a heat warning in the southwest 10 a.m., followed by the rest of the capital city at 2 p.m., according to the Korea Meteorological Administration.

This was the latest heat warning ever issued in Seoul in September since the introduction of the heat warning system in 2008. The previous record was set just last week, on Sept. 10.

A heat wave warning is issued when the highest sensible temperature is expected to reach 35 degrees Celsius or higher for two or more consecutive days or when major damage is anticipated from the heat.

Temperatures in the southern cities of Tongyeong and Gimhae reached 34.6 degrees and 36.9 degrees, respectively, setting new records for September daily highs.

As of 3 p.m., 166 out of the nation's 183 weather watch areas, or 91 percent, were under heat advisories. There were 125 heat warnings and 41 heat watches in effect.

Overnight, many regions experienced tropical nights, which occur when the lowest temperature remains at 25 degrees or higher.

The lowest temperature in Seoul overnight was 26.5 degrees, 9.2 degrees above the average low for the day, setting a new record for the latest tropical night.

Additionally, Incheon, Daejeon and Jeju experienced tropical nights, with the number of tropical nights in Jeju rising to 72 this year.

The heat wave is expected to subside starting Friday, the weather agency said. (Yonhap)