Hamdeok Beach in the southern resort island of Jeju is crowded with people, Sunday. Yonhap

Korea is forecast to see its hottest summer on record as the Earth has just experienced its warmest day in recent history, the national weather agency said Sunday.

The Korea Meteorological Administration (KMA) issued heat wave alerts and advisories for most parts of the country on Sunday, except for some mountainous areas of Gangwon Province and Jeju Island.

A heat wave warning is issued if the daily maximum temperature of 35 degrees Celsius or higher lasts for more than two days. It is also issued when there is a concern that a prolonged heat wave will cause significant damage to a large number of areas.

Seoul has been under heat wave alerts since Thursday. Its lowest temperature between Saturday night and Sunday morning was 27.3 degrees.

The KMA said the heat wave is continuing as a high temperature and dry high pressure — originating from Tibet — are located in the upper atmosphere of Korea, while the North Pacific High and typhoons that landed in southern China affect the middle and lower layers together, blowing hot and humid air.

Korea is seeing an increased number of tropical nights nationwide under these weather conditions.

A tropical night is defined as a night when the temperature remains above 25 degrees Celsius between 6 p.m. and 9 a.m. the next day.

Korea experienced an average of 4.9 days of tropical nights this year as of Thursday.

The figure is greater than 1.8 days, the average between 1991 and 2020 until July 25, and even higher than 4.1 days posted in 2018 when the nation suffered record heat.

According to the Korea Disease Control and Prevention Agency (KDCA), the country had 856 cases of heat-related illnesses from May 20 to July 26, including 253 between July 22 and 26.

The KDCA is on high alert as the number of heat-related illness cases could go beyond the figure registered in 2018 when 4,526 people fell victim to scorching heat, with 48 of them dying.

"People are advised to refrain from going outdoors between 10 a.m. and 4 p.m. on the days when heat wave warnings and alerts are issued and drink water often," the KDCA said in a statement.

Meanwhile, the Copernicus Climate Change Service (C3S), the EU's climate monitoring organization, said the average temperature of the world's surface reached 17.15 degrees Celsius on July 22.

The figure is the highest since the C3S began climate observations in 1940. It broke the previous record, which was set only a day before, at 17.08 degrees.