It’s official: South Korea recorded its hottest September on record this year, with the state weather agency recording the highest average temperature, the most heat wave days and most tropical nights last month.

According to the Korea Meteorological Administration, Korea saw average temperatures reaching up to 24.7 degrees Celsius last month, the highest September average since weather records were first tracked nationwide in 1973. The previous highest September average was just last year at 22.6 C. Only a year later, the monthly average recorded 2.1 C higher.

September’s average daytime highs and morning lows also recorded 29.6 C and 20.9 C, respectively, well above the average temperatures of 27.1 C and 19 C.

Out of the 66 local weather observatories nationwide, 46 of them observed record-breaking daytime temperatures. For example, on Sept. 19, Miryang in South Gyeongsang Province, Jeongeup in North Jeolla Province and Boryeong in South Chungcheong Province all set record-breaking high temperatures, going a little above 37 C. Gwangju also broke its high temperature record on the same day with daytime temperatures reaching as high as 35.8 C.

Korea also saw up to six heat wave days in June, which was 5.8 days more than the number of heat wave days in June recorded on average. Heat wave days in Korea are defined as days with daytime temperatures reaching above 33 C or more.

Seven cities, including Seoul, Icheon in Gyeonggi Province and Seosan in South Chungcheong Province, recorded heat wave phenomena for the first time in September this year. Wando-gun in South Jeolla Province also saw the highest number of September heat wave days, at up to 13 days in total.

Additionally, Korea saw up to 4.3 tropical nights on average in September this year, which was also 4.2 days more than the number of tropical nights recorded on average. Tropical nights occur when temperatures remain above 25 C from 6 p.m. to 9 a.m. the next day.

Jeju Island, which saw the highest number of tropical nights recorded during the summer, also saw the highest number of tropical nights last month, with up to 19 nights observed. This was followed by Busan with 15, Incheon with 10 and Seoul with nine tropical nights. Four other cities, including Chuncheon in Gangwon Province, observed their first September tropical nights on record.

According to the KMA, the intense heat in September was due to increased convective activity in the North Indian Ocean from early to mid-September, resulting in the Tibetan high pressure system developing stronger than usual above the Korean Peninsula.

“As the Tibetan high pressure system enhanced the high pressure flow in the upper atmosphere, Korea saw clearer days with strong sunrays, resulting in high temperatures,” a KMA official told The Korea Herald.

The KMA added that increased convective activity near the Philippines also contributed to the late heat. As low pressure formed over the Southeast Asian country because of the weather phenomenon, this caused the North Pacific high pressure system to develop east of Korea, bringing hot, humid air to the country.

Meanwhile, Korea saw up to 241 millimeters of rain last month, which was 85.9 mm more than the average precipitation from previous years. According to the KMA, September this year was seen as the 12th wettest September on record since 1973.