Snow, hail in April — what’s driving the chill?

After nationwide warm spring temperatures accompanied by blooming flowers, South Korea saw unseasonably cold temperatures from Saturday until Monday, bringing strong gusts of wind, rain and even snow to many parts of the country.

The cold weather continued into Tuesday, accompanied by strong winds.

Over the weekend, eastern parts of Gyeonggi Province, as well as inland regions and mountainous regions in Gangwon Province, saw accumulated snowfall of up to 5 centimeters. As strong wind advisories were issued in Jeju Island, flights were canceled, and up to 52 safety incidents, such as fallen traffic signs and exterior walls of buildings as well as broken windows, were reported to fire authorities as of Monday.

Nationwide high temperatures through Monday only reached 8 degrees Celsius, an 18 C difference in average temperatures compared to temperatures recorded just the week before by the Korea Meteorological Administration.

According to the state weather agency on Tuesday, the sudden drop in temperatures and extreme weather conditions over the weekend were caused by a cutoff low-pressure system situated in the upper atmospheric layer.

A cutoff low-pressure system forms when a jet stream -- a band of strong winds in the upper atmosphere -- flows in a highly wavy pattern. As this type of low-pressure system traps extremely cold air below 30 C coming in from the Arctic, it destabilizes the atmosphere, making temperatures between the upper and lower layers of the atmosphere differ by 40 C to 50 C.

Other than extreme temperatures, the destabilization in the atmosphere causes strong wind gusts as well as rain with thunder and lightning.

The cutoff low is also often preceded by a phenomenon known as atmospheric blocking, which occurs when the low and high pressure systems become stationary in the upper atmosphere, weakening the normal west-to-east flow of weather systems. Atmospheric blocking was also behind Korea’s unusually long cold waves this winter. Many parts of the country saw cold waves for as long as eight consecutive days -- a deviation from the usual cold wave cycle of three days followed by four relatively warmer days.

Though cutoff lows and atmospheric blocking are natural atmospheric processes that occur regardless of the season, some meteorologists were quoted in local media reports saying that blocking events may occur more frequently than they did in the past due to climate change.

With the influence of the high-pressure system passing along the southern coast of Jeju Island, Korea saw clear skies Tuesday. However, average temperatures remained 2 C to 8 C lower than the previous year’s temperatures, with wind gusts of up to 55 kilometers per hour.

As the warm southwesterly winds begin to flow into the Korean Peninsula along the high-pressure system, temperatures will return to previous years’ averages from Wednesday, with low temperatures expected to range between 1 C and 10 C and high temperatures expected to range between 18 C and 26 C.