This morning, January 23, the temperature at Mau Son dropped to minus five degrees centigrade.
Hoang Quoc Huy, head of Mau Son Meteorology Station in Loc Binh District, said the frost on the mountain will last for several days.
Many tourists are flocking to visit the mountain.
Due to the thick frost, the road to Mau Son Mountain has become more slippery, but this has not discouraged visitors.
According to Tran Son Hai from Loc Binh District, the frost began at Mau Son on Monday afternoon.
The peak of Mau Son Mountain is 1,180 metres above sea level. It is some 30 kilometres East of Lang Son City.