All northern provinces were advised to brace for more cold weather over five days, as temperatures are dropping with a low pressure zone now centred over China covering the region.
"Cold weather will continue in the North from Dec 18 to Dec 22. After that it will be a bit warmer," Pornnapha Thongthep, of the weather bureau in Chiang Mai, told FM100.5 news programme.
"The region has warmer weather during the daytime, but temperatures drop at night," she asaid.
Temperatures would fall by another 2 deg Celsius over the next five days, she said.
Chiang Rai and Phayao were the coldest provinces on Tuesday with temperatures there plunging to 7 deg Celsius. In Doi Ang Khang, in Chiang Mai, the ground temperature was minus 1 deg Celsius, the department said.
The cold season in the North will stretch into mid-February.
Ms Pornnapha said the weather this year was colder than last year.