At around 8 am, thick fog enveloped many streets in both central and suburban areas of Hanoi. Road users reported mist so dense it felt like light drizzle, with visibility reduced to about 100 metres, making travel difficult.
Fog in Hanoi commonly appears during seasonal transitions, especially from winter to spring. During this period, high humidity and sharp drops in temperature overnight and in the early morning cause water vapour to condense into tiny droplets suspended near the ground.
Temperature inversion during the cold season also limits air circulation, trapping moisture and fine dust at lower levels, resulting in thicker and longer lasting fog. Weak winds or calm conditions further allow fog to accumulate for several hours.
An air quality monitoring station on Giai Phong Street recorded an Air Quality Index of 170 on Thursday morning, classified as unhealthy. Another station at 556 Nguyen Van Cu Street reported poor air quality.
According to global air quality monitoring platform IQAir, Hanoi ranked fourth among the world’s most polluted cities on February 5, with an AQI of 193. Delhi, India, topped the list with an index of 225. Several monitoring sites in Hanoi recorded very unhealthy levels, with the Ha Dong area reaching an AQI of 336, considered hazardous.
Foggy conditions combined with air pollution in Hanoi are forecast to persist for about three more days before gradually easing as a new cold air mass moves into northern Vietnam. The Department of Environment forecast poor air quality in Hanoi, Phu Tho and Tuyen Quang today, with conditions expected to improve to moderate levels tomorrow.