YINCHUAN - Sunday morning brought heavy fog to the city of Yinchuan, the capital of Northwest China's Ningxia Hui autonomous region, stranding over 1,500 passengers after flights were delayed due to low visibility.

The thick fog reduced visibility to just 100 meters at the Hedong Airport, far below the 800-meter minimum needed for planes to safely take off and land, said a spokesman with the Ningxia Airport Company.

Visibility in some parts of the city was lowered to 50 meters, according to Yinchuan's meteorological observatory, which issued a fog warning Sunday morning.

Police closed five highways in the area Sunday morning and are restricting traffic on other highways in the province, as visibility has been reduced to as low as 5 meters in some areas, according to Ningxia's traffic police department.