Dense fog advisory in effect until 9 a.m.

A dense fog advisory has been issued until 9 a.m. by the National Weather Service.

The advisory is for most northern Illinois counties, including Cook, DuPage, Kane, Lake and McHenry.

Visibility will be less than a quarter mile, officials said.

Areas of low visibility will cause hazardous driving conditions.

People are urged to reduce speeds when driving, use headlights and leave plenty of distance between you and other cars.