fog noun definition in Sky topic from the Oxford Advanced Learner's Dictionary

fog

 noun: Sky topic
a thick cloud of very small drops of water in the air close to the land or sea, that is very difficult to see through Dense/thick fog is affecting roads in the north and visibility is poor. freezing fog Patches of fog will clear by mid-morning. We get heavy fogs on this coast in winter. The town was covered in a thick blanket of fog. The fog finally lifted (= disappeared).