What is fog?

Now fog can be divided into several color grades, from yellow, orange and red. When these warning signals appear, their meanings are different. In terms of visibility, the yellow warning signal is 12 hours, or the fog will not dissipate in the future 12 hours. Orange means coming in six hours, and the child will not dissipate for the next six hours. Red is very close, that is, two hours. Judging from the standard of visibility, when the yellow warning appears, it means that the visibility due to fog will be around 200-500 meters in the future. Within the visibility range of 200-50 meters, the warning signal is orange. When the visibility is less than 50 meters, the warning signal is red.