Why is there dense fog?

In daily life, we often encounter this phenomenon: water vapor in the air condenses to form tiny water droplets suspended in the air, which reduces the visibility of the ground. This weather phenomenon is called fog in meteorology. Only when the moisture content in the lower air is high and the temperature drops suddenly at night, the moisture will condense into fog. There is a folk proverb: "If the fog is heavy in the morning, the weather will be fine for a long time." "It is foggy in the morning, even though you are drying rice." People often see the saying that fog is "ten fogs and nine sunny days". Fog is actually formed by small water droplets in the air adhering to dust in the air, so the appearance of dense fog means that there is more dust in the air, which is harmful to our human health. For example, fitness in foggy days is not worth the loss. Because pollutants in the air combine with water vapor in the air, it will become difficult to spread and settle, which makes most pollutants gather at the height where people often move. So the air pollution in foggy days is much more serious than usual. Another reason is that the particles that make up the fog core are easy to be inhaled and stay in the human body, and the harm of harmful substances to the human body is further aggravated by inhaling a lot of air when exercising in foggy days. Therefore, people should reduce the time of outdoor activities in foggy days. Even outdoors, they should remember to wear scarves and masks to protect their skin, throat and joints. The elderly, children and other weak people should pay more attention to protection. Fog not only seriously affects our human health, but also affects road traffic safety. Because the visibility is greatly reduced, many vehicles can't be used normally. Such as vehicle speed limit, ship navigation restriction, aircraft grounding and so on. Therefore, fog often brings inconvenience to our daily life. I would like to remind all friends to listen to the weather forecast, know the weather information in time and take preventive measures in advance.