How to distinguish foggy days from cloudy days

From the definition of meteorology, cloudy days and foggy days are still very different. First of all, the key to determining a cloudy day is to look at the cloud cover. The sky is basically covered by clouds, and it is cloudy without sunshine. The judgment of fog and haze is mainly from the perspective of visibility, and the two are not the same thing. Fog is a water vapor condensate composed of a large number of tiny water droplets or ice crystals suspended in the air near the ground, which is often milky white, making the horizontal visibility less than1km; Haze refers to the phenomenon that a large number of extremely fine particles float evenly in the air, making the horizontal visibility less than 10 km. These particles mainly come from nature and human activities. In addition, cloudy days mainly refer to high altitude, and fog-haze days refer to the situation near the ground.

"In fact, fog is not easy to appear on cloudy days." In particular, the radiation fog we often say is formed by the radiation cooling of the surface at night, which often occurs at night and in the morning when the sun is shining, the breeze is blowing gently, the water vapor near the surface is rich and stable, or there is an inversion. However, the situation of smog is slightly different, because the weather conditions when smog occurs are characterized by stable air mass, and cloudy days and haze days may cross, that is, sometimes the public will think that cloudy days and fog-haze are confused. This is because when smog appears, the visibility near the ground decreases, and people's vision looks gloomy and the illumination is poor, which is similar to the impression given by cloudy days.

In addition, the blue sky does not necessarily appear in sunny conditions, but is caused by physical effects such as absorption, reflection and scattering of light by air and particles in the air. Therefore, when the sky is not blue, it is not necessarily cloudy or foggy. We should treat fog and haze more rationally and objectively and take corresponding measures to deal with it, instead of just "smelling it".