Definition of low visibility weather

Meteorological visibility is defined as the maximum horizontal distance that a person with normal vision can see and identify the target (black medium size) from the sky background under the weather conditions at that time. Meteorological visibility is one of the basic items of meteorological element observation, and it is an important issue that directly affects human life, production and transportation.

visibility

The attenuation of incident light by air molecules in the atmosphere is relatively regular. Aerosols in the atmosphere also have a great influence on the propagation of visible light. The distribution of aerosols is complex and changeable, and cloud droplets in the atmosphere will also attenuate and scatter light.

The actual observation shows that in the atmosphere, the number density of aerosol particles and small droplets with radius from tens of microns to tens of microns will drop sharply with the increase of radius. The smaller the radius, the greater the particle number density. These particles have a strong scattering effect on visible light, but also have a complex absorption effect, especially for small particles, which is quite remarkable.

Due to human activities, thermal convection, wind and turbulence, the content of small particles in the lower atmosphere will change greatly. When the content of small particles in the atmosphere is large, the attenuation coefficient of the atmosphere may be one or two orders of magnitude larger than that of "clean" air molecules. At this time, the sky will be cloudy, and we can't see the distant objects clearly. This is the visibility problem.

Visible distance

We can always see the object clearly when we are close, but when the object is gradually away from us, it becomes blurred. It is not until the object reaches a certain distance that we can identify the object from the background with the naked eye. The maximum distance we can "see" an object is called the visible distance.