How far can a healthy person with normal vision see? The data shows that in clear weather, the atmospheric visibility is 20 kilometers to 30 kilometers. In the polar regions where the air is very clean, humans can see things 70 kilometers away. However, in daily life, people rarely feel that they have such good eyesight. This is related to the current weather conditions and air quality. The higher the air quality, people can see farther away, and the lower the air quality, people can’t see distant objects clearly.
What is Atmospheric Visibility?
Visibility refers to the maximum distance at which a person with normal vision can clearly see the appearance of the target object in the sky background during the day. At night, visibility is the distance at which a person can clearly see a luminous point. Take m (meter) as the unit. Visibility is an important indicator that helps meteorologists understand atmospheric stability and vertical structure.
Visibility depends on the height at which the observer sees the target object. Atmospheric visibility has three types, it is horizontal visibility, squint visibility, and vertical visibility.
What factors affect atmospheric visibility?
1. Fog and vapor
Fog is the most important meteorological factor affecting atmospheric visibility. Diffuse heavy fog has a huge impact on visibility, which can reduce visibility to less than 1 kilometer, while sudden fog can reduce visibility to more than ten meters. Fog is tiny water droplets suspended in the atmosphere close to the ground. These water droplets can gather together to form visible aggregates, thereby reducing atmospheric visibility. Fog is mostly liquid water or ice crystals, which scatter light without affecting the wavelength, so the fog looks milky or bluish-white. Areas with continuous rain or a sudden drop in temperature are often accompanied by foggy weather.
2. Dust
In addition to fog, dusty weather can also seriously affect visibility. Dust and fine sand in the atmosphere can block vision. The stronger the wind, the more dust in the atmosphere. According to different wind levels, there are five types, namely floating dust, blowing sand, sandstorm, strong sandstorm, and extremely strong sandstorm. In floating dust weather, dust and fine sand float evenly in the air, and the horizontal visibility is less than 10 kilometers. When it develops into a sandstorm, the strong wind blows a large amount of dust and sand on the ground, making the air very cloudy, and the horizontal visibility is less than 1 km. When it became a particularly strong sandstorm, the strong winds would blow up the dust and sand on the ground, making the air particularly cloudy, and the horizontal visibility was even less than 50 meters. Hurricane weather is very likely to cause sandstorms, resulting in reduced atmospheric visibility.
3. Haze
Airborne particles such as dust, sulfuric acid, nitric acid, and organic hydrocarbons can also cloud the air, blurring vision and causing reduced visibility. If the horizontal visibility is less than 10,000 meters, the visual impairment caused by this aerosol system is called haze. Haze often occurs together with fog because it can also cause reduced visibility. Compared with fog that dissipates when exposed to sunlight, haze is less effective in reducing visibility, but it can last a whole day or last for many days without dissipating. Haze scatters more light with longer wavelengths, so it appears yellow or orange-gray.
Fog and dust are natural phenomena, but the haze is usually caused by serious environmental pollution. Haze is mainly aerosol particles, which can appear at any time. The fine aerosols in the haze can directly enter and adhere to the human respiratory tract and lung lobes, affecting human health. Haze is mainly caused by human activities such as industry, power generation, and transportation. Natural haze from dust, wildfires, etc. also occurs in many parks. Environmental protection departments usually install visibility sensors to monitor important roads, thereby reducing casualties.
How do pollutants affect visibility?
Air pollution produces pollutants such as tiny particles and aerosol particles. Aerosol particles suspended in the atmosphere affect solar radiation through scattering and absorption, changing its intensity and transmission direction, so people will see reduced atmospheric visibility.
Particles in the atmosphere absorb and scatter light, affecting atmospheric visibility. Particles scatter light and then absorb some of the light that is about to reach the eye. The more particles in the air, the more light is scattered and absorbed, and the less visibility. Particles containing so4-2 and particles containing NO3- will scatter more light, especially in wet conditions.
Why Atmospheric Visibility is important?
Visibility is very important for road safety. Navigation, aviation, and ground transportation all have strong demands for monitoring atmospheric visibility. The low visibility on the road can easily cause car accidents, and the water vapor conditions on the sea surface are sufficient, so fog is more likely to form, affecting the progress of ships. Learning visibility knowledge and scientific monitoring can avoid loss of life and property.
