Factors affecting atmospheric visibility

Atmospheric visibility refers to the maximum horizontal distance at which an object can be seen and identified against the sky background in a given environment. It is influenced by various factors, mainly including:

1. Aerosol particles and small cloud droplets

Aerosol particles and small cloud droplets have a significant impact on the transmission of visible light. As their radius increases, their number density decreases sharply, while the number density of smaller particles is greater. These particles have a strong scattering effect on visible light, as well as a certain absorption effect, especially for smaller particles, where this absorption effect is more pronounced. Therefore, the content of aerosol particles and small cloud droplets in the atmosphere directly affects the atmospheric attenuation coefficient, which in turn affects atmospheric visibility.

2. Target and Background Brightness Contrast

Whether an object can be seen depends on the brightness contrast between the target and the background, as well as the eye's ability to discern this contrast. Under normal daytime illumination, if the angle of view of the target is not less than a certain size, the contrast sensation threshold for an observer with normal vision is close to 0.02. However, at dawn or dusk, the contrast sensation threshold can reach above 0.5, which is why objects that can be seen during the day may not be visible at dusk.

3. Other Factors

Visibility is also affected by various factors such as the duration of the target object, its position in the field of view, and the physical and mental condition of the person. In meteorological research, to ensure comparability, meteorological visibility is defined as the maximum horizontal distance at which a person with normal vision can see and recognize a target object against the sky background under prevailing weather conditions. Nighttime light visibility refers to the maximum horizontal distance at which a certain intensity of light can be seen and identified.

To sum up, atmospheric visibility is a complex phenomenon influenced by multiple factors. Understanding these influencing factors is of great significance for meteorological observation and forecasting.

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