What is the definition of visibility in foggy days?

Atmospheric visibility is an index reflecting atmospheric transparency. It is generally defined as the maximum horizontal distance on the ground where people with normal vision can see the outline of the target clearly under the weather conditions at that time. There is also a definition of the minimum distance at which the last feature of the target disappears. Generally speaking, for the same goal, the visibility determined by these two definitions is different, and the latter is greater than the former. Visibility is a meteorological factor that has an important influence on aviation, navigation, land transportation and military activities. In the aviation field, visibility defined by the former is generally used.

The main factors affecting visibility are atmospheric transparency, light intensity and visual threshold. Atmospheric visibility is closely related to the weather conditions at that time. When rainfall, fog, haze, sandstorm and other weather processes occur, the atmospheric transparency is low, so the visibility is poor.

Atmospheric visibility can generally be measured by visual inspection, and can also be measured by measuring instruments such as atmospheric transmittance instrument and laser visibility automatic measuring instrument. At present, the observation of visibility is mainly based on manual visual inspection, which is relatively poor in standardization and objectivity. The atmospheric transmission instrument directly measures the transmittance of the air column through the light beam passing through the air column between two fixed points, thus calculating the visibility value. This method requires the beam to pass through the air column for a long time, and the reliability of measurement is affected by the stability of hardware systems such as light source. Generally, it is only suitable for the observation of visibility below medium, while in low visibility weather such as rain and fog, due to complex conditions such as water vapor absorption, a large error will be caused. Automatic laser visibility measuring instrument calculates visibility by measuring atmospheric extinction coefficient by laser, which is objective and accurate. However, this kind of instrument is expensive, the maintenance cost is high, the operation is complicated, and it is difficult to observe normally in rainy and foggy days, so it is difficult to popularize. China University of Science and Technology developed an instrument system for automatically measuring visibility with digital camera. The digital camera directly shoots the images of the selected target and its background, and then transmits the images from the image acquisition card to the computer. By analyzing and processing the collected images, the visibility value can be obtained automatically. The visibility measured by this method is close to that measured by lidar.