Visibility refers to the maximum distance that a person with normal vision can recognize a target from the background.That is, in the daytime, the sky close to the horizon is used as the background to clearly see the outline of the ground gray target with a visual angle greater than 20 degrees and identify what it is. At night, the luminous points of the target light can be clearly seen.The unit is m (meter).The visibility is mainly determined by two factors: ① the brightness difference between the target and its background.The greater (smaller) the difference, the greater (smaller) the visibility distance.However, the brightness difference usually does not change much. ②Atmospheric transparency.The gas layer between the observer and the target can reduce the aforementioned brightness difference.The worse (better) the atmospheric transparency, the smaller (larger) the visibility distance.Therefore, the change of visibility mainly depends on the quality of atmospheric transparency.Weather phenomena such as fog, smoke, sand dust, heavy snow and drizzle can make the atmosphere turbid and less transparent.According to the needs of aviation, visibility can be divided into two categories: ① ground visibility, which reflects the horizontal visibility distance near the ground.The visibility in different directions is often different.Typical values of visibility are reported by general meteorological stations“Effective visibility”(The visibility distance that can be reached in more than half of the field of vision of the station).During navigation activities, the visibility on the runway and the minimum visibility in a certain direction are mainly mastered. ②Aerial visibility is also called "flight visibility".Visibility is one of the conditions that determine the opening or closing of an airport, and it is also one of the bases that determine the aircraft to operate according to visual flight rules or instrument flight rules during takeoff and landing.Low visibility makes visual flight difficult and is a meteorological condition that affects safe takeoff and landing.[1]
It refers to the maximum horizontal distance that a person with normal vision can see and recognize the outline of the target (black, moderate size) from the sky background under the weather conditions at that time;At night, it is the maximum horizontal distance of the luminous points that can see and determine a certain intensity of light.The unit is expressed in meters or kilometers.Visibility is a weather indicator to understand the stability of the atmosphere and the vertical structure, and is an extremely important factor to protect the safety of transportation.[2]
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The visibility is mainly determined by two factors: ① the brightness difference between the target and its background.The greater (smaller) the difference, the greater (smaller) the visibility distance.However, the brightness difference usually does not change much. ②Atmospheric transparency.The gas layer between the observer and the target can reduce the aforementioned brightness difference.The worse (better) the atmospheric transparency, the smaller (larger) the visibility distance.Therefore, the change of visibility mainly depends on the quality of atmospheric transparency.Weather phenomena such as fog, smoke, sand dust, heavy snow and drizzle can make the atmosphere turbid and less transparent.
According to the needs of aviation, visibility can be divided into two categories: ① ground visibility, which reflects the horizontal visibility distance near the ground.The visibility in different directions is often different.Typical values of visibility are reported by general meteorological stations“Effective visibility”(The visibility distance that can be reached in more than half of the field of vision of the station).During navigation activities, the visibility on the runway and the minimum visibility in a certain direction are mainly mastered. ②Aerial visibility is also called "flight visibility".Visibility is one of the conditions that determine the opening or closing of an airport, and it is also one of the bases that determine the aircraft to operate according to visual flight rules or instrument flight rules during takeoff and landing.Low visibility makes visual flight difficult and is a meteorological condition that affects safe takeoff and landing.[3]
Ground visibility refers to the maximum distance that can distinguish the contour of the ground gray target with a visual angle greater than 20 degrees in the daytime with the sky close to the horizon as the background.For example, if an object with a width of 58 meters and a distance of 10 kilometers from the eye point has a visual angle of 20 degrees, and a person with normal vision can just see it with the naked eye, then the visibility is 10 kilometers.Aerial visibilityIt refers to the visibility of ground or air targets observed through the cockpit glass when flying in the air.
b. Use 1000 on a matte backgroundCandela The maximum distance that the left and right lights can see and recognize.
(2) Effective visibility: refers to the maximum horizontal distance that can be reached within more than half of the field of vision around the observation point.CAAC observation and reportEffective visibility。
(3) Dominant visibility: refers to the maximum horizontal distance that can be reached within half or more of the visual field around the observation point.
(4) Runway visibility: refers to the maximum distance from one end of the runway along the runway direction to identify the runway itself or the target approaching the runway (designated runway edge lights at night).
(5) Vertical visibility: refers to the vertical visual range in turbid coal.
(6) Inclined visibility: refers to the maximum distance that can be recognized when observing the obvious objects on the ground that are not covered by clouds from the cockpit of the aircraft in flight (the specified lights are at night).Visibility when viewed from above the ground syncline is also calledInclined visibility。
(7) Minimum visibility: refers to the minimum visibility distance when the visibility varies from direction to direction.
(8) Runway visual range: at the aircraft's touchdown point, from the average height of the pilot's eyes, along the takeoff line and the landing direction, the farthest distance of the runway and runway sign lights and markers can be seen clearly.
Observation method
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The atmospheric visibility can be measured by visual inspection, atmospheric transmission instrument, laser visibility automatic measuring instrument, etcMeasuring instrumentMeasurement.At present, the visibility observation is mainly based on manual visual observation, which is relatively poor in standardization and objectivity.The atmospheric transmission meter is used to measure the gas column directly through the beam passing through the atmospheric column between two fixed pointstransmissivityIn this way, the visibility value can be calculated. This method requires the beam to pass through a sufficiently long atmospheric column. The reliability of the measurement is affected by the working stability of the light source and other hardware systems. Generally, it is only applicable to the observation of visibility below the medium level. In low visibility weather such as rain and fog, large errors will be caused due to complex conditions such as water vapor absorption.The laser visibility automatic measuring instrument is based on laser measurementAtmospheric extinctionThe method of coefficient to calculate visibility is relatively objective and accurate, but this instrument is expensive, expensive to maintain, complex to operate, and difficult to carry out normal observation in rainy and foggy days, so it is difficult to promote.University of Science and Technology of ChinaThe instrument system for automatically measuring visibility by digital camera method is developedvideo cameraDirectly take the image of the selected target and its background, and then transfer the image from the imageAcquisition cardIt is transmitted to the computer, and the visibility value can be automatically obtained by analyzing and processing the acquired image.The visibility value measured by this method is close to that observed by lidar.General visibility shall be classified as follows:
7. The visibility is less than 0.3km. The visibility of heavy fog is extremely poor
8. The visibility is less than 0.1km, and the visibility of dense fog is extremely poor
9 Visibility less than 100m is generally considered as zero
environmental effect
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climatic conditions
visibility
The visibility can reach 70-100km in the Arctic or mountainous areas with particularly clean air. However, the visibility is usually due toair pollutionAnd moisture.Climate conditions affecting visibility include haze (dry), fog (wet), rainfall, snowfall, etc.
When haze weather occurs, a large number of extremely fine dry dust particles float in the air, making the atmosphere turbid and the visibility less than 10km.It makes bright objects in the distance slightly yellow and red, and dark objects slightly blue.According to different types of dust particles, it can be divided into dust haze, sand haze, salt haze, etc[4]。Haze seriously affects people's horizontal visibility, thus affecting transportation.
The research shows that the main reason for the reduction of atmospheric visibility caused by environmental fog is the reduction of visibility due to the scattering and absorption of light by fog droplets[5]。The visibility in fog is determined by the number density of droplets and the water content of fog.
Various particle pairs in the atmosphereAtmospheric Extinction Coefficient And visibility. In addition to aerosol particles, the impact of precipitation particles on visibility cannot be ignored. Due to the characteristics of precipitation particles, weather conditions and other factors, the relationship between visibility and precipitation intensity is not unique.Rainfall and snowfall have different impacts on visibility. In contrast, the impact of rainfall on visibility is relatively easy to determine, while the impact of snowfall on visibility is relatively complex, mainly because the types of snowflakes or ice crystals are complex and changeable, and have different degrees of impact on atmospheric extinction coefficient.[6]
Aviation Applications
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Visibility is closely related to flight activities. Poor visibility is a serious visual range obstacle in flight activities, which directly causes difficulties in visual flight and even endangers flight safety.The visibility predicted by the air force meteorological observatory is usually 1, 2, 4, 6, 8, 10 and more than 10 km.When the visibility is below 4km, it is called complex weather;When the visibility is greater than 10km, it is considered that the visibility is good.Visibility is also the most critical determinant for the opening or closing of the airport.Airports with poor ground navigation equipment will be closed when the visibility is less than 0.8km.the international civil aviation organizationAccording to the navigation equipment of the airport, three types of minimum standards for aircraft entering the airport for landing are specified, including the minimum standards for non precision straight approach, the minimum standards for visual circling approach, the minimum standards for Class I precision approach, the minimum standards for Class II precision approach and the minimum standards for Class III precision approach.At present, the weather standards for precision approach uniformly stipulated by ICAO are as follows:
Class I: runway visual range 800m, decision height 60m;
Class II: runway visual range 400m, decision height 30m;
