According to the World Meteorological Organization (WMO) observation guidelines, atmospheric visibility is a physical quantity of atmospheric transparency (or turbidity), which described by distance from the point of view to the target object, which can be seen and identified.
Meteorological Visibility at Day refers to the maximum distance, that a suitable scale black object can be seen and identified , when observed against scattered light background such as fog.
Meteorological Visibility at Night refers to the maximum distance from which an appropriate scale of black object can be seen and recognized, or a object can be seen and recognized from medium intensity luminous.
Assuming that the overall illumination increases to the normal daytime level. The latter definition is more in line with the actual operation of night visibility measurement.
It must be emphasized that the criterion used is to identify the target, not just to see the target.
Visibility Sensor Principle
Here we will show you the principles.
How is visibility measured in weather?
Koschm ieder put forward the basic theory of visibility measurement in 1924. Assuming that the atmosphere is uniform, that is, the horizontal extinction coefficient of the atmosphere is constant, the law of variation of atmospheric transmittance and brightness contrast between the object and the background with distance is Koschm ieder’s law. According to Bouguer Lambert’s law,
That is to say, the basic formula of atmospheric horizontal visibility is derived.
This is the original formula of visibility measurement. It can be seen that the core problem of measuring visibility is how to accurately detect the extinction coefficient of the atmosphere.
In 1876, Allard proposed the law of attenuation of light emitted from a point source of known intensity, thus supplementing the measurement of night visibility and RVR.
Visibility Sensor Measuring Method
(1) Transmission method
The transmission method is to measure the transmittance or extinction coefficient of a long air column (such as transmitter, telephotometer, etc.).
(2) There are two kinds of scattering methods.
Scattering method is to measure the light scattering of a small volume of air. According to the measured scattering angle, it can be divided into forward scatterer, backward scatterer and total scatterer.
Strictly speaking, there is no true total scatterer, but the angle range of its measurement is very wide. Because of its poor measurement accuracy and insensitive response, it is seldom used in practice.
Visibility Sensor Measuring Instrument
Transmission visibility meter and scattering visibility sensors are widely used.
Transmittance visibility sensor, also known as transmissivity sensor, its working principle is based on the transmittance measurement of light radiation in atmospheric transmission.
The transmitter is generally divided into two types: double-ended Type and single-ended Type. Single-end transmitter is to install the transmitter and receiver at the same end of the baseline. Its principle is the same as that of double-end transmitter. Only the reflector is needed at the other end of the baseline, which makes the measuring optical path turn to round-trip optical path.
Many airports adopt double baseline system, i. e. two receivers, which are used to measure low visibility and high visibility respectively.
The working principle of the Scattering Visibility sensor: The transmitter emits infrared radiation and irradiates the adjacent atmospheric sample, The suspended particles in the sample scatter in all directions against the radiation.
The receiver detects the scattering signal in one direction, and determines the total scattering coefficient, according to the relationship between the scattering signal and the total scattering quantity at a specific angle, and then obtains the extinction coefficient.
Finally, the visibility is inverted according to the formula.
What Visibility Sensor Can Measure
Horizontal Visibility Monitoring
The horizontal visibility along the horizon is monitored mainly by forward scattering visibility sensor.
Vertical Visibility Monitoring
Visibility values perpendicular to the horizon are monitored mainly by a forward laser transmission visibility sensor, also known as a cloud altimeter.
Misunderstanding of Visibility Sensor
Is the monitoring distance of the visibility sensor true?
Of course not, because the main principle of visibility sensor is forward scattering method, and its monitoring area is small, the premise of its monitoring is to follow the assumption of scattering isotropy, that is, to simulate weather conditions within the monitoring range is “uniform and consistent”. However, in actual weather conditions, the irregularity of aerosol particles can not be ignored. This will introduce an error source to the monitoring error. Think about it. The monitoring distance is 30Km. If the small instrument can know visibility information beyond 30Km, unless they have special functions.
Transmission Visibility Sensor
Because the transmission visibility is separated by transceiver and receiver, can the actual visibility be measured?
In principle, this is the case, but in fact, the transmission visibility sensor is mainly used on both sides of the airport runway, and the monitoring distance is limited, but the measured value is the real visibility value, rather than through the algorithm simulation conversion. But in the real monitoring environment, it is difficult to find two points at the full level, and at the same time, to get the visibility, because it requires a lot of power consumption.
Visibility affects all forms of traffic: roads, sailing and aviation. Meteorological visibility refers to transparency of air: in dark, meteorological visibility is still the same as in daylight for the same air.
Visibility: The Seeing of Near and Distant Landscape Features reviews the science of visibility from how to measure it quantitatively to its impacts by one of the foremost experts in the field. Carefully designed pedagogy allows a diversity of readers, from regulators to researchers to use this book to further their understanding of the field.
This topical volume of the Journal of Pure and Applied Geophysics utilizes new information not previously accessible for fog related research. It focuses on surface and remote sensing observations of fog, various numerical model applications using new parameterizations, fog climatology, and new statistical methods. The results presented in this special issue come from research efforts in North America and Europe.
Visibility sensor is an instrument that provides real-time visibility data output by means of instrument simulating human eyes.
What Is The Monitoring Distance of Visibility Sensor?
In fact, the monitoring distance is not the actual distance, but the ideal value calculated by the model.
It refers to the distance values that can be detected under the same meteorological conditions, especially in mountainous areas where the fog is obvious, and the measured values can not represent the actual visibility of the local area.