Visibility Training

8/11/2019 Visibility Training

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Visibility andVisibility Measurement



Visibility

Definition: “the greatest distance at which selected objectscan be seen and identified”

human observation

• depends on

• the objects (size, colour, shape, …)

• background of the objects (brightness, colour, …)

• the observer (eyesight, alertness, …)

• the atmosphere (weather)



Uses of visibility information

• Operational use (“how far would I be able to see”)

• traffic

• air

• road

• sea / lake / river

•

other• recreation (?)

• Air mass characteristics (“what is the optical state of the

atmosphere”)• synoptic meteorology

• climatology

• telecommunication...



Standards and recommendations

WMO:

• standardization of internationally distributed weather

information• observation methods, measurements (accuracy, range),

reporting

• synoptic meteorology, climatology, aviation (see ICAObelow)

ICAO:

• observing and reporting practices specific to the needsof international aviation

•

all weather message and observation matters coordinatedwith WMO, ICAO does not act as an expert on meteorology

• some practical aspects defined by ICAO only (measurementsites, frangibility, …)

• Runway Visual Range is specific to aviation, ICAO has an

active role in RVR recommendations



Standards and recommendations

FAA:

• standards for some aviation related weather

observations & equipment in the USA• airport automatic weather stations (AWOS), RVR in the USA

• has no official status internationally, but is influentialdue to the role of a forerunner (and general US influence)

NOTE:

• WMO and ICAO do not have any executive power, theyonly issue recommendations and standards

• national organizations are responsible for the implementation• local rules and specifications take precedence

• WMO & ICAO do not issue any approvals for instruments

• no international standards exist for any other

applications than meteorology and aviation!



Visibility observations

• WMO recommends observing large black (dark) objectsagainst sky, e.g. mountains or tall buildings

• when the objects and background are always similar theobservations will be more consistent

• At night the observed objects have to be light sources(lamps)

• e.g. flight warning lights of towers or tall buildings



Day visibility

• Visibility during day is limited by the contrast between the

object and the background• contrast decreases when the distance to the objects gets

longer

• think about a foggy day: if you try to see objects which

are quite far away, you will only see uniform gray fog -the contrast of the objects has decreased so low thatthey can not be distinguished from the fog

• The formula for contrast visibility is called Koschmieder’s

law



Night visibility

• Night visibility is limited by the intensity of the light whichreaches the eye of the observer

• light intensity is attenuated by the atmosphere (fog,snow, …)

• If the intensity is too low, the observer’s eye will not detectthe light

• eyes adapt to darkness, the detection threshold dependson the general level of illumination (i.e. how dark thenight is)

• The formula for the visibility of light sources is calledAllard´s law



Problems of visibility observations

• Application: operational visibility (“how far would I be ableto see?”):

•

the real experienced visibility depends on the observer,the target and the background• consider e.g. a car driver: the “official” visibility observations

are made by a another person in a different place with adifferent target

• Application: optical properties of the atmosphere

• observed visibility is subjective, while the optical state ofthe atmosphere should be an objective, physical

quantity• night visibility is not well defined (or the definition is not

generally accepted)

• visibility observations are dependent on background

luminance (night / twilight / day)



Solution (?): MOR

• Meteorological Optical Range (MOR) has been defined bythe WMO as the basic parameter to express the optical stateof the atmosphere

• MOR can (and should) be used instead of human visibilityobservations (WMO recommendation)

• MOR corresponds closely with human visibilityobservations (but only day observations)

• definition based on contrast visibility (Koschmieder’s law)

• The word “visibility” is often used when talking about MOR

• e.g. FD12 Visibility Meter is actually a MOR meter...

• MOR has been defined as a purely physical quantity

• objective, can be measured

• Other, application specific visibility values can be derivedfrom MOR

• e.g. RVR



Definition of MOR

• MOR can be measured by measuring the attenuation of light

• attenuation is mainly caused by scattering, to a small

degree also by absorption (in smoke, dust, …)

100 %

intensity

5 %

intensity

MOR

MOR is defined as the distance where the intensity of a lightbeam has been attenuated to 5% of the original intensity



Runway Visual Range

• ICAO definition:

• “The range over which the pilot of an aircraft on thecenter of the runway can see the runway surface

markings or the lights delineating the runway oridentifying its center line”

• how far the pilot can see the runway lights or the runwaysurface when the aircraft is about to touch down?

• RVR can be a human observation (at CAT I airports), butinstruments are strongly recommended

• instrument RVR (IRVR) is required in many countries at

CAT I airports as well• Annex 3 wording has changed into a clearly stronger

recommendation in the latest edition

• human RVR can not meet e.g. the requirements for

updating frequency nor consistency



RVR system

• RVR is either the visibility of the runway surface (contrastvisibility = MOR) or the visibility of the runway lights

• RVR system calculates both and reports the highervisibility value as RVR

• MOR is used as the runway surface visibility

• MOR (visibility) sensor is needed

• Runway light visibility depends on:

•

runway light intensity• attenuation of light in the air (MOR)

• illumination threshold = human eye detection threshold(depends on the darkness of the night)



RVR system

• Runway light intensities are stored in the RVR computer

• the values stored are the nominal (100%) values

•

the air traffic controller may use lower than 100%intensities, a special device (RSI) is needed to monitor thesetting in use

• Attenuation of light is measured by a MOR (visibility) sensor

•

transmissometer of forward scatter sensor• The illumination threshold is estimated from the darkness ofthe night, which is measured by a background luminancesensor

• RVR computer calculates the RVR from the measured values(MOR and background luminance) and the stored lightintensity table



Standard visibility measurements - 1

WMO - synoptic meteorology

• the reported value should be MOR (day and night)

•

reporting range in SYNOP: below 50 m to over 70 km• not always covered completely by instruments or human

observations

• accuracy:

• requirement: +/-50m below 500m, +/-10% above 500m• “typical instrument accuracy”: +/-10 - 20%

• measurement height not specified, should berepresentative

• e.g. human eye height

• other heights may be needed due to environmentalreasons, e.g. heavy snow conditions




ICAO - airport observations

• the reported value should be MOR

•

range: below 50 m - over 10 km• accuracy:

• “desirable”: +/-50 m up to 600 m, +/-10% between 600 mand 1500 m, +/-20% above 1500 m

• “attainable”: +/-50 m up to 500 m, +/-10% between 500 mand 2000 m, +/-20% above 2000 m

• measurement height not specified, should berepresentative




ICAO - RVR

• range: below 50 m - 1500 m

•

locally in some countries RVR is measured up to 2000 m• accuracy:

• “desirable”: +/-10 m up to 400 m, +/-25 m between 400 mand 800 m, +/-10% above 800 m

• “attainable”: +/-25 m up to 150 m, +/-50 m between 150 mand 500 m, +/-10% above 500 m and up to 2000m

• measurement height not exactly specified, should berepresentative of pilot’s visibility from 5 meters height

• 2.5 m height often used (pilot at 5 m is viewing runwaylights on ground)



Measuring MOR

• Transmissometer: a direct measurement of the attenuation

• Transmissometer estimates the distance where the intensitywould reach 5% of the original value

100%

intensity

? %

intensity



Measuring MOR

• Scatter sensor: an indirect measurement of the attenuation

• The scatter sensor measures a part of the scattered light andestimates the amount of all scattered light -> attenuation

• the estimate can be accurate, if the weather type is known

• the accuracy of a scatter sensor may depend on weathertype



Transmissometer versus forward scatter

TRANSMISSOMETER:

• measurement principle the same as human vision

•

accurate in all weather• easy to calibrate

• limited dynamic range (2 decades)

• fail-safe: loss of signal corresponds to low visibility

• higher cost: two units (Tx and Rx)

• more difficult installation (heavy foundation, alignment)

• typically needs more maintenance (MITRAS not

necessarily)• larger sample volume - more representative and safer

measurement



Transmissometer versus forward scatter

FORWARD SCATTER:

• measures visibility indirectly

•

needs corrections for linearity & precipitation• needs chain of calibrators and standards

• critical to manufacturing and calibration repeatability andtolerances

• large measurement range (3...4 decades)

• fail-unsafe: loss of signal corresponds to high visibility

• lower cost: single unit

• easy installation

• less maintenance

• small sample volume - less representative measurement

Visibility Training

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