Visibility
for
Aeronautical Purposes
Visibility for Aeronautical Purposes
Objectives
• Give a definition of the visibility for aeronautical
purposes and the available means for its
observation
• Recognize the atmospheric causes of reduced
visibility with a special focus on fog
• Establish a link with the flight rules constraints
in term of visibility and ceiling
• Know the criteria and accuracy of the visibility
and ceiling parameters in reports and forecast
information (messages and charts)
Visibility for Aeronautical Purposes
2
Table of Contents 1/2
I - INTRODUCTION
–General Remarks
–Influence on Safety
–Influence on Efficiency
II - WHAT VISIBILITY ?
–Meteorological Requirements
–Meteorological Visibility
–Meteorological Optical Range (MOR)
–Daytime Visibility
–Visibility at Night
–Visibility for Aeronautical Purposes
–Prevailing Visibility
–Problems relating to human observation
–Runway Visual Range RVR
–RVR measurement
–Location of transmissometers
–Vertical Visibility
–Slant and Flight Visibility
III -ATMOSPHERIC TURBIDITY
–Origins of Atmospheric
Turbidity
–Precipitations
–Snow Drift
–Mist
–Fog
–Clouds
–Haze
–Drifting or Blowing Dust or
Sand
–Dust or Sand Storm
–Smoke
IV – FOCUS ON FOG…
–Radiation Fog
–Advection Fog
–Coastal Fog
–Upslope Fog
–Evaporation Fog
–Frontal Fog
Visibility for Aeronautical Purposes
3
Table of Contents 2/2
V - FLIGHT RULES
–VFR Rules
–Visual Meteorological Conditions VMC
–Special VFR Flight
–LVP and LVTO Procedures
–IFR Approach: Operational Minimums
–IFR Procedures
– Take Off Procedures
–Approach procedures
VI - METEOROLOGICAL REPORTS AND
FORECASTS
–Meteorological Reports
–Criteria for issuance of local special
reports and SPECI
–Criteria for the inclusion of change
groups in TAF
–Criteria for the inclusion of
meteorological elements in TREND
forecasts
–AIRMET Information
–Significant Weather Chart (low
level)
–Accuracy of Measurements on
Surface for Operational Needs
–Accuracy of Forecast for
Operational Requirements
VII -ANNEXES
–annex 1 – factors of
vulnerability(1/4)
–annex 2 - Characteristics of the eye
(1/4)
–annex 3 – contrast threshold
–annex 4 - the scatter meter
–annex 5 – illuminance threshold
–Glossary
–Bibliography
Visibility for Aeronautical Purposes
4
I - INTRODUCTION
Poor visibility is one of the more restricting
phenomena for aviation, especially for approach and
take-off phases.
Visibility for Aeronautical Purposes
General Remarks
• Poor visibility has a direct relation to the safety
and the efficiency of the flight operations.
• Atmospheric turbidity, which is the cause of
reduced visibility, is due to the presence of
aerosols (hydrometeors and lithometeors) in the
atmosphere.
• The value of the visibility (what a human eye
can see…) is not only due to the transparency of
the atmosphere but also to more subjective
information as the quality of this human eye, the
availability of marks, the ambient light,…
Visibility for Aeronautical Purposes
6
Influence on Safety
The prevention of collisions of all types, between
aircrafts on the ground or in-flight and relief and
aircraft (Controlled Flight Into Terrain), ensured by a
large number of systems and procedures, also
requires good knowledge of visibility conditions
“On 27th March 1977, in poor visibility conditions, two
B747 collided into each other on the only runway of
Tenerife airport, killing 578 people. One of the aircraft
was taxiing up the runway, while the other was taking off.
The visibility of about 500 metres did not enable this
collision to be avoided, whose cause was a
misunderstanding between the crews and the airport
traffic control”.
Visibility for Aeronautical Purposes
7
Influence on Efficiency
The efficiency of flight operations depends on the
factors of vulnerability to poor visibility of all the
actors operating directly or indirectly in aviation.
– The equipment level of the aerodrome, the capacities
of the airport, the qualifications of the crew, the onboard equipment of the aircraft take part in the
efficiency of flight operations. (see annex 1)
Visibility for Aeronautical Purposes
8
II - WHAT VISIBILITY ?
the visibility value given by the same observer at
the same place and at the same time may be
different according to synoptic observation,
climatological observation or aeronautical
observation
Visibility for Aeronautical Purposes
Meteorological Requirements
• The concept of visibility is used extensively in
meteorology in two distinct ways.
– Firstly, identification of air-mass characteristics,
especially for the needs of synoptic meteorology and
climatology. Here, visibility must be representative of
the optical state of the atmosphere.
– Secondly, it is an operational variable which
corresponds to specific criteria or special applications.
For this purpose, it is expressed directly in terms of
the distance at which specific markers or lights can
be seen.
Visibility for Aeronautical Purposes
10
Meteorological Visibility
• Visibility was first defined for meteorological
purposes as a quantity to be estimated by a
human observer:
– the estimation of visibility is affected by many
subjective and physical factors (state of atmosphere,
observed object, time of observation, eye of observer,…) see annex 2
– the essential meteorological quantity, which is the
transparency of the atmosphere, can be measured
objectively, and is represented by the meteorological
optical range (MOR).
Visibility for Aeronautical Purposes
11
Meteorological Optical Range (MOR)
• MOR: the distance traveled by a beam of parallel
light generated by an incandescent light at a
color temperature of 2700K so that the light
flow intensity is reduced by 95%
0
d = MOR
d=0.05 0
Visibility for Aeronautical Purposes
this value of 95 % is based
on the mean value of the
contrast threshold of
individuals see annex 3
12
Daytime Visibility
• Daytime visibility
(the greatest range at which a black
object of appropriate dimensions located close to the ground can
be identified when seen against a bright background) is
represented by MOR.
– MOR is insensitive to day/night variations
– It can be deduced from any measurement made by
scatter meter see annex 4
Visibility for Aeronautical Purposes
13
Visibility at Night
• Visibility at night (the greatest distance at which lights of
moderate intensity can be seen and identified) is not simply
related to MOR.
– It depends not only on MOR and the intensity of the
light, but also on the illuminance at the observer’s eye
from all other light sources see annex 5
Distance of light perception D (in metres) of a lamp of 100
candelas depending on the value of the meteorological
optical range (MOR).
At dawn, the MOR underestimates the meteorological visibility
for low visibility values and overestimates it for mean visibility
values. Consequently, the same visibility estimated at 500 metres
in daylight by an observer (using visual contrast as the
meteorological optical range) changes to more than 1,200 metres
at night (by using the visual perception of light emissions).
Visibility for Aeronautical Purposes
14
Visibility for Aeronautical Purposes
• Visibility for aeronautical purposes (ICAO
definition) corresponds to the higher of the two
following values
– The longest distance at which you can see and
recognize a black object of appropriate dimensions
located near the ground when observed on a luminous
background (day visibility)
– The longest distance at which you can see and identify
lights of an intensity close to 1,000 candelas when
observed on an unlighted background (night or foggy
weather visibility)
This ‘visibility for aeronautical purposes’ is in fact a ‘visual range’
(MOR), involving subjective elements such as the visual performance of
a human eye and artificial lights.
Visibility for Aeronautical Purposes
15
Prevailing Visibility
In aviation reports visibility should be representative of the
aerodrome and provide an indication of changes in direction.
•Prevailing visibility : visibility value (according to the definition of
visibility for aeronautical purposes) which is reached or exceeded
– within at least half the horizon circle or
– within at least half of the surface of the aerodrome.
These areas may comprise contiguous or non-contiguous sectors.
•a minimum visibility should be indicated if
– minimum visibility < 1500 m or
– minimum visibility < 50% of prevailing visibility
Notes: this value may be estimated by a human observer and/or by instrument
systems. The quality of this value depends on the available observation means at the
aerodrome. A few examples of the difficulty to obtain an objective value:
Visibility for Aeronautical Purposes
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Problems relating to human observation
• With a human observer at a fixed point (meteorological
station) the observation is based on an overview that
covers a large volume of the atmosphere but with
limitations related to how objects or lights can be
detected by the human eye:
if the observer is located in a foggy area
with a visibility of 300 m, he does not see
anything beyond those 300 m.
if partial fog is located 2000 m from the
observer with a visible mark at 2000 m,
the observer indicates a visibility of
2000 m, even though the visibility in the
partial fog is much less
fog
300 m
fog 300 m
aerodrome area
aerodrome area
In these 2 cases, the visibility in the whole area is therefore unknown !
Visibility for Aeronautical Purposes
17
Example with a human observer
• What a human observer sees from a fixed point,
using known landmarks, without using an
instrument:
•prevailing visibility
4,000 m
•minimum visibility
1,600 m in W sector
Visibility for Aeronautical Purposes
18
Example with scatter meters
• fully automated measurement of visibility using
scatter meters and without human intervention:
•prevailing visibility 1,500 m
•minimum visibility 500 m in
W sector
note: in this case, prevailing visibility is
the visibility value given by at least 50%
of the sensors (with 3 or 5 sensors it is
the median value).
Visibility for Aeronautical Purposes
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Example with human observer and scatter
meters
• by associating both human observation and
scatter meters:
•prevailing visibility
4,000 m
•minimum visibility
500 m in W sector
Visibility for Aeronautical Purposes
20
As a conclusion…
• In short, information depending on the visibility
observation resources available in the airfield.
For the same
weather
situation
what a human
observer sees
prevailing
visibility
4,000 m
1,500 m
4,000 m
minimum
visibility
1,600 m
in sector W
500 m
in sector W
500 m
in sector W
fully automated by associating
measurement
both human
of visibility
observation
and scatter
meters
Visibility for Aeronautical Purposes
21
Don't forget that:
ICAO Annex 3, paragraph 4.1.9 states that:
"Due to the variability of meteorological elements
in space and over time, and due to the limitations
of observation techniques and the inevitable
imprecision of certain elements, the recipient of
the information must accept that the precise
value of any of the elements indicated in an
observation message is the best possible
approximation of true conditions as they existed
at the time of the observation."
Visibility for Aeronautical Purposes
22
Runway Visual Range RVR
for take-off and landing, the visibility should be
representative of the touchdown zone of the
runway.
ICAO definition:
“distance at which a pilot on board of an
aircraft located in the axis of runway may see
marks or lights limiting or marking the axis”
RVR is the regulatory parameter (minimum operational
values) in case of reduced meteorological visibility on
airports
Instrumented systems based on transmissometers or forward-scatter meters
shall be used to assess runway visual range on runways intended for precision
instrument approach and landing operations
Visibility for Aeronautical Purposes
23
RVR measurement
• RVR is the object of a measurement by a transmissometer
- background luminance sensor assembly
• To calculate RVR we need:
– The value of the transmissive power (T) of the air,
attenuation of a light beam, on a known distance (B) measured
by the transmissometer
– The value of the luminance of the horizon or sky in the
direction opposite the sun and given by the background
luminance sensor
– At night the value of the intensity of the runway lights
–measurement range from
50 m to 2000 m
T=(d/ 0)=e-B
0
Visibility for Aeronautical Purposes
d
24
Location of transmissometers
• The number of sensors along a runway depends
on the approach category of this runway
(instrument landing and approach procedures)
Runway visual range assessments shall be representative of:
a) the touchdown zone of the runway intended for nonprecision or Category I
instrument approach and landing operations;
b) the touchdown zone and the mid-point of the runway intended for Category II
instrument approach and landing operations;
c) the touchdown zone, the mid-point and stop-end of the runway intended for
Category III instrument approach and landing operations.
Visibility for Aeronautical Purposes
25
Vertical Visibility
• Vertical visibility - the maximum visibility
obtained by looking vertically upwards into a
surface-based obstruction such as fog or snow.
– in aviation reports when unable to observe clouds
(instrument measurement with a ceilometer)
in the case of aerodromes with precision approach runways,
sensors for cloud amount and height of cloud base should
be sited to give the best practicable indications of the
height of cloud base and cloud amount at the middle marker
site of the instrument landing system or at aerodromes
where a middle marker beacon is not used, at a distance of
900 to 1200 m (3 000 to 4 000ft) front the landing
threshold at the approach end of the runway.
Visibility for Aeronautical Purposes
26
Slant and Flight Visibility
• Slant visibility - visibility observed by looking
forward and downwards from the cockpit of the
aircraft.
• Flight visibility - the average range of visibility
at any given time forward from the cockpit of an
aircraft in flight.
– the pilot in command is responsible for the estimation
of visibility in flight
This is visibility as
seen by the pilot,
often worse,
sometimes better,
than ground visibility.
Visibility for Aeronautical Purposes
27
III - ATMOSPHERIC TURBIDITY
Atmospheric transparency is the property of the
medium, which enables a stream of incident
radiation to pass through it.
Turbidity is the reduced transparency of the
atmosphere to radiation (especially visible) caused
by absorption and scattering by solid or liquid
particles: the aerosols
Visibility for Aeronautical Purposes
Origins of Atmospheric Turbidity
• Atmospheric turbidity has multiple origins,
according to the nature and density of aerosols
• They are broken down in two large categories:
– hydrometeors: water particles either liquid or solid (ice)
– lithometeors: solid and non-aqueous particles (significant
when they spread over a large area with visibilities < 5 km)
cloud
snow
rain
mist
hail
sand
volcanic ash
drizzle
fog
Visibility for Aeronautical Purposes
29
dust
smoke
Precipitations
• some visibility values depending on intensity
nature intensity
visibility (#)
rain
no reduction
4 to 10 km
# 1 km
50 to 500 m
drizzle light
moderate
DZ
heavy
3 to 8 km
1 to 3 km
< 1 km
snow
1 to 2 km
200 m to 1 km
# 50 m
light
moderate
RA heavy
severe
SN
light
moderate
heavy
Visibility for Aeronautical Purposes
30
Surprising situations with heavy rain showers!
•
•
•
Beware of heavy rain which may brutally reduce (as for a shower)
visibility to a few 50 meters!
Such rainfalls are the origin of many aircraft accidents
A classical scenario where the startle reaction is disconcerting

The runway is in sight
Flight path is stabilized
When you add other problems such as wind shear, gusty tail or cross
wind, the situation becomes delicate!
Visibility for Aeronautical Purposes
31
Surprising situations with snow
•
•
•
A moderate snowfall reduces visibility to less than 1 km.
Under a heavy snowfall, a visibility of less than 50 meters is not
rare.
In these conditions, during take off and landing phases, according
to aircraft speed, forward visibility may be close to zero !
Lake Erie. © Corey Paul
CC BY licence - http://www.flickr.com/photos/redgoober4life/97670373
Visibility for Aeronautical Purposes
32
Snow Drift
• Accumulated snow may be lifted by wind and
becomes then a snow drift, forming snow banks
blowing snow
(> 2m) BLSN
drifting snow
(< 2m) DRSN
Trans Canada in January. © Richard Smith
CC BY licence - http://www.flickr.com/photos/smith/134216/
Visibility for Aeronautical Purposes
33
Mist
• Visibility between 1000 and 5000 meters
• Made up of microscopic droplets or ice crystals
suspended in the atmosphere.
Relative
humidity is then
between 70 and
100%.
With mist the visibility value depends on
the route followed by the pilot according to
the position of the sun, especially at
sunrise and at sunset.
For example: if the pilot in a downwind leg
during a circuit have the sun in his back, in
final he will face the sun with a sudden
drop in visibility.
Visibility for Aeronautical Purposes
34
Fog
•
Visibility of less than 1000 meters
•
Fine droplets or ice crystals suspended in the
atmosphere
•
Amount of condensed water: > 0.5 g/kg dry air
hydrometeor in
Relative humidity is
equal to 100%.
Visibility for Aeronautical Purposes
35
A surprising situation with fog
• A classical scenario with a startle reaction
Beware of thin
fog patches !
Visibility for Aeronautical Purposes
36
Clouds
Visibility within clouds depends on:
- the size of the droplets or crystals
- the cloud's water content
As a general rule
- the visibility value within clouds increases with height
- at a given level the visibility value is better within stable
clouds than within unstable clouds
1000 meters in a cirrus
30 to 1000 meters in medium and low clouds
10/30 to 100 meters in CBs
Visibility for Aeronautical Purposes
37
Haze
• Haze
(HZ,
)
many small suspended solids, opalescent aspect, giving a reddish
or yellowish shade to far away objects
• Sand or dust haze (SA, DU,
)
Suspended dust or sand lifted from ground before the
observation of a storm (leaves a visible deposit on surface)
bad visibility
Caution, haze
is frequently
distributed in
layers
?
HAZE HZ
good visibility
Visibility for Aeronautical Purposes
38
Drifting or Blowing Dust or Sand
• Dust or sand, raised from the ground to small or
moderate height by a sufficiently strong and
turbulent wind
Drifting Dust or Sand :
(low (< 2m) DRDU, DRSA,
)
Blowing Dust or Sand :
(high (> 2m) BLDU, BLSA,
)
In this case the visibility is sensibly
reduced
Visibility for Aeronautical Purposes
Credit U.S. Geological Survey
Department of the Interior
39
Dust or Sand Storm
• dust or sand strongly lifted by strong and
turbulent wind up to very high heights and that
may travel considerable distances
Dust or sand storm
(SS, DS,
Visibility for Aeronautical Purposes
40
)
Smoke
• Widespread smoke produced by industrial areas
or forest fire
(FU,
)
May also form an
"industrial" fog
diverse combustion particles suspended
Visibility for Aeronautical Purposes
41
IV – FOCUS ON FOG…
One of the major causes of aircraft delay.
Safety and efficiency (economics) importance for
airline companies, airport managers and air traffic
controllers when fog occurs over an airport.
Information about fog disappearing as important as
on fog appearing.
Visibility for Aeronautical Purposes
Radiation Fog
• air cooling in contact with cold ground
Favorable meteorological
conditions:
•Clear sky or few clouds at night
•High air relative humidity
•Low wind speed (1 to 3 kts)
•Over land, firstly in valleys
•At night or early morning with
significant temperature inversion
•In anticyclones, ridges and cols.
x10m
Not too thick (less than 300 feet),
often local, dissipates with strong wind
or after sun rise (2 to 3 hours).
Visibility for Aeronautical Purposes
43
Advection Fog
• Advection of warm air on a colder surface
Favorable meteorological conditions:
•A warm and moist air mass moving
above a colder surface.
•Air mass dew point greater than
ground temperature
•High air humidity content over a
thickness of a few decameters
•Wind speed from 3 to 10 kts
•In the warm area of a disturbance
(tropical maritime air)
•A humid air invasion over snow-covered
ground
5 to 10kt
© : Bernhard Mühr
Thick (up to more than 1,000 ft),
wide outspread (region, country…),
persistent, its dissipation is
uncertain during a short winter day.
Wind stop or rotation is a
dissipation factor. Wind
x100m strengthening may transform it into
stratus.
Visibility for Aeronautical Purposes
44
Upslope Fog
• Forced ascent of air
implying expansion and
cooling
• This is nothing more than an
orographic cloud
• May also occur when the wind
starts to stir up fog which had
been confined to low ground
condensation level
Visibility for Aeronautical Purposes
45
Evaporation Fog
• Saturation caused within a cold and stable air
mass by rapid evaporation from an underlying
warm water surface
•At small scale, occurs over lakes, rivers,…
•Much more common in artic regions known as « artic
or sea smoke »
Water temperature > air temperature
Visibility for Aeronautical Purposes
46
Coastal Fog
• Maritime advection with sea breeze
Often the result of an evaporation fog made over the
sea and carried by the sea breeze toward the coast
Visibility for Aeronautical Purposes
47
Frontal Fog
• By mixing of the two air masses present
• By increase of the moisture in the cold air
resulting from evaporation of rain
© : Bernhard Mühr
Visibility for Aeronautical Purposes
48
V - FLIGHT RULES
Visual Flight Rules (VFR) :
–flight in Visual Meteorological Conditions (VMC)
Instrument Flight Rules (IFR) :
–flight in Instrument Meteorological Conditions (IMC)
Meteorological conditions expressed in terms of visibility,
distance from cloud, and ceiling, less than the minima
specified for Visual Meteorological Conditions.
Visibility for Aeronautical Purposes
VFR Rules
• “See and Avoid”
• VFR flights shall not take off or land at an
aerodrome within a control zone, or enter the
aerodrome traffic zone or traffic pattern:
a) when the ceiling is less than 1 500 ft ; or
b) when the ground visibility is less than 5 km.
Ceiling: The height above the ground or water of the base of
the lowest layer of cloud below 20 000 feet covering more than
half the sky (BKN or OVC).
Visibility for Aeronautical Purposes
50
Visual Meteorological Conditions VMC
controlled airspace
distance from clouds
• Horizontally 1500 m
• Vertically 300 m
flight visibility 8 km
FL100
flight visibility 5 km
msl
Visibility for Aeronautical Purposes
51
Visual Meteorological Conditions VMC
uncontrolled airspace
distance from clouds
• Horizontally 1500 m
• Vertically 300 m
flight visibility 8 km
FL100
3000 ft asfc
flight visibility 5 km
3000 ft asfc
•
•
clear of clouds and in sight of the surface
flight visibility: the greatest of the following 2 values:
 1500 m
 Distance covered in 30 s in flight
msl
Visibility for Aeronautical Purposes
52
Special VFR Flight
• A VFR flight cleared by air traffic control to
operate within a control zone in meteorological
conditions below VMC.
– minimum visibility values: the greatest of the following
2 values 1500 m or distance covered in 30 s in flight
• the air traffic controller judges special VFR
flight in force if
– ground visibility less than 5 km
– or ceiling less than 1500 ft
• the pilot can ask for special VFR if he estimates
in meteorological conditions below VMC.
Visibility for Aeronautical Purposes
53
LVP and LVTO Procedures
• Low Visibility Procedures :
– procedures applied to an aerodrome in order to provide
safety during cat II and III precision approach and
Low Visibility Take Off (take off on a runway where the RVR is
less than 400 m)
Example of Roissy/Charles de Gaulle airport where LVP occurrence is one of the major
causes of aircraft delay and where airport authorities need accurate forecasts to
manage airport traffic.
Low Visibility Procedures when RVR is less than 600m or when ceiling is less than
200ft :
• On average, LVP conditions occur about 40 days a year
• decrease of landing and take-off airport capacity (reduced by a factor 2).
• If LVP is not planned in advance, ready to land planes are obliged to wait in
stacks (till 5).
• Effects may persist during several hours after the end of the LVP procedure
• Huge economic importance for airline companies, airport managers and air
traffic controllers.
Visibility for Aeronautical Purposes
54
IFR Approach: Operational Minimums
•
Each aerodrome publishes its own minimums but they can not be
lower than those below
Approach with Instrument Landing System
Precision
operational
minimums
non
precision
Decision
Height
=Minimum
Descent
Height
RVR
VIS
CAT I
CAT II
CAT
IIIA
CAT
IIIB
CAT
IIIC
 200 ft  100 ft < 100 ft
< 50 ft
no DH
x
 550 m
 350 m
 200
m
< 200 m
 50 m
no RVR
 800 m
 800 m
x
x
x
x
MDH= the lowest height above the aerodrome elevation used in establishing compliance with
the .appropriate obstacle clearance criteria.
Visibility for Aeronautical Purposes
55
IFR Procedures
• According to the availability of a Runway Visual
Range (RVR) or a Visibility (VIS)
– for take off
– for approach and landing
Visibility for Aeronautical Purposes
56
Take Off Procedures
RVR
yes
RVR  op. min.
yes
clear to take
off
no
available
VIS
yes
no
available
no
VIS  op. min.
take off
yes
no
clear to take
off
RVR: Runway Visual Range
VIS: Visibility
Visibility for Aeronautical Purposes
assessment by the
pilot
RVR/Visibility along
the runway  op.
min.
yes
no
clear to take
off
take off
57
Approach procedures
Non
precision
no
Precision
Cat I
RVR
transmitted*
Conversion
VIS  RVR
RVR
transmitted*
RVR  minimums
RVR = available
visibility multiplied by:
lights working
approach
yes
yes
VIS available
no
Precision
Cat II and III
daytime
night
time
HI approach
and runway
lights
1,5
2,0
all other types
of lights
1,0
1,5
no light
1,0
no apply
no
yes
continue with
the approach
Approach
*RVR transmitted: may be sometimes measured by human
observation
Visibility for Aeronautical Purposes
58
VI - METEOROLOGICAL REPORTS AND
FORECASTS
The accuracy and the criteria for issuance of
visibility and ceiling in meteorological reports and
forecasts are directly linked to the thresholds of
operational values for flight operations.
Visibility for Aeronautical Purposes
Meteorological Reports
In local routine and special reports and METAR and SPECI,
visibility, RVR and cloud base height are reported as above
if
step applicable
VIS < 800 m : 50 m
800 m < VIS < 5 000 m : 100 m
5 000 m < VIS < 10 km : 1 km
VIS > 10 km : none, given as 10 km
RVR < 400 m : 25 m
400 < RVR < 800 m : 50 m
800 < RVR < 2 000 m : 100 m
Cloud Base < 5 000 ft:
100 ft
(reference level: aerodrome
elevation)
Visibility for Aeronautical Purposes
60
•Any observed
value which does
not fit the
reporting scale
in use shall be
rounded down to
the nearest
lower step in the
scale.
Criteria for issuance of local special
reports and SPECI
When one of these parameters is improving and changes to or passes through
one or more of the following values, or when it is deteriorating and passes
through one or more of the following values
Visibility :
• 800 m, 1,500 m, 3,000 m, (5,000 m if numerous VFR flights)
RVR :
• 150m, 350m, 600m, 800m
Cloud Base Height of clouds of BKN or OVC nebulosity :
• 100 ft, 200 ft, 500 ft, 1,000 ft, (1,500 ft if numerous VFR flights)
Nebulosity of a cloud layer with a base below or equal to 1,500 ft
• from [BKN or OVC] to [SCT, FEW or SKC] or conversely
Vertical Visibility :
• 100 ft, 200 ft, 500 ft or 1000ft
METAR 130500Z LFxx VRB02KT 900 R32/0700 FG BKN008 10/10 Q1020=
Thresholds:
VIS
800m
RVR
600m
CEILING
500ft
SPECI 130523Z LFxx VRB02KT 700 R32/0500 FG OVC004 10/10 Q1020=
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61
Criteria for the inclusion of change groups
in TAF
When one of the following parameters is forecast to improve and change to
or pass through one or more of the following values, or when it is
forecast to deteriorate and pass through one or more of the following
values
Visibility :
• 150 m, 350 m, 600 m, 800 m, 1,500 m, 3,000 m, (5,000 m if numerous VFR
flights)
Cloud Base Height of clouds of BKN or OVC nebulosity :
• 100 ft, 200 ft, 500 ft, 1,000 ft, (1,500 ft if numerous VFR flights)
Nebulosity of a cloud layer with a base below or equal to 1,500 ft
• from [BKN or OVC] to [SCT, FEW or SKC] or conversely
Vertical Visibility :
• 100 ft, 200 ft, 500 ft or 1000ft
TAFxxxx 220500Z 220615 32006KT 9999 SCT015 TEMPO 1115 32015G30KT
3000 SHSN BKN010=
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62
Criteria for the inclusion of meteorological
elements in TREND forecasts
When one of the following parameters is forecast to improve and change to
or pass through one or more of the following values, or when it is
forecast to deteriorate and pass through one or more of the following
values
Visibility :
• 150 m, 350 m, 600 m, 800 m, 1,500 m, 3,000 m, (5,000 m if numerous VFR
flights)
Cloud Base Height of clouds of BKN or OVC nebulosity :
• 100 ft, 200 ft, 500 ft, 1,000 ft, (1,500 ft if numerous VFR flights)
Nebulosity of a cloud layer with a base below or equal to 1,500 ft
• from [BKN or OVC] to [SCT, FEW or SKC] or conversely
Vertical Visibility :
• 100 ft, 200 ft, 500 ft or 1000ft
METAR xxxx 101300Z 11002KT 0500 R33R/0700 FZFG VV003 M01/M01
Q1020 TEMPO 0100 VV001=
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AIRMET Information
Surface Visibility
• Widespread areas where visibility is reduced to
less than 5,000 m and weather phenomenon at
the origin of visibility reduction
– coding
• SFC VIS
• + visibility
• + weather phenomenon in question
LIMM AIRMET 1 VALID 110230/110830 LIMMLIMM MILANO FIR SFC VIS 1200M BR FCST PO VALLEY STNR NC=
Visibility for Aeronautical Purposes
64
Significant Weather Chart (low level)
• Surface Visibility (m)
• Clouds
– amount, height (ft) of base and type
•
•
•
•
•
•
•
•
•
Widespread Fog
Mountain Obscuration
Widespread Blowing Snow
Severe Sand or Dust Haze
Widespread Sandstorm or Dust storm
Widespread Haze
Widespread Mist
Widespread Smoke
All types of precipitation
Visibility for Aeronautical Purposes
65
Accuracy of Measurements on Surface for
Operational Needs
Desirable accuracy
Practicable accuracy
Visibility
as far as 600 m:  50 m
between 600 and 1,500 m: 10%
above 1 500 m: 20%
as far as 500 m:  50 m
between 500 and 2,500 m: 10%
above 2,000 m: 20%
Runway
Visual
as far as 400 m:  10 m
between 400 and 800 m:  25 m
above 800 m: 10%
as far as 150 m:  25 m
between 150 and 500 m:  50 m
above 500 m: 10%
Clouds
height
up to 330 ft:  33 ft
above 330 ft: 10%
up to 3,300 ft:  33 ft
above 3,300 ft up to 10,000 ft:
100 ft
Range
Visibility for Aeronautical Purposes
66
Accuracy of Forecast for Operational
Requirements
Desirable accuracy
Minimum percentage
of cases where
these limits should
not be exceeded
TEND
TAF
Visibility
As far as 700 m:  200 m
between 700 m and 10 km:  30%
90
80
Clouds
height
Up to 400 ft:  100 ft
between 400 and 3,000 ft:  30%
90
70
Visibility for Aeronautical Purposes
67
VII - ANNEXES
Visibility for Aeronautical Purposes
Annex 1 – Factors of vulnerability(1/4)
The aircraft
Aircraft speed is a major factor of vulnerability and this parameter
influences in-flight paths near and on the ground. The trajectory
of an aircraft flying at high speeds is framed into a large
atmospheric volume. If visibility does not enable the whole
volume to be taken in, methods other than flight visual piloting
are required. Even taxiing will have to be assisted in the worst
visibility conditions.
Thus, the presence (or absence) of on-board equipment and its
quality, enabling to pilot without any visibility, are essential
factors of vulnerability. They are naturally taken into account in
regulations concerning flight in poor weather conditions.
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69
Annex 1 – factors of vulnerability
(2/4)
Pilots
Through various qualifications including instrument flight, pilots
gradually obtain the know-how essential for operating in complete
safety in poor visibility conditions. In the cases of very poor
visibility, the pilot can even be replaced (without however
completely being removed from the circuit) by automated
devices, which are the only to be able to calculate a path in these
extreme conditions.
The pilot (or crew) is often vulnerable due to:
•
absence of qualification adapted to the situation of visibility
•
lack of training for this situation
•
over-confidence or tiredness
•
lack of awareness of the meteorological phenomena involved.
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70
Annex 1 – factors of vulnerability
(3/4)
Control and air traffic services
Procedures adapted to poor visibility conditions enable the operation
to be carried on. The quality (or lack thereof) and the pertinence
of these procedures are factors of vulnerability. It should be
remembered that globally, through these often time-consuming
procedures, air traffic is very sensitive to the visibility
conditions prevailing in a region, even on a single strategic site.
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71
Annex 1 – factors of vulnerability
(4/4)
Airports
Local climatology is a factor of vulnerability. Some regions are
particularly prone to fog.
The level (nature and quality) of the equipment installed on the
ground and adapted to the procedures of poor weather
conditions is a factor of vulnerability.
The layout of facilities and the urban and geographic environment
are other factors of vulnerability.
When all these factors are combined, the traffic of an airport can
be reduced by 50%, as soon as there are poor visibilities and
following the implementation of Low Visibility Procedures (LVP).
Back to slide 7 
Visibility for Aeronautical Purposes
72
Annex 2 - Characteristics of the Eye
• The eye perceives contrast especially by day
time
• The eye perceives light sources especially by
night
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73
(1/4)
Annex 2 - Characteristics of the Eye
(2/4)
• Beware of white-out situations
You cannot
distinguish
obstacles
any more!
Lac Tremblant nord, Québec. © Robbie Sproule
CC BY licence - http://www.flickr.com/photos/robbie1/6433092/
Visibility for Aeronautical Purposes
74
Annex 2 - Characteristics of the Eye
(3/4)
For a given atmospheric transparency, what an observer can see
with a variable background luminance...
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75
Annex 2 - Characteristics of the Eye
(4/4)
• What an observer can see with light sources of
variable intensity
1,500 m
1,500 m
1,500 m
Formula 0
By a clear night, the
observer can
distinguish the
powerful light of the
stadium and the
hotel sign
Formula 0
Formula 0
700 m
By foggy weather,
lights 1,500 m away
are distinguished,
but not those 700m
away! What is the
visibility?
700 m
700 m
And after the end of
the game?
Back to slide 10 
Visibility for Aeronautical Purposes
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Annex 3 – Contrast threshold
According to individuals (age, tiredness,…) contrast threshold Cx/Co
is between 0.01 and 0.1 with an average value of 0.05
La = background luminance
Lo = object luminance
contrast Co=(La-Lo)/La
Apparent contrast at distance X
Cx=Co.e-X (: atmospheric extinction coefficient)
Koschmieder’s Law
Back to slide 11 
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77
Annex 4 - The scatter meter
• Measures the scattering of light by a small
volume of atmosphere
• Provides estimates of MOR with a range from
about 10 m to more than 10 km (depends on the type of
instrument)
Transmitter (light source)
0 (transmitted flux)
aerosols
2m
d (diffused flux)
Clear from obstacles and
light sources
Receiver (photosensitive sensor)
Visibility for Aeronautical Purposes
Back to slide 12 
78
Annex 5 – Illuminance threshold
• When lights are dominant (visibility based on
illumination threshold): the visibility depends on
the transmission factor of the air, on the
intensity of the lights, and on the observer’s
(and pilot’s) threshold of illuminance, which itself
depends on the background luminance
L = background luminance
I = intensity of source
The illuminance threshold of the eye is a continuous
function dependent on the background luminance
Allard’s Law
Back to slide 13 
Visibility for Aeronautical Purposes
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Glossary 1/2
Photometric qualities defined by the International Electrotechnical
Commission (IEC, 1987):
(a) Luminous flux (symbol: F (or F), unit: lumen) is a quantity derived from
radiant flux by evaluating the radiation according to its action upon the
ICI standard photometric observer;
(b) Luminous intensity (symbol: I, unit: candela or lm sr-1) is luminous flux
per unit solid angle;
(c) Luminance (symbol: L, unit: cd m-2) is luminous intensity per unit area;
(d) Illuminance (symbol: E, unit: lux or lm m-2) is luminous flux per unit
area.
•
The extinction coefficient (symbol s) is the proportion of luminous flux
lost by a collimated beam, emitted by an incandescent source at a colour
temperature of 2 700 K, while travelling the length of a unit distance in
the atmosphere. The coefficient is a measure of the attenuation due to
both absorption and scattering.
•
The luminance contrast (symbol C) is the ratio of the difference
between the luminance of an object and its background and the
luminance of the background.
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80
Glossary 2/2
•
•
•
•
The contrast threshold (symbol e) is the minimum value of the
luminance contrast that the human eye can detect, i.e. the value
which allows an object to be distinguished from its background. The
contrast threshold varies with the individual.
The illuminance threshold (Et) is the smallest illuminance, at the
eye, for the detection of point sources of light against a background
of specified luminance. The value of Et, therefore, varies according
to lighting conditions.
The transmission factor (symbol T) is defined, for a collimated
beam from an incandescent source at a colour temperature of 2 700
K, as the fraction of luminous flux which remains in the beam after
traversing an optical path of a given length in the atmosphere. The
transmission factor is also called the transmission coefficient. The
terms transmittance or transmissive power of the atmosphere are
also used when the path is defined, i.e. of a specific length (e.g. in
the case of a transmissometer). In this case, T is often multiplied
by 100 and expressed in per cent.
Candela : the basic unit of luminous intensity adopted under the
“Système International d'Unites” ; equal to 1/60 of the luminous
intensity per square centimetre of a black body radiating at the
temperature of 2,046 kelvin
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Bibliography
• WMO – Guide to Meteorological Instruments and Methods
of Observation
• WMO – International Meteorological Vocabulary
• ICAO – Annex 3 – Meteorological Service for
International Air Navigation
• ICAO – Annex 2 – Rules of the Air
• ICAO – Annex 6 – Operation of Aircraft
• ICAO – Doc 9426_5 - Terms and References
• ENM/ENAC –2002- Météorologie Aéronautique
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The End
Visibility for Aeronautical Purposes