Aerodrome Weather Observer
Antarctic Supplement
Bureau of Meteorology Training Centre
Aerodrome Weather Observer: Antarctic Supplement
© Commonwealth of Australia 2016
This work is copyright. Apart from any use as permitted under the
Copyright Act 1968, no part may be reproduced without prior written
permission from the Bureau of Meteorology.
Aerodrome Weather Observer
Antarctic Supplement, Version 4.0, August 2016
Cover photo: CASA 212 VH-VHB departs Casey Skiway. L. De Pauw.
Aerodrome Weather Observer: Antarctic Supplement
Contents
Introduction............................................................................................................................... 2
1
Hazards associated with depth perception and the horizon ............................................ 3
1.1
1.2
1.3
1.4
1.5
2
SPECI Criteria ................................................................................................................. 7
2.1
2.2
3
Surface contrast................................................................................................... 3
Horizon definition ................................................................................................. 3
White-out ............................................................................................................. 4
Surface Contrast and Horizon Definition Characteristics ..................................... 5
Reporting and Recording Surface Contrast and Horizon Definition .................... 6
Surface contrast................................................................................................... 7
Horizon definition ................................................................................................. 7
Further Antarctic specific practices.................................................................................. 8
3.1
3.2
CAVOK ................................................................................................................ 8
Reporting Cloud ................................................................................................... 8
Notes ........................................................................................................................................ 9
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Aerodrome Weather Observer: Antarctic Supplement
Introduction
Antarctica offers a very unique and challenging operating environment for the aviation
industry. This environment can present hazards to aircraft operations that generally are not
experienced on the mainland. Observers who perform observations for aeronautical
purposes in the Australian Antarctic Territory must understand the additional reporting needs
of Antarctic pilots and forecasters.
Antarctic Compendium
The Bureau of Meteorology has produced the Antarctic Compendium. This document
specifies some of the additional practices and procedures applicable to Antarctic operations,
including aviation observations.
The current version can be found on the intranet at the following address:
http://www.tas.bom.gov.au/reg_progs/ros/reports/compend/index.htm
This Antarctic Supplement is derived from the Antarctic Compendium.
Note: As of August 2016, the Compendium is undergoing a rewrite.
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Aerodrome Weather Observer: Antarctic Supplement
1
Hazards associated with depth perception and the horizon
Much of the aircraft operations in Antarctica are performed under the Visual Flight Rules
(VFR). One of the fundamental aspects of flying an aircraft under the VFR is the ability of the
pilot to be able to clearly define the natural horizon to assist with both navigation and control
of the aircraft. The degree of depth perception available from visual cues plays an equally
important role for a pilot, particularly in the approach and landing phases of flight. This also
applies for aircraft operating under the Instrument Flight Rules (IFR).
The human body employs several mechanisms to help with orientation, whether one is
standing on the surface of the earth or sitting in the flight deck of an aircraft. The visual
system (the eyes), the vestibular system (inner ear, or balance system) and the
proprioceptive system (associated with muscles and joints) all play an important role in
spatial orientation.
The visual system is the most ‘reliable’ system used by pilots in flight to help with orientation
and aircraft control. Approximately 80% of orientation cues come from this system. For an
aircraft operating under the VFR, the inability for a pilot to clearly define the position of the
horizon can have disastrous consequences.
Phenomena such as rain and drizzle, fog and mist, and smoke and haze all have the
potential to obscure the horizon and interfere with depth perception. There are some
additional phenomena unique to environments such as that in Antarctica that can also have
an effect.
1.1
Surface contrast
Features on a snow surface can normally be visually seen and recognised. This includes
large mounds and hill contours, sastrugi (ripples, wind ridges, and barchans), drifts, gullies,
and tracks.
The ability to perceive snow–covered orographic features depends on the shadows that they
cast; such forms become indistinguishable when cloud cover prevents shadows being cast.
Without any visual stimulation it is common to incorrectly evaluate an incline: one may walk
up and down hills without realising it. Furthermore, it is known that an individual attempting to
follow a straight path unaided will veer. Judgments of the distance and orientation of objects
in the field of view is severely handicapped.
The extent to which features on the snow surface can be distinguished is known as the
surface contrast and is reported as good, fair, poor or nil.
Note: Some texts may refer to the term surface definition. This term has the same meaning
as surface contrast.
1.2
Horizon definition
Horizon definition can be simply defined as the ease with which the horizon can be defined.
The importance of reporting such has been outline in previous paragraphs.
Horizon definition is reported as good, fair, poor, or nil.
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Aerodrome Weather Observer: Antarctic Supplement
1.3
White-out
White-out is an optical phenomenon that occurs in uniformly overcast conditions over a
snow–covered surface. It is caused by a heavy cloud cover over the snow, in which the light
coming from above is approximately equal to the light reflected from below, and which is
characterized by absence of shadows, invisibility of the horizon, and only very dark objects
can be identified.
It is associated with diffuse (uniform), shadowless illumination that causes a lack of surface
contrast and reduced horizon definition.
White–out conditions can occur while visibility (i.e. transparency of the air) remains good.
While total white–out results from nil surface contrast and horizon definition there are
degrees of this effect. Partially reduced horizon definition and surface contrast, for example,
can occur under a broken cloud layer.
White-out is not a reportable phenomenon in an Aerodrome Weather Report.
White-out on the Ekström Ice Shelf, Weddell Sea, Antarctica. Photo: Hannes Grobe
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Aerodrome Weather Observer: Antarctic Supplement
1.4
Surface Contrast and Horizon Definition Characteristics
SURFACE CONTRAST
GOOD
FAIR
POOR
NIL
HORIZON DEFINITION
Snow surface features such as The horizon is sharply defined by
sastrugi, drifts and gullies are easily shadow or contrast. The horizon is
identified by shadow.
distinct with an obvious difference
between land (snow) and sky.
The sun is usually unobscured.
Surface features are clearly defined
for as far as the eye can see.
Snow features can be identified by The horizon may be identified, although
contrast. No definite shadows exist. the contrast between sky and snow is not
The sun is usually totally obscured. sharply defined.
Surface features become indistinct at
distances of more than a few
kilometres.
Snow surface features (e.g. skidoo
tracks) cannot readily be identified
except from close up (within 50
metres). The sun is usually totally
obscured.
The horizon is barely discernable: in
other
words,
the
sky
can
be
discriminated from land but no distinct
horizon is visible.
Snow surfaces cannot be identified. Total loss of horizon: the snow surface
No shadows or contrast exist. Dark merges with the whiteness of the sky.
coloured objects appear to float in the
air. The sun is totally obscured,
although the overcast sky may exhibit
considerable glare. The glare appears
to be equally bright from surface
reflection and from all directions.
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Aerodrome Weather Observer: Antarctic Supplement
1.5
Reporting and Recording Surface Contrast and Horizon Definition
Each aerodrome weather observation should include a visual evaluation of the surface
contrast and the horizon definition and be reported via the applicable method (MetConsole,
two-way radio, etc). Note that any directional variation can also be reported and recorded as
required.
Plain Language AWR form:
SURFACE CONTRAST

(tick)
HORIZON DEFINITION (tick)
GOOD
GOOD
FAIR

FAIR
POOR
POOR
NIL
NIL
MetConsole Data Entry:
SFC GOOD HZN FAIR N TO SE
A37 Register:
The A37 Register can be altered to include two ruled columns in the existing Remarks
column, labelled ‘SFC’ and ‘HZN’. The assessment of the surface contrast and horizon
definition can then be entered in the appropriate column.
6
SFC
HZN
GOOD
FAIR
FAIR
POOR
N to SE
ReWx/Wind Shear/Remarks
Aerodrome Weather Observer: Antarctic Supplement
2
SPECI Criteria
The criteria for the issue of Special (SPECI) Reports for both mainland and Antarctic
observations is outlined in Tables 6.1 and 6.2 of the Aeronautical Services Handbook (ASH).
The following SPECI issue requirements for Antarctic operations are in addition to those
outlined in the ASH.
2.1
Surface contrast
When the following conditions are observed a SPECI is to be issued:
•
•
2.2
25% or more of snow surface features have poor or nil contrast
50% or more of snow surface features have poor or nil contrast
Horizon definition
When the following conditions are observed a SPECI is to be issued:
•
•
25% or more of the horizon has a poor or nil definition
50% or more of the horizon has a poor or nil definition
Surface contrast / horizon definition SPECI issue examples:
A SPECI shall be immediately issued if a poor surface contrast changes from 15% to 40% of
surface features.
A SPECI shall be immediately issued if a nil horizon definition changes from 30% to 75% of
the horizon.
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Aerodrome Weather Observer: Antarctic Supplement
3
Further Antarctic specific practices
3.1
CAVOK
The Aeronautical Services Handbook, Ch7, 2.7.12: states:
“…..Note that CAVOK is not used in Antarctica due to adverse flying conditions
created by cloud above 5000ft.”
While ASH Ch7 refers to the provision of aerodrome forecasts, the Meteorological Authority
recommends that CAVOK is also not to be used in weather reports in Antarctica.
Staff providing weather observations and reports shall consult with the Antarctic
Meteorological Section regarding MetConsole data entry and the use of CAVOK at their
location.
3.2
Reporting Cloud
The official rule (1-3-5 Cb/TCu) shall normally be used to report clouds.
Pilots or forecasters may request the report to include all cloud irrespective of the cloud
amounts. Observers shall consider using the plain language field to report such additional
information.
Staff should consult with the Antarctic Meteorological Section regarding these requirements.
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Aerodrome Weather Observer: Antarctic Supplement
Notes
9