METWSG/2-SN No. 6
17/2/09
METEOROLOGICAL WARNINGS STUDY GROUP (METWSG)
SECOND MEETING
Montréal, 19 to 21 May 2009
Agenda Item 5: Content and issuance of SIGMET
5.4: Use of closed lines of coordinates and location indicators to describe the area in
SIGMET/AIRMET
PROPOSALS TO AMEND LOCATION INFORMATION IN SIGMET AND AIRMET
TEMPLATES
(Presented by Colin Hord)
SUMMARY
This study note discusses the proposals to amend location information in SIGMET
and AIRMET templates.
1.
INTRODUCTION
1.1
The first meeting of the Meteorological Warnings Study Group noted that the Air
Navigation Commission, endorsed EANPG Conclusion 48/39, application and development of the
template for SIGMET and AIRMET, which called for the consideration of the amendment of the template
for SIGMET and AIRMET to allow only the use of a closed line of coordinates, location indicators of
waypoints or airports in order to describe the area of the phenomena in a SIGMET and/or AIRMET.
1.2
The group was aware that the format for SIGMET and AIRMET messages is set out by
Table A6-1. Template for SIGMET and AIRMET messages and special air-reports (uplink) included in
Appendix 6 of Annex 3. In this regard the group noted that the proposal to introduce a new way to
describe the phenomena in SIGMET and/or AIRMET had been raised by IATA when EANPG was
discussing ways to improve the compliance with operational requirements. The requirement had been
stated in order to improve the efficiency of automatic handling of information. However, the group felt
that the proposal by EANPG would only take into account the needs of one type of users (i.e. users with
automatic data processing). In addition it was indicated that the proposal would imply the removal, from
the template, of the geographical features well known internationally from which no adverse comments
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had been received and would cause problems to other type of users (i.e. ATC units). Therefore, the group
agreed that this issue needed further study and agreed to the following action:
Action agreed 1/4— Description of the area of phenomena in
SIGMET/AIRMET
That, an ad hoc working group composed by Steve, Patrick,
Nigel (Rapporteur) and Tom be tasked to assess the need of
change the method to describe the area of the phenomena in a
SIGMET/AIRMET in time for the METWSG/2 Meeting.
2.
DISCUSSION
2.1
Table A6-1 of Amendment 74 to Annex 3 defines the content and order of elements for
SIGMET and AIRMET messages. In addition, paragraphs 1.1.5 and 2.1.5 of Appendix 6 state that such
messages shall not contain unnecessary descriptive material.
2.2
In considering the need to change the method for describing the area of a phenomenon in
a SIGMET or AIRMET, it is worthwhile considering more fully what the benefits and limitations of the
existing arrangements within Table A6-1 are, whilst also considering the potential benefits and limitations
of any proposed changes may be.
Perceived benefits and limitations of the existing arrangements
2.3
Table A6-1 currently permits a Meteorological Watch Office (MWO) to describe the
location of a phenomenon in a SIGMET or AIRMET in one of five ways, namely:
a) Nnn[nn] Wnnn[nn] or Nnn[nn] Ennn[nn] or Snn[nn] Wnnn[nn] or Snn[nn] Ennn[nn];
b) N OF Nnn[nn] or S OF Nnn[nn] or N OF Snn[nn] or S OF Snn[nn] or [AND] W OF
Wnnn[nn] or E OF Wnnn[nn] or W OF Ennn[nn] or E OF Ennn[nn];
c) [N OF, NE OF, E OF, SE OF, S OF, SW OF, W OF, NW OF] [LINE] Nnn[nn] or
Snn[nn] Wnnn[nn] or Ennn[nn] – Nnn[nn] or Snn[nn] Wnnn[nn] or Ennn[nn];
d) [N OF, NE OF, E OF, SE OF, S OF, SW OF, W OF, NW OF, AT] nnnnnnnnnnnn; or
e) WI Nnn[nn] or Snn[nn] Wnnn[nn] or Ennn[nn] – Nnn[nn] or Snn[nn] Wnnn[nn] or
Ennn[nn] – Nnn[nn] or Snn[nn] Wnnn[nn] or Ennn[nn] – [Nnn[nn] or Snn[nn]
Wnnn[nn] or Ennn[nn] – Nnn[nn] or Snn[nn] Wnnn[nn] or Ennn[nn]]
2.4
One notable benefit of the existing arrangements expressed in paragraph 2.3 i) is the fact
that the location of a phenomenon can be described with very few characters or words. For example, “N
OF N5500” clearly describes that the phenomenon is expected to occur “North of 55 degrees 00 minutes
North” (within the FIR for which is has been issued). Noting the earlier remarks in paragraph 2.1, this is
considered of benefit for the originating MWO and the end user, since the information contained in the
SIGMET or AIRMET can be sufficiently succinct. However, by the same token, the use of an
abbreviation such as “N OF N5500” requires prior knowledge of the exact boundary of the FIR (either in
a manual sense or an automated sense). Using a term such as “N OF N5500” does not clearly define how
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far east or west the phenomenon is expected to extend, nor indeed how far North. Therefore, unless the
boundary of the FIR is well known by an operator, this could be considered a limitation of the existing
arrangements.
2.5
Use of a term such as “N OF nnnnnnnnnnnn” (refer 2.3 iv)), where nnnnnnnnnnnn is
used to describe a location or geographic feature that is well known internationally, whilst being noted for
its brevity, could be considered to lead to ambiguity by an end user. For example, “N OF EGLL”
describes that a phenomenon is expected to occur, or has occurred, “North of London Heathrow Airport”,
but it does not exactly express how far north. Furthermore, does it imply “North of the line of latitude of
London Heathrow airport” or does it imply “North of the co-ordinate point for London Heathrow
airport”? Such aspects could be viewed as open to interpretation by those generating the messages and
those making decisions based on the messages, and therefore considered a limitation within the existing
arrangements.
2.6
Terminology such as described in 2.3 v) can actually overcome some of the limitations
expressed in paragraphs 2.4 and 2.5. For example, using “WI N5000 E00000 - N5500 E00000 - N5500
W01000 - N5000 W01000” will allow an operator to know that the phenomenon is contained within a
polygon bounded by the co-ordinates described. Operations outside of these co-ordinate points are not
likely to lead to an encounter. However, it is worth noting that the character length of the message
consequently increases by using such terminology. At the present time, ICAO Annex 3 provisions appear
to only permit a minimum of three and a maximum of five co-ordinate points when the abbreviated term
‘WI’ is used – refer to 2.3 v) above. Whilst this ensures that the length of a SIGMET or AIRMET
message is not overwhelming for an operator, it can also lead to smoothing – i.e. going directly from
point A to point C, rather than from point A to point C via point B. Such limitations within the existing
rules could result in unnecessary flight restrictions being placed on certain parts of the FIR by airspace
controllers. If the intention of the current SIGMET and AIRMET provisions in Annex 3 is actually to
permit the use of more than 5 co-ordinates to define a closed polygon, then a footnote is recommended to
Table A6-1 stating such a practice.
2.7
The method for describing speed of movement in a SIGMET or AIRMET can be
described in one of two ways, namely:
a) MOV N [nnKMH] or MOV NE [nnKMH] or MOV E [nnKMH] or MOV SE
[nnKMH] or MOV S [nnKMH] or MOV SW [nnKMH] or MOV W [nnKMH] or
MOV NW[nnKMH] (or MOV N [nnKT] or MOV NE [nnKT] or MOV E [nnKT] or
MOV SE [nnKT] or MOV S[nnKT] or MOV SW [nnKT] or MOV W [nnKT] or
MOV NW [nnKT]); or
b) STNR
2.8
One notable benefit of using the terminology described in paragraph 2.7 above is that
brevity and succinctness of the messages is maintained. For example, “MOV N 15KT” could imply that
the phenomenon, which has had its location defined by one of the terms listed in paragraph 2.3 above, is
expected to “move north at 15 knots”. However, is the speed of movement defining the speed of
movement of the boundary within which the phenomenon is occurring (see A below), or is it describing
the speed of movement of the phenomenon within the boundary (see B below)?
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FIR boundary
FIR boundary
A
B
15 KT
55N
55N
15 KT
Graphical SIGMET provisions are shown in Appendix 1 to Annex 3. Of note, MODEL SGE
(extract below) could be considered to imply that the boundary of the phenomenon is moving
bodily NE at 40KT (or KMH?):
However, the MODEL SVA for a graphical volcanic ash SIGMET does not include a speed of
movement, and instead simply provides an observed (solid line) and forecast position (dashed
line) at the start and end of the period of validity of the SIGMET:
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2.9
The current structure of Table A6-1 can be limiting from a forecasting perspective since
the provisions do not allow for more than one directional speed of movement. One way to overcome this
could be to provide an observed or forecast position at the start of the SIGMET validity period, and a
forecast position at the end of the SIGMET validity period. For example, “….SEV MTW OBS N OF
LINE N5500 AT 1200Z …… FCST 1600Z N OF LINE N6000 E00000 – N5600 W01000”. This
working practice is presently permitted for SIGMET related to Tropical Cyclones and Volcanic Ash;
however there is no such provision for the other weather phenomenon listed at paragraph 1.1.4 of
Appendix 6 to Annex 3, or for AIRMETs. If provisions were to be introduced, in our example the
operator would know that the phenomenon was observed north of 55 degrees north at the start of the
SIGMET validity period (1200Z), but by the end of the SIGMET validity period (1600Z) it was expected
to line north of line through 60N 00E to 56N 10W. However, careful consideration should be given to
whether MWOs would have the capability to undertake such a recommendation.
Perceived benefits and limitations of the proposed arrangements:
2.10
The proposal from EANPG Conclusion 48/39 is to consider allowing only the use of a
closed line of coordinates, or location indicators of waypoints or airports, in order to describe the area of
the phenomena in a SIGMET and/or AIRMET. To put this into perspective, the following would only be
permissible to describe the location of a phenomenon in SIGMET and AIRMET messages:
a) WI Nnn[nn] or Snn[nn] Wnnn[nn] or Ennn[nn] – Nnn[nn] or Snn[nn] Wnnn[nn] or
Ennn[nn] – Nnn[nn] or Snn[nn] Wnnn[nn] or Ennn[nn] [– Nnn[nn] or Snn[nn]
Wnnn[nn] or Ennn[nn] – Nnn[nn] or Snn[nn] Wnnn[nn] or Ennn[nn]]; or
b) iWI nnn[nn] – nnn[nn] – nnn[nn] [– nnn[nn] – nnn[nn]]; or
c) iWI nnnn – nnnn – nnnn [– nnnn – nnnn].
where part i) describes the use of a closed line of co-ordinates (as is currently provisioned); part ii)
describes the use of a closed line of waypoints; and part iii) describes the use of a closed line of airports.
Given that the current provisions within Annex 3 only appear to permit the use of 3, 4 or 5 co-ordinates to
describe the location of a phenomenon, the same principle has been applied in the examples at parts i) to
iii) above. An airport identifier will always be 4 characters in length, whilst a waypoint will either be 3 or
5 characters. It may be wiser perhaps use ccc[cc] or cccc to describe the use of waypoints or airport IDs.
This would ensure that there is no confusion with the practices that are permissible for defining
latitude/longitude points in SIGMET or AIRMET messages.
2.11
It must be remembered that the vast majority of FIRs in the world do not conform to
perfect North-South or East-West oriented boundaries. Indeed, it is only really oceanic FIR boundaries
that do follow a rule resembling this. Most FIR boundaries are non-linear, following the contour of a
national or international border. To eliminate many of the provisions listed in Table A6-1 (and shown at
paragraph 2.3 above), and instead to only allow for those listed at paragraph 2.10, could make the
describing of a phenomenon particularly challenging for the respective MWO. As outlined already, the
current SIGMET/AIRMET provisions for describing a closed line of co-ordinates only permit 3
(maximum 5) co-ordinates to be used.
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Consider the Switzerland FIR (LSAS) shown below. Supposing SEV MTW (severe mountain
waves) were expected south of the red dashed line at N4700. At the present time, the MWO
would simply be able to state “….SEV MTW FCST S OF N4700….”. However, under the new
proposals outlined in 2.10 above, the MWO would have to try to describe the area within a
polygon with no more that 5 turning points. This could be virtually impossible for a non-linear
FIR boundary, for example:
As the example above shows, to try to describe the phenomenon that extends only as far as the
Swiss border with Austria, Italy and France would be very difficult, without part of the closed
line of co-ordinates causing the polygon to encroach into a neighbouring FIR, or without the use
of a larger number of polygon turning points. Given that SIGMET and AIRMET should ideally
be succinct, it is perhaps questionable whether this would be possible given the apparent
limitations that the proposals introduce.
2.12
The use of waypoints to describe locations within SIGMET and AIRMET is not currently
standard practice. Most MWO have no day-to-day association/involvement with waypoints, and therefore
each MWO would need to be provided with a global database of waypoints that included the waypoint
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name, abbreviation (ID) and latitude/longitude as a minimum. Such a database would need to be
carefully managed and distributed (or made available) to all MWO, and updated when revisions are made.
2.13
As outlined in paragraph 2.10, a waypoint will either be composed of 3 or 5 characters.
In the case of a waypoint over a land area, the waypoint will typically be denoted by a 3-letter
combination – for example, “DVR” is the abbreviation for the Dover waypoint in the UK. Over oceanic
areas, the situation is somewhat different, and instead uses 5 characters (letter and number combination).
The following table summaries waypoint notation for oceanic routes, based upon an example at 60N and
60S:
Latitude / Longitude range
Current SIGMET / AIRMET notation
Waypoint notation
60N 180W to 60N 100W
N60[00] W180[00] to N60[00] W100[00]
60N80 to 60N00
60N 099W to 60N 001W
N60[00] W099[00] to N60[00] W001[00]
6099N to 6001N
60N 000E to 60N 099E
N60[00] E000[00] to N60[00] E099[00]
6000E to 6099E
60N 100E to 60N 179E
N60[00] E100[00] to N60[00] E179[00]
60E00 to 60E79
60S 180W to 60S 100W
S60[00] W180[00] to S60[00] W100[00]
60W80 to 60W00
60S 099W to 60S 001W
S60[00] W099[00] to S60[00] W001[00]
6099W to 6001W
60S 000E to 60S 099E
S60[00] E000[00] to S60[00] E099[00]
6000S to 6099S
60S 100E to 60S 179E
S60[00] E100[00] to S60[00] E179[00]
60S00 to 60S79
Waypoints “60N80” and “60N00” are examples of northern hemisphere waypoints at N6000
W18000 (60’00’N 180’00’W) and N6000 W10000 (60’00’N 100’00’W). Those unfamiliar with
waypoint notation are encouraged to look closely at how the notation switches according to
northern or southern hemisphere, and east or west orientation relative to 100W and 100E lines of
longitude.
Waypoint notation is quite different to the current practice used to denote co-ordinates in
SIGMET and AIRMET messages. To introduce waypoints into Annex 3 provisions would
require careful consideration since the MWOs would require detailed guidance and/or training in
how waypoints are composed and applied.
A further consideration is to note that oceanic waypoints cannot be described more accurately
than to the whole degree. At the present time, a co-ordinate within a SIGMET/AIRMET can be
described to the nearest degree and minute – e.g. N5538 W13002. In oceanic waypoints, such a
point would probably have to be rounded to N5600 W13000, and termed ‘56N30’ in waypoint
notation.
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2.14
The use of airports to describe locations within SIGMET and AIRMET is currently
allowed within Annex 3 provisions – however, only to the extent that an airport identifier (ID) could be
used in the “N OF nnnnnnnnnnnn” context (for example, “N OF EGLL”). Current provisions do not
allow for airport identifiers to be used for turning points of a polygon – for example, “WI EGLL – EGHI
– EGCC – EGGW” (“within area bounded by London Heathrow airport to Southampton airport to
Manchester airport to London Luton airport”). It could be considered that there could be a certain amount
of merit introducing such a practice, since the area of the phenomenon could be easily described in
abbreviated plain language. However, what happens if the phenomenon was expected to occur in a region
sparse of airports – such as the 70% of the world that is covered by oceans? Could the SIGMET contain a
mix of airport IDs with traditional latitude/longitude points or waypoints? Furthermore, would the use of
airport IDs be limited only to international airports, or would/should it include domestic and military
installations?
2.15
Whilst the use of waypoints and airport IDs could be viewed as beneficial, the limitations
that exist in their application in SIGMET and AIRMET messages could be considered detrimental if
noting the remarks in 2.11 to 2.14 in particular. In addition, to only permit the use of a closed line of coordinates could lead to innumerable issues for those generating the messages and for those trying to
interpret the information quickly, easily, and above all accurately. One possibly approach could be to
permit the use of waypoints or airport IDs in a form such as “50NM N of 60N80” or “50NM N of
EGLL”. But again, this would need to be carefully considered. The proposal for introducing radial
distance from the waypoints is to avoid over forecasting or otherwise closing off usable airspace because
there is no alternative available by using waypoints to be more precise in describing the location of hazard
and its forecasted movement.
3.
LIST OF RECOMMENDATIONS
3.1
The following list of recommendations is for the consideration of the Met Warnings
Study Group with the sole intention of aiding further debate rather than being a definitive list of
requirements. There are clearly aspects within the current SIGMET and AIRMET provisions that are
beneficial and limiting from a provider and operator perspective. A similar remark could be attributed to
the tabled proposal.
3.2
A closed line of co-ordinates is already permissible in SIGMET and AIRMET messages
according to Table A6-1. For example, “WI N5000 E00000 - N5500 E00000 - N5500 W01000 - N5000
W01000”. However, the proposal by the EANPG is to eliminate the other (traditional) forms of
describing a location or area within an FIR – for example, “N OF 50N”. Noting the comments above, in
particular paragraph 2.11 regarding non-linear FIR boundaries, the study group may wish to agree that to
eliminate the present means of describing location may be detrimental to MWO’s whose responsibility it
is to best describe the location of a phenomenon without any unnecessary descriptive material.
3.3
As described in paragraph 2.8 above, movement of a phenomenon can occasionally be
challenging to describe in a SIGMET or AIRMET, especially if the phenomenon is changing spatially
through time. Therefore, perhaps of more value, would be to allow the use of a closed line of coordinates to describe the location of the phenomenon at the start of the validity period, and a forecast
location at the end of the validity period of the SIGMET or AIRMET. For example, observed position
WI N5000 E00000 - N5500 E00000 - N5500 W01000 - N5000 W01000 at 1200Z, forecast position WI
N5200 E00500 - N5700 E00500 - N5700 W01500 - N5200 W01500 at 1600Z. Such a provision is
already included for SIGMET messages that relate to Tropical Cyclones and Volcanic Ash Clouds.
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Recommendation 1:
The study group may wish to remove reference to locations or geographic features well known
internationally (refer to recommendation 6 below), but maintain all the other Table A6-1 standards for
describing location in a SIGMET or AIRMET, including a closed line of co-ordinates.
Recommendation 2:
The study group may wish to discuss further the maximum number of co-ordinates that are permissible
when describing location within a SIGMET and AIRMET, as outlined in paragraphs 2.6 and 2.11 above.
Recommendation 3:
Noting the remarks in paragraph 2.8, the study group may wish to seek further clarification from ICAO
on the interpretation of speed of movement (MOV) when used in text and graphical forms of SIGMET
and AIRMET.
Recommendation 4:
In addition to the current provisions for TC and VA, the study group may wish to consider the
introduction of an observed or forecast location at the start of the SIGMET/AIRMET validity period, and
a forecast location at the end of the validity period, for all other phenomena described at paragraph 1.1.4
of Appendix 6 to Annex 3. Careful consideration should be given to whether MWO would have the
capability to undertake such a recommendation. In addition, the group should carefully consider whether
such a practice is limited to a closed line of co-ordinates alone, or whether is could/should be applicable
to the other forms of describing location in Table A6-1 (e.g. “N OF….”); and whether there would
continue to be a need for speed of movement (“MOV”) information.
3.4
Noting the comments in paragraphs 2.13 relating to waypoint co-ordinates, in particular
that their abbreviated form differs from current SIGMET/AIRMET co-ordinate standards, that oceanic
waypoints have to be to the nearest whole degree, and that there would be a need for a routinely updated
global database, the study group may wish to agree that it would be unwise to progress with this proposal.
Recommendation 5:
The study group may wish to consider rejecting the proposal to introduce the use of waypoints in
SIGMET and AIRMET messages.
3.5
Whilst noting the comments in paragraph 2.14, in particular that internationally known
airport IDs are already permissible within SIGMET and AIRMET messages and will therefore be familiar
to all concerned, the study group may wish to agree that there are also limitations within the current
provisions. International airport IDs can only be used in the context of “N OF ….”, “S OF ….” etc. They
cannot be used in, say, a closed set of co-ordinates. As previously described, “N OF EGLL” does not
make clear exactly how far north of London Heathrow airport the phenomenon is expected to occur, or is
occurring. In addition, what happens in those parts of the world where international airports are sparse
(e.g. oceanic areas or sparsely populated continental areas)?
3.6
The use of geographic features that are well known internationally could be ambiguous.
What one person may consider to be ‘well-known internationally’ may not be ‘well-known’ by someone
else. Indeed, their exact location could be ambiguous. For example, most people would agree that
London is ‘well-known internationally’, but what is the exact location of London? Is it N5132 W00005
(city centre), or is it N5128 W00027 (Heathrow airport)? Such ambiguity could lead one to the
conclusion that geographic locations should be removed from Annex 3.
Recommendation 6:
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The study group may wish to consider eliminating the current Annex 3 provisions that permit the use of
airport IDs and geographic features that are well known internationally.
4.
4.1
ACTION BY THE METWSG
The METWSG is invited to:
a) note the information in this paper; and
b) consider the recommendations provided in Section 3.
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