AMOSSG/4-SN No. 11

21/8/03

AERODROME METEOROLOGICAL OBSERVING SYSTEMS STUDY GROUP (AMOSSG)

FOURTH MEETING

Montreal, 15 to 18 September 2003

Agenda Item	5:	Aerodrome observation requirements
5.1:	Reporting requirements and criteria

ADDITIONAL REGULATION WITH RESPECT TO THE FM 15 METAR, FM 16 SPECI AND FM51 TAF CODE, REGARDING THE DESCRIPTOR "FREEZING" (FZ) IN CASES OF FOG (FG) AT AN AERODROME

(Presented by Dennis Hart)

SUMMARY
Current coding practice of present weather in METAR/SPECI and Local MET Report makes no distinction between different types of fog on an aerodrome and their different operational consequences when the temperature is below zero. These different situations are all encoded as Freezing Fog (FZFG). This study note suggests a possible solution to give a clear description of the fog type even in combination with low temperatures.

1.	INTRODUCTION

1.1	In a normal winter period the weather situation can be described by:

—	Fog is observed or is forecasted over the aerodrome;

—	The observed or forecasted fog is not covering the airport in total, so one of the descriptors MI, PR or BC applies, and

—	The observed air temperature is below zero or is forecasted below zero.

1.2	In this weather scenario the existing encoding practice, regulated by ICAO Annex 3 and the WMO No306 Manual on Codes is not sufficient enough for a correct and complete description of the weather conditions at the aerodrome. The regulation is limiting the operational use of the aerodrome and can result in encoding problems by men and/or machine.

2.	DISCUSSION

2.1	When MIFG, PRFG or BCFG and T < 0º C applies present weather in METAR/SPECI, Local MET Report and TAF should be encoded as FZFG (see Appendix A). However, this encoding practice can result in ambiguous situations.

2.2	In the described cases of fog it is obvious that only a part of the aerodrome is covered by fog but encoding FZFG supposes that freezing fog covers the complete aerodrome. Even in cases of shallow fog, MI, and a temperature below zero the fog should be encoded as FZFG. The possible user interpretation can be that the whole aerodrome is covered -or forecasted to be covered- by freezing fog.

2.3	This coding practice limits the operational use of an aerodrome. In the situation with fog and a temperature above zero an aerodrome (partially) can be used due to the clear distinction between areas with or without fog and the height of the fog. However, this distinction cannot be made when the temperature is below zero. Operational limitations, such as LVP, will be put in place for the whole aerodrome where only a part of the aerodrome encounters fog or shallow fog.

2.4	The obligation to give a correct and complete description of the weather conditions, observed or forecasted, on an aerodrome has to be fulfilled. To overcome the mentioned operational limitations, the observer and forecaster have to make a choice how to encode present weather in the case of an observed or forecasted fog situation with temperatures below zero. The observer and forecaster are limited by the coding practice in encoding the "real" present weather situation at an aerodrome.

2.5	Differences between observers, forecasters and States are possible in dealing the described encoding limitation. For example, a currently used encoding practice in some States is the deletion of the descriptor FZ, in cases where the temperature group indicates that the air temperature is below zero. The combination of partial fog (PRFG) combined with a temperature group TT/TdTd (M01/M03) can still indicate that the observed partial fog at the aerodrome is freezing.

2.6	When the observing practice regarding prevailing visibility will be in implemented (Amendment 73 ICAO Annex 3), the operational advantage of this practice is often limited in cases where there is partly fog and temperature below zero due to the current present weather encoding practice. As a result of the implementation of prevailing visibility can be in contradiction with the current practice regarding the encoding of freezing fog. Again this can lead to differences between observers, forecasters and States in dealing with this encoding limitation.

3.	CONCLUSION

3.1	To overcome self-interpretation by observer, forecaster and States in cases of encoding different types of fog during temperatures below zero, a uniform and clear modification of FM 15 METAR, FM16 SPECI, FM51 TAF and Annex3 should be implemented.

3.2	It is proposed to add the descriptor FZ to indicate freezing conditions, even when the descriptors MI, PR or BC already apply and are encoded. This will result in the combinations FZMIFG, FZPRFG or FZBCFG (see Appendix).

3.3	In line with this proposed modification it can be worthwhile to study the operational need for the descriptor FZ (freezing) in case of mist (BR) and temperatures below zero. The current practice doesn't allow this present weather type (FZBR). At the moment, the distinction between freezing fog and mist is based on a threshold for visibility, 1000 meter, and not for the "freezing" abilities of water droplets which is more an operational limit.

4.	ACTION BY THE GROUP

4.1	The group is invited to:

a)	to note the content of this paper;

b)	to agree on the proposal stated in paragraph 3.2;

c)	to agree on the proposal stated in paragraph 3.3;

d)	to urge the implementation of the proposed modification(s) in applicable WMO and ICAO regulations, and

e)	to undertake any further action, if necessary.

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