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KADS 072350Z 31030G20KT 13SM SKC 24/02 A2980

I’ve never seen a negative gust factor before. Is that a real thing (and if so, what does it mean), or did someone just make a typo?

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WMO standard defines a gust as the maximum wind speed exceeding the "mean speed" by 5 m/s (10 knots) during the 10-minute interval. With this definition, I would never expect to see a negative gust factor.

However, the FAA defines it differently (FMH-1):

5.4.3 Wind Speed. The wind speed shall be determined by averaging the speed over a 2-minute period.

5.4.4 Wind Gust and Squall. The wind speed data for the most recent 10 minutes shall be examined to evaluate the occurrence of gusts or squalls. Gusts are indicated by rapid fluctuations in wind speed with a variation of 10 knots or more between peaks and lulls.

Thus, by my interpretation, a U.S. airport METAR can report gust speeds below the mean reported wind speed since it only requires a variation of 10 knots, not an exceedance of 10 knots.

However, reading further in the same document:

5.6 Summary of Wind Observing and Reporting Standards

Wind speed: 2-minute average speed in knots is reported.
Wind gust: The maximum instantaneous speed in knots in the past 10 minutes is reported.

It seems impossible to have a 2-minute average speed which is higher than the maximum instantaneous speed over the past 10 minutes. So following that, it would seem to indicate an error in the measuring or reporting equipment.

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  • $\begingroup$ FAA definition seems reasonable, eg. if you have a constant 30 kts headwind that occasionally drops to 0 for a few seconds, that would be important to know $\endgroup$
    – Radu094
    Apr 8 at 19:03
  • $\begingroup$ Updated with additional info. I think the definition is somewhat incongruous with the reporting standards. $\endgroup$
    – hemp
    Apr 8 at 19:08
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    $\begingroup$ It doesn’t say “AUTO”, and the tower should have been open at that time, so my money is on human (rather than equipment) error. $\endgroup$
    – StephenS
    Apr 8 at 20:14
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I would say must be a typo, or a non-standard report.

ICAO defines gust in Appendix 3,section 4.1.5.2 c) as an increase of mean reported speed by 10kts or more

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  • $\begingroup$ That is not the definition of gust. Per the document linked, that is a criteria used for determining when to issue a SPECI METAR. $\endgroup$
    – hemp
    Apr 8 at 18:44
  • $\begingroup$ Sorry, wrong copy+paste inserted wrong link. Edited and corrected right away, but internet is fast...see link now $\endgroup$
    – Radu094
    Apr 8 at 18:47
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Gusts lower than the posted wind speed will never be posted on the METAR in the US. By definition per the FAA, the wind speed will be posted as a mean average. Where as the gust will be posted as the maximum wind speed during that time period. You calculate the gust factor by subtracting the mean average from the maximum wind speeds. Since the maximum of anything will be greater than its average, there can never be a negative gust factor.

Another point to make is that since the wind speed posted is a mean average and the gust posted is the maximum, the gust factor works in roughly both directions from the wind speed. In the case of winds reported as 31030G20KT, this would make me believe it to be a typo. Or, the straight line wind gusts were strong enough to temporarily blow in the opposite direction. The question would be why the METAR does not show that as the wind shifting between the directions.

5.4.3 Wind Speed.
The wind speed shall be determined by averaging the speed over a 2-minute period. At designated stations, Table 5-1 shall be used to estimate wind speeds when instruments are out of service or the wind speed is below the starting speed of the anemometer in use.
5.4.4 Wind Gust and Squall.
The wind speed data for the most recent 10 minutes shall be examined to evaluate the occurrence of gusts or squalls. Gusts are indicated by rapid fluctuations in wind speed with a variation of 10 knots or more between peaks and lulls. Squalls are indicated by a sudden onset of wind where the speed increases by at least 16 knots and is sustained at 22 knots or more for at least a minute. The speed of a gust or a squall shall be the maximum instantaneous wind speed.
5.4.5 Peak Wind Speed.
Peak wind data shall be determined with wind speed recorders. The peak wind speed shall be the maximum instantaneous speed measured since the last routine METAR.

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    $\begingroup$ Never? I posted one in the Q. $\endgroup$
    – StephenS
    Apr 9 at 14:02
  • $\begingroup$ Yes, that would make me think it was a typo. Or, winds gusts so strong that it blew in the exact opposite direction without shifting through the other directions. Even though the winds have been past two or three days, I would vote for typo. Would it be possible for you to post the source of the METAR (not the ADS ATIS, but the service you used to receive it)? Maybe it was a transcription error and is different from other sources. $\endgroup$
    – Dean F.
    Apr 9 at 14:09
  • $\begingroup$ @StephenS - Nevermind. I pulled the historic METARs from the Aviation Weather Center. It appears to be a definite anomaly. KADS 080047Z 28009G16KT 10SM SKC 23/01 A2983. KADS 072350Z 31030G20KT 13SM SKC 24/02 A2980. KADS 072252Z 28015G23KT 13SM SKC 24/02 A2980. KADS 072147Z 29014G25KT 10SM SKC 25/01 A2979. KADS 072047Z 30014G27KT 10SM SKC 25/M01 A2980. KADS 071949Z 30015G23KT 13SM SKC 25/00 A2982. KADS 071848Z 29013G24KT 13SM SKC 22/03 A2985. KADS 071747Z 30014G25KT 10SM SKC 21/04 A2986. KADS 071650Z 32014G20KT 10SM SKC 20/05 A2986. $\endgroup$
    – Dean F.
    Apr 9 at 14:17
  • $\begingroup$ @StephenS - I wish Airpark Dallas had a METAR. I doubt the guys I know there would have been at the airport at that time for an on the ground POV. But, neither Mesquite nor McKinney shows a similar anomaly. KTKI 072353Z 29010G19KT 10SM CLR 23/04 A2979 RMK AO2 PK WND 27026/2334 SLP086 T02280044 10244 20200 52002. KHQZ 072350Z 27016G25KT 10SM CLR 23/02 A2981 $\endgroup$
    – Dean F.
    Apr 9 at 14:37
  • $\begingroup$ I don’t see anything similar at KDAL, KDFW or KDTO either. My first thought was human error, but I wanted to check in case there was some weird rule I wasn’t aware of. $\endgroup$
    – StephenS
    Apr 9 at 14:48

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