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.
I would say must be a typo, or a non-standard report.
ICAO defines gust in Appendix 3,section 22.214.171.124 c) as an increase of mean reported speed by 10kts or more
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.