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I'm currently building a weather station from scratch. It measures temperature, pressure, humidity, wind speed and direction. The end result shall be a weather station that sends out METAR weather messages so pilots can see live weather in our area.

Now I'm at the point where I'm writing the software and I have only little knowledge about meteorology. More specifically I ask myself when a meteorologist speaks about "gusts". I know it's a wind speed peak, but I'm a bit puzzled how much faster compared to the average wind speed a gust must be to be actually called gust? I was searching for some definitions, but I didn't really find anything useful.

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A US definition is:

Wind Gust: Is a sudden, brief increase in speed of the wind. According to U.S. weather observing practice, gusts are reported when the peak wind speed reaches at least 16 knots and the variation in wind speed between the peaks and lulls is at least 9 knots. The duration of a gust is usually less than 20 seconds.

So it must be 9 knots above the non-gust background.

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  • $\begingroup$ I see where you got this. However do you think this can be also applied elswhere? I mean ins't there a worldwide standard? $\endgroup$ – binux May 23 '17 at 18:34
  • $\begingroup$ @binux I can't answer that, but there is another US definition here ofcm.gov/publications/fmh/FMH1/FMH1.pdf saying 10 knots between peak and lull is required. $\endgroup$ – DavePhD May 23 '17 at 18:47
  • $\begingroup$ Okay so since this is the easiest to implement I will accept your answer. It makes most sense to keep it simple. Based on @JeopardyTempest answer it will need some custom adjustments. $\endgroup$ – binux May 24 '17 at 22:36
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Doesn't appear to be great sources out there for some of this info! (But then, that's what StackExchange has great promise in being!)

In the US, this manual for the official instrument setup, as well as this old manual on how to take observations gives quite a bit of good detail on how wind observations are done...

In the US, wind speed is the most recent 2 minute average.

A thorough explanation for gusts for ASOS, which is about as readable as syrup:

In the manual procedure, a gust is reported when an observer sees rapid fluctuations in sensor wind speed indications with a variation of 10 knots or more between peaks and lulls during the 10-minutes before the observation. The reported gust is taken from the maximum “instantaneous” wind speed observed during this period. The average 2-minute wind is used to report wind direction and wind speed. Conceivably, an average 2-minute wind speed as low as 3 knots (observed in the last minute) may be reported with a gust of 10 knots (observed in the last 10 minutes). Observations of 5 knots with gusts of 10 to 15 knots, however, are the more common minimum values reported.

The ASOS algorithm also relies on a 10-minute observation period to determine gusts, but uses it in a different way. Once every 5 seconds, the ASOS computes the greatest 5-second average wind speed (and corresponding direction) during the past minute, and once each minute stores this information in memory for 12 hours.

Once every 5 seconds the ASOS computes the current 2-minute average wind speed and compares it with the greatest 5-second average wind speed during the past minute. If the current 2-minute average wind speed is equal to or greater than 9 knots and the greatest 5-second average wind speed (during the past minute) exceeds the current 2-minute average speed by 5-knots or more, then the greatest 5-second average speed observed during the past minute is stored in memory as a gust for 10 minutes. Once every 5 seconds, the ASOS compares the highest gust stored in memory for the past 10 minutes with the current 2-minute average wind speed. If the difference between the two is 3 knots or more, the current reported wind speed is greater than 2 knots, and the highest gust exceeds the minimum 5-second wind speed in the past 10 minutes by 10 knots or more, then the highest gust stored in memory is designated as the reportable gust. This value is appended to the current wind direction and speed reported in the OMO, computer-generated voice messages, and the METAR/SPECI reports. The minimum gust speed reported by ASOS is 14 knots. Wind speeds from 3 knots and 11 knots may be reported with gusts to 14 knots. For example, a 2-minute average wind of 240 degrees at 10 knots with gusts to 20 knots is reported as:

“24010G20KT.

Of course, the wind speed rules aren't standardized globally, as this WRAL meteorologist discusses. In fact, they aren't even entirely consistent in the US, as the Hurricane Center uses a 1-minute average for sustained winds for hurricane intensity, and 3-5 second period for gusts.

So, even having done coursework on meteorological measurement systems, it's about as muddled to me now as it is to you. Probably report the most recent 2 minute average as the actual wind, and the highest instantaneous or 5-second average as the gust? (with requirement that it exceeds set criteria as DavePhD's answer alludes to, if you wish)

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    $\begingroup$ That's interesting. Especially since this definition is with a METAR context. However this seems to be far too complicated to implement for my basic use. My ultimate goal is it to give pilots in my area a hint on the weather. Gusts therefore are a very important element. Taking your last statement I will probably stick to the answer found on the website of the US weather service. However I probably will lower the bar when I count a temporary wind as gust. Still - I dislike that the METAR format didn't also specify how its values are computed... $\endgroup$ – binux May 23 '17 at 18:43
  • $\begingroup$ @binux Agree with all you said :-) $\endgroup$ – JeopardyTempest May 23 '17 at 22:07

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