Aviation Stack Exchange is a question and answer site for aircraft pilots, mechanics, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
I have never seen the "VRB" (for wind direction variations < 60°; e.g. VRB12003KT) and "G" annotation (for gusts > 10KT wind speed difference; e.g. 12003GT30KT) being combined in a METAR.
Until now I have just seen:
Why does this never happen? Is there a rule and is it not possible or does it just never happen?
Yes, it is possible for a METAR to contain both VRB and G. VRB10G25KT is valid.
The reason it rarely happens is because, as wind speed increases, major variations in wind directions tend to disappear.
As other answers say, yes, this is valid. It appears approximately 150K times in our database of 85 million METARs[1]. This database is built from data provided by NOAA and covers roughly 16 months.[2]
Here are several real examples from February 2019.
KBML 011110Z AUTO VRB06G18KT 7SM OVC029 M16/M22 A3007 RMK AO2 T11611217
KSZY 011115Z AUTO VRB06G11KT 10SM FEW050 BKN070 OVC110 08/M04 A3020 RMK A01
ENST 011120Z VRB13G28KT CAVOK M02/M11 Q1011 RMK WIND 300FT 11027G39KT
LFBH 011130Z AUTO 16009KT 9999 FEW042 OVC068 07/05 Q0985 TEMPO VRB15G30KT 3000 SHRA BKN020CB
LFRS 011130Z AUTO 20009KT 9999 FEW016 BKN200 07/05 Q0983 TEMPO VRB15G25KT 2000 TSRA BKN010 BKN018CB
85% of METARs containing this pattern are marked as AUTO. 11% of the time the pattern occurs after TEMPO, though it could also occur elsewhere in the METAR.
I did a quick, simple pattern match to obtain these numbers so they are approximate.
[1] - This database is part of the VOCUS product suite. I work for the company that makes VOCUS. We use weather data to analyze risk prior to flight, and to provide post flight analysis.
[2] - I have an archive of an additional 201 million older METARs if anyone is really interested in numbers against a larger data set.
To elaborate on a previous answer, large changes in wind direction on short timescales tend to be caused by thermal updrafts passing through the area, tending to "suck" the wind toward their centers.
When the overall windspeed is high, the (relatively) small changes in wind velocity (speed and direction) caused by this phenomenon tend to be dwarfed by the overall prevailing wind.
In addition, strong wind tends to suppress thermal convection, at least near the ground.
On a windy day, there still can be some significant changes in wind speed, caused by turbulent "rotors" (tumbling masses of air) intermittently mixing the stronger wind aloft down to the surface, but these tend not to involve extreme changes in wind direction. This intermittent turbulent mixing will be more pronounced on a sunny day (or with an unstable airmass) than on a cloudy day (or with a stable airmass).
It is useful to know that even on a windy day, there can be some significant change in wind direction as the turbulent "rotors" make the wind speed increase and decrease, and it typically happens in a systematic way-- due to a phenomenon called the Eckman spiral, the wind aloft tends to be more westerly (in the northern hemisphere, and more southerly in the southern hemisphere) than the wind at the surface. So as the wind gusts up to a stronger value, it typically "veers" in the direction that is aligned with the wind higher up. This is good to know if you are a pilot-- and is absolutely essential to know if you are a racing sailor.
Similarly, as your aircraft rises up through the air after take-off, it will typically (in the northern hemisphere) tend to "feel" a temporary increase in apparent wind from the right (or a temporary decrease in apparent wind from the left), which may have some noticeable affect on handling. And the reverse on landing. And all the reverse in the southern hemisphere. And all disappearing or being greatly reduced near the equator. Since these effects are dependent on a marked change in wind direction with altitude, they tend to be more pronounced on a cloudy day or with a stable airmass, and less pronounced on a sunny day or with an unstable airmass, when the airmass tends to be more "churned up" by turbulent mixing.
It is possible as @tim-medora indicated in his answer. Part of the reason this is possible is in the timeframe which each part of the wind information is gathered. Wind direction and speed is averaged over two minutes while gusts are the maximum gust over the previous ten minutes.
Office of the Federal Meteorological Handbook - Surface Weather Observations and Reports
Wind Direction.
The wind direction shall be determined by averaging the direction over a 2-minute period. When the wind direction sensor(s) is out of service, at designated stations, the direction may be estimated by observing the wind cone or tee, movement of twigs, leaves, smoke, etc., or by facing into the wind in an unsheltered area.
Variable Wind Direction
The wind direction may be considered variable if, during the 2-minute evaluation period, the wind speed is 6 knots or less. Also, the wind direction shall be considered variable if, during the 2-minute evaluation period, it varies by 60 degrees or more when the average wind speed is greater than 6 knots.
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.
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.
VRBmeans there is no recognizable prevailing wind direction at all and so the lettersVRBreplace the direction. If there is a prevailing direction, the12003KT 090V150form is used. $\endgroup$