The wind group of a METAR, laid on a runway, gives two numbers: what the wind pushes along the axis — headwind or tailwind — and what it pushes across. That is what the site's runway plan draws, runway by runway, from the Dublin report taken here as an example.
Reading the wind group
21015KT: the wind comes from 210, in true degrees, at fifteen knots. G introduces gusts; VRB a wind without direction; 00000KT calm. A second group, 160V240, says between which directions it varied.
A wind is named by where it comes FROM: a wind from 210 blows towards 030. On a runway 21, whose heading is close to 210, it blows head-on.
METAR EIDW 101030Z 21015KT 160V240 9999 -RA FEW017 SCT020 BKN022 18/14 Q1007 NOSIG
The calculation
The angle between the wind and the runway decides everything. The headwind component is the speed multiplied by the cosine of that angle; the crosswind, the speed multiplied by its sine. At Dublin, runway 16 has a true heading of 163°: the angle with a wind from 210 is 47°, the headwind 15 × cos 47° = 10 knots, the crosswind 15 × sin 47° = 11 knots, from the right.
Without a calculator, the rule of thirds is enough in flight: at 30° of angle, the crosswind is half the wind; at 45°, seven tenths; at 60°, almost all of it; at 90°, all of it. And for the headwind, the reverse.
On the opposite runway, 34, the same wind blows from behind — 10 knots of tailwind — and the crosswind changes side.
True and magnetic
The wind direction of a METAR is true, relative to geographic north. The number painted on a runway is magnetic, rounded to ten degrees: Dublin's 16 points to 163° true, and at New York runway 04 points to 031°. The site takes the true headings of the source when it gives them; when only the number remains, it writes it with a "≈".
The difference matters little in western Europe, a few degrees; it matters in the north and in America, where the variation exceeds ten or twenty degrees.
Gusts and variable wind
The components are also computed on the gust: 14013G24KT on a runway 10 gives 10 knots of headwind on average and 18 in gusts. The site shows them side by side, and the light end of the bar is what the gust adds.
A variation sector, 160V240, is drawn as an arc on the rose; the components stay those of the mean direction. A VRB or calm wind has no components: it has no direction.
Choosing the runway
The best aligned runway is the one where the wind is most head-on: that is geometry, and it is the one the site chooses on arrival. It is not necessarily the runway IN USE: traffic, noise, terrain and works decide too, and only the ATIS or the controller says so — on the site's ATIS listening, the announced runway replaces the best aligned one.
The crosswind limit is your aircraft's, the "demonstrated crosswind" of the flight manual, and your experience's. The report gives the number; the decision stays yours.