METAR wind and your runway

The METAR wind is true

From the Aeronautical Information Manual, on reading the wind group of a METAR:

The first three digits are the direction the wind is blowing from, in tens of degrees referenced to true north, or “VRB” if the direction is variable. The next two digits is the wind speed in knots, or if over 99 knots, the next three digits. If the wind is gusty, it is reported as a “G” after the speed followed by the highest gust reported. [AIM 7-1-29 b 5]

So 20015KT is wind from 200° true at 15 knots.

Runways and the tower’s wind are magnetic

Runways are identified by numbers that indicate the nearest 10-degree increment of the azimuth of the runway centerline. For example, where the magnetic azimuth is 183 degrees, the runway designation would be 18; for a magnetic azimuth of 87 degrees, the runway designation would be 9. Wind direction issued by the tower is also magnetic and wind velocity is in knots. [AIM 4-3-6 a]

ATIS and the automated stations speak their wind in magnetic degrees too:

Wind direction is reported relative to magnetic north in ATIS as well as ASOS and AWOS radio (voice) broadcasts. [AIM 7-1-10 d 3 (c) note]

Converting true to magnetic

The Pilot’s Handbook of Aeronautical Knowledge, on converting a true direction for flight:

…the true direction must be converted into magnetic direction for the purpose of flight. This conversion is made by adding or subtracting the variation indicated by the nearest isogonic line on the chart. [PHAK p. 16-7]

Remember, if variation is west, add; if east, subtract. [PHAK p. 16-8]

That rule is written for courses and headings. Applying it to a wind direction is the same arithmetic: with westerly variation you add it to the true wind direction, with easterly you subtract it, and you keep the result between 1° and 360°. This calculator does not do the conversion; do it first and type the magnetic direction.

How much it changes the answer

An illustration, with a made-up variation of 15° west: the report says 20015KT and the runway is 18 (180°).

• Typed in as reported, 200°: angle 20°, crosswind = 15 × 0.3420 = 5.1 → 5 knots.
• Converted first, 200° + 15° = 215° magnetic: angle 35°, crosswind = 15 × 0.5736 = 8.6 → 9 knots.

Same report, same runway, 3.5 knots apart. The size of the gap depends on the variation and on how the wind and the runway line up.

When the METAR says VRB

The AIM quoted above says VRB is used “if the direction is variable”. There is then no single direction to enter, so one set of components cannot be worked out from the report alone.

Use it

The calculator takes the direction in the same degrees as the runway. For the sums behind it, see the formula.