Computing Magnetic Wind Direction
The National Weather Service (NWS) has received several inquiries concerning the computation of magnetic wind reports from the Automated Surface Observing System (ASOS). ASOS encodes wind reports with respect to true north in all METAR and SPECI reports, the 5-minute observations, and for use in the daily weather summary. Magnetic winds are broadcast from the Ground-To-Air (GTA) radio, appended to the 5-minute observations, and available on several video displays.
ASOS computes the true 2-minute average wind, adds or subtracts the magnetic declination for the site, and then rounds the wind direction to the nearest 10 degrees. If the site has an east magnetic declination it is subtracted from the true direction and a west declination is added to the true direction. A way to remember this rule is: AEast is least (subtracted declination) and west is best (added declination).@
In Figure 1 the magnetic declination is 10 degrees to the east. If the wind is measured having a direction of 360 degrees true, the magnetic wind would have a magnetic direction of 350 degrees, i.e., 360-10 = 350. In other words, if you were using magnetic north as your frame of reference, the wind would be blowing from a direction 10 degrees west of magnetic north, i.e., 350 degrees magnetic. Likewise, a wind with a true direction of 090 degrees would have a magnetic direction of 080 degrees magnetic.
Figure 2 shows the case where the declination is 15 degrees to the west. If the wind has a direction of 360 degrees true and if your frame of reference is magnetic north, then the wind is really blowing from the direction of 015 degrees magnetic. Keep in mind that at this site true north is 015 degrees magnetic.
The last case is a site with a magnetic declination of zero degrees. In this case true north and magnetic north are the same and no corrections are necessary.
June 21, 1999