Quantcast Magnetic Wind Direction

 
  
 
Magnetic Wind Direction A magnetic wind direction is a direction based on the 360° azimuth circle with the 0/360° azimuth radial aligned  with  magnetic  north.  Magnetic  bearings  are used by tactical weather observers in the field when determining wind directions by using a magnetic compass for reference. The magnetic wind directions thus obtained are converted to true wind directions by adding  or  subtracting  the  appropriate  magnetic declination for the location. If, for instance, a charted magnetic declination is "7° west," this means that magnetic north is 7° west of actual or True North, and that 7° must be subtracted from the wind direction obtained to convert it to true wind direction. When a location  has  a  declination  east  of  true  north,  the correction must be added to the magnetic direction. As long as the tactical observer is stationary (not in a moving vehicle), no correction need be applied to the observed wind speed. WIND SPEED Wind Speed is the average rate of air motion, or the distance air moves in a specified unit of time. The instantaneous wind speed is the speed of the air at any moment.  The  instantaneous  wind  speed  will  usually show  minor  fluctuations  over  time.  Fluctuations between  the  highest  instantaneous  speed  and  the  lowest instantaneous speed are averaged to obtain mean wind speed. Mean wind speed is the arithmetic or graphical average wind speed during the period of observation, which is normally 2 minutes. For example, wind speeds on a recorder chart during a 2-minute observation period may constantly vary between 24 and 32 knots. The average, 28 knots, is the mean wind speed. Mean wind speed is the value observed and reported for "wind speed" in all meteorological observations. All U.S. military weather observations use nautical miles  per  hour,  or  knots  (kt)  as  the  standard  for measuring  observed,  reported,  and  forecast  wind speeds. Unless stated otherwise, the U.S. National Weather Service commonly uses statute miles per hour (mph) for all winds speeds, since the public is most familiar with that measurement. Overseas, meters per second (m/s) is the most frequently used measurement. Navy and Marine Corps observers will frequently need to convert wind speeds from one measurement system to  another.  Wind  speeds  are  normally  observed  and reported to the nearest whole knot. Occasionally, you may see reference to wind speeds on the Beaufort wind scale,  such  as  "force  1  winds"  or  "winds  3  to  4, becoming 5 by night." Force is not always stated, but is assumed. The Beaufort wind scale is included and cross-referenced  to  standard  wind  speeds  in  the  table  in Appendix  V. Wind speeds aboard ship are affected by ship movement. If the ship is heading into the direction from which the wind is blowing, the observed wind speed across the deck will be greater than the actual wind speed. On the other hand, if the ship is traveling with the wind, the observed wind speed over the deck will be less than the actual wind speed. For this reason, the winds across the deck, as measured on an anemometer, are called relative wind speeds; the wind speed is relative to the motion of the ship. Relative wind speed is converted to true wind speed, which would be the actual wind speed  if  measured  at  a  stationary  location.  You  can convert relative wind speed to true wind speed by using the CP-264/U true wind computer (see chapter 2), or a maneuvering board may be used. Many "descriptive" terms are used to identify wind speed. Some are light breeze, fresh breeze, gentle breeze, moderate breeze, or fresh gale and storm. These terms are part of an accepted scale of nautical wind speeds that may be directly related to wind speed measurements.  These  descriptive  names  are  included  in Appendix V. Others, such as brisk or sultry, although acceptable in literature, have only a vague relationship to measured wind speeds and should not be used. Only two descriptive terms may be used in military surface weather observations for wind speeds. They are light, abbreviated LGT, meaning the wind speed is 10 knots or less, and calm, meaning there is no detectable motion of the air. WIND  CHARACTER In addition to wind speed and wind direction, most observations  require  a  determination  of  wind  character. Wind character is a description of how the wind (speed or  direction)  changes  during  the  specified  period.  Wind speed gusts, the peak wind gust, wind speed squalls, and variable  wind  direction  are  all  included  in  wind character, and should be noted during an observation. Gust A wind gust is a rapid fluctuation in wind speed with a variation between peaks and lulls of 10 knots or more. Gusts are normally observed in the 10-minute period prior to the actual time of observation. Gusts increase  the  difficulty  of  controlling  aircraft  during takeoff and landing. The gust spread (the difference in knots between the normal lulls and peaks), if large enough, may cause problems for rotary wing aircraft by 1-44


 


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