Wind Direction Conventions

Several conventions are used to report wind direction. As a weather observer, you must be familiar with the relationship between these direction-reporting conventions. Wind Direction Conventions Wind directions are expressed in azimuth bearings or by the 8-point or 16-point compass. In addition, the wind direction may be a true, relative, or a magnetic wind direction. Wind directions are normally observed to the nearest 5° of azimuth, but reported (and forecast) to the nearest 10°. POINTS OF THE COMPASS.—Points of the compass as represented in figure 1-30 are normally used only to express wind directions in general weather forecasts. They are not used in aviation observations or forecasts. The standard for expressing wind direction in most general public weather forecasts and military forecasts is the 8-point compass. It uses the cardinal points of the compass (north, east, south, and west) as well as the inter-cardinal points (northeast, southeast, southwest, and northwest). General marine area forecasts may also use intermediate compass points, such as North-Northeast, East-Northeast, and East- Southeast. When not specified as "relative" or "magnetic" directions, the points of the compass refer to "true" directions. When wind direction is critical to the safe conduct of an operation or exercise, such as routine aviation weather operations, parachute operations, or the employment of weapons systems, both observed and forecast wind directions should be provided using azimuth bearings. AZIMUTH BEARINGS.—In surface aviation weather observations, wind direction is always reported using a 360° azimuth circle with 000°/360° representing True North. Figure l-30 also shows an azimuth bearing circle. Note that the 0/360 azimuth bearing is aligned with True North, the North Pole. Directions are expressed in degrees of azimuth progressing clockwise through 090° representing due East, 180° representing due South, 270° representing due West, and 360° representing due North. In meteorology, an azimuth of 000° is used only when no wind is blowing, while 360° means the wind is from the North. True Wind Direction True North is represented on a globe as the North Pole. All directions relative to True North may be called "true bearings." Since the majority of azimuth bearings for wind directions and navigational bearings are required to be oriented to True North, a wind direction or navigational bearing lacking designation is assumed to be a "true" bearing. A true wind direction is a wind direction measured with respect to True North. The wind equipment installed at all shore weather stations should be oriented to True North. Therefore, wind direction and wind speed obtained from the equipment is considered to be a true wind direction and a true wind speed. Relative Wind Direction A relative bearing uses the current direction that an object (such as a ship or an aircraft) is facing as the 0/360° azimuth alignment. On a ship, a line taken through the centerline of the ship directly over the bow represents the relative bearing 0/360°. The relative azimuth bearings proceed clockwise with directly off the Starboard Beam representing 090°, dead astern representing 180°, directly off the Port Beam representing 270°, and back to the bow at 360°. Wind direction aboard ship is observed by relative bearing and is called a relative wind direction. The relative wind direction (and relative wind speed) may be manually converted to a true wind direction (and true wind speed) by using the CP-264/U true wind computer, a maneuvering board, or an aerological plotting board. Do not confuse relative wind with apparent wind (the relative wind speed with the wind direction reported using true bearing vice relative bearings). Apparent winds have no application in meteorological observations. Figure 1-30.—Points of the compass and azimuth bearings. 1-43