24 MANUAL OF MARINE METEOROLOGICAL OBSERVATIONS 



3311.2, The true speed of the wind is j^reater than the apparent whenever 

 the apparent direction is aft of the beam. 



3311.3. The true speed of the wind is less than the apparent whenever the 

 true direction is forward of the beam. 



3320. SURFACE WIND AT SEA--FORM 1209. This chart is designed 

 to aid in the computation of the true wind from the apparent wind (see Fig. 2). 

 If a supply of the forms is desired for use aboard ship, they may be obtained 

 from the Weather Bureau when requested in accordance with 111301. 



3321. A wind scale, in knots, is printed on concentric circles that are 

 drawn around the center of the chart. If the wind speed exceeds the printed 

 scale, a new scale may be made by doubling or tripling the printed one. Wind 

 direction for every 10 degrees is printed on lines that radiate from the center 

 of the chart. The wind direction in degrees off the bow is plotted with refer- 

 ence to the black degree markings (top figures) printed at the ends of the 

 radial lines. A separate slide, with a ship's-speed scale similar to the 

 wind- speed scale used on the chart, is furnished with the forms. With the 

 slide, the ship's speed is laid off vertically downward from the plotted point 

 that represents the wind speed and the direction off the bow. The point 

 plotted at the ship's speed has coordinate values representing the true wind 

 speed and the true wind direction with respect to the ship's heading. The 

 wind direction, in degrees off the starboard bow (0° to 360°), is read from 

 the green- colored figures (bottom figures). The ship's heading in degrees 

 must be added to the wind direction, as read from the chart, in order to 

 have the true wind direction with respect to north (360°). 



3322. Form 1209 is used as follows: 



3322.1. Select the radial line coresponding to the angle, in degrees off the 

 bow (port or starboard), from which the wind is blowing and plot a point on it 

 at a distance from the center (scale printed on concentric circles) equal to the 

 speed of the apparent wind in knots. 



3322.2. Place the (arrow point) of the ship's speed slide at the point 

 plotted in 3r3322.1 and measure the ship's speed vertically downward toward 

 the bottom of the chart. Plot a point on the chart opposite the value on the 

 slide corresponding to the ship's speed at the time of the wind observation. 



3322.3. Read the wind speed corresponding to the point plotted in J3322.2, 

 estimating for the values between the concentric circles. This value is the 

 true wind speed. 



3322.4. Read the angular value, from the figures printed in green (bottom 

 figures), corresponding to the radial line from the center through the 

 point plotted in 313322.2. Add the value of the angle to the ship's heading in 

 degrees to obtain the true wind direction. If the total exceeds 360°, subtract 

 360° from it. 



