INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 37 



any swell that may be prevailing. The difference in the appearance 

 of the ocean surface between forces 4 and 5 is principally one of 

 degree. However, with force 5 spnme tends to be blown from the 

 breaking wave crests, whereas no such tendency is noticeable with 

 force 4. No attempt will be made here to describe the appearance 

 of the sea surface with wdnds of greater strength, because these occur 

 usually in the neighborhood of cyclonic storm centers where the rapid 

 changes of direction and velocity do not permit the sea to reach a 

 state of equilibrium with respect to the wind at any given time. 



The motion of the air is never perfectly uniform, but is subject 

 to incessant variations in direction and velocity. These variations 

 are designated as "turbulence" or "gustiness." The occurrence of 

 unusual gustiness should always be noted in the weather report. Un- 

 usual gustiness is reported in the radio message by adding 33 to the 

 appropriate code number for the wind direction. It should not be 

 reported by describing or coding the state of weather as "squally." 

 Gusts are distinguished from squalls by their brief duration, by the 

 absence of any special accompanying cloud formation and by the fact 

 that the wind does not vary appreciably from its average direction. 

 Unusual gustiness may be defined as the condition prevailing when 

 the wind temporarily exceeds its average force by a full figure or 

 more on the Beaufort scale but continues essentially (i. e., to within 

 two points) from the same direction. Care should be taken that the 

 average force of the wind is recorded, and not its force during the 

 gusts. 



The apparent and the true direction and force of the wind.- — -The 

 apparent direction and velocity of the wind is the resultant of 

 two motions, that of the vessel and that of the air. As an example 

 of this, take the case of a vessel steaming westw^ard 20 knots, and 

 let the true direction of the wind be due north, or 8 points off the 

 starboard bow, its true velocity 20 knots. Let A D (fig. 12), repre- 

 sent the true direction and velocity of the wind; B D the direction 

 (west) and the velocity (20 knots) of the wind created by the 

 motion of the steamer. Then G Z>, the resultant of A D and B Z>, 

 will be the apparent direction and velocity of the wind as observed 

 aboard the steamer; i. e., the wind, while its true direction is due 

 north (8 points off the starboard bow), and its true velocity 20 knots, 

 will apparently be NW. (4 points off the starboard bow), and will 

 have an apparent velocity of 28 knots. 



The true direction of the wind is thus always farther from the 

 bow than the apparent direction. 



The true velocity of the wind is greater than the apparent as long 

 as the apparent direction is aft of the beam. 



The true velocity of the wind is less than the apparent as long as 

 the true direction is forward of the beam. 



Having observed the apparent force and direction of the wind, 

 in points off the bow, the true force and direction may be taken 

 from the table on pages 116 and 117. 



The following examples will serve to illustrate the manner of 

 using the table : 



1. Let the true course and speed of the vessel be SSW., 20 knots, the apparent 

 force of the wind, (dead cahii). The true direction of the wind is NNE. 

 (16 points off the bow) ; its true force is 5. 



