84 MANUAL OF MARINE METEOROLOGICAL OBSERVATIONS 



9040. Distinction between SEA and SWELL. 



9041. The system of waves raised by the local wind blowing at the 

 time of observation is usually referred to as "SEA." 



9042. Those waves not raised by the local wind blowing at the time of 

 observation, but due either to winds blowing at a distance or to winds 

 that have ceased to blow, are known collectively as "SWELL." 



9043. Usually one swell train dominates the rest, but occasionally two 

 swell trains crossing at an angle may be observed. These are referred 

 to as "CROSS SWELLS." 



9050. For additional wave observation techniques, see W.B. Training 

 Paper No. 14, "Observing and Reporting Sea and Swell." 



9100. WAVE DIRECTION 



9110. Wave direction, like wind direction, is the direction from which 

 the waves are coming. The wave direction is determined with reference 

 to true north (360°), and is recorded to the nearest ten degrees in tens of 

 degrees; e.g., 237° is recorded as 24. 



9120, Wave direction may be determined by eye observation or, more 

 accurately, by sighting from a compass along the wave crests and adding 

 or subtracting 90°. Ship's heading can also be used as a guide to determine 

 the direction from which the waves are approaching. The higher the 

 observation point, the easier it is to determine wave direction. The 

 average of several observations should be used as the reported wave 

 direction. 



9200. WAVE PERIOD 



9210, GENERAL, The wave period is the interval in seconds between 

 the passage of two successive crests of well- formed waves past a fixed 

 point (see Fig, 11). The interval can be timed with a stop watch or an 

 ordinary watch with a second hand. 



9220. OBSERVATION, Determine the period as follows: 



9221. Select a distinctive patch of foam or a small floating object at 

 some distance from the ship. Select a new patch of foam from time to 

 time as the old one falls astern. 



9222. Note the elapsed time to the nearest second between the moments 

 when the object is on the crest of the first and of the last well- formed 

 wave in the group. Also note the number of crests that pass under the 



