34 Observations and Measurements of Ocean Waves 



motion and the various relations between it and the generating force of the 

 wind. We wish to call special attention to this valuable collection of wave 

 photographs. 



Waves, in the open ocean, are generated by winds which change their 

 direction very often and suddenly, especially outside the region of the trade 

 winds. Therefore, simple uniform wave systems over large areas cannot be 

 expected. These wave systems, with their great velocity of propagation, travel 

 great distances from their source. These waves, which were not generated 

 in the area under observation by the wind prevailing at that particular moment, 

 but are coming from great distances, are called swell. They are not subjected 

 any longer to the direct influence of the wind and can, therefore, be con- 

 sidered as free waves. By their roundish, and long-streched profile, which they 

 maintain despite their length and height, they look different from the real 

 wind waves. These wind waves, under the direct influence of the wind, have 

 considerably shorter wave lengths and shorter crests, which break very easily; 

 their profile is unsymmetrical and steeper on the front than at the rear side. 



Long and high swells from the region of the trade winds of the two 

 hemispheres travel into the equatorial zones of calm, and occasionally 

 intersect here (cross sea). No less typical are the high and long swells, which 

 are almost permanently present in the calm area of the Hors latitudes, in 

 the polar border regions of the trade winds of both hemispheres. North-west 

 storms (in the Northern Hemisphere) which originate in the rear of atmospheric 

 low-pressure regions whip wind waves into mighty seas which gradually 

 move out from their source of origin. Eventually, they travel as far as the 

 north-east trade wind region and sometimes even into the Southern 

 Hemisphere. It is not difficult for an observer aboard a ship, especially if 

 he has some experience, to distinguish between "sea" and "swell". When 

 the sea is rough, one wave system will generally surpass the others by its 

 dimensions and thus strike the eye ; frequently they have a different direction 

 of propagation, so that, by observing them carefully, they are easily dis- 

 tinguishable. 



In principle, it is not difficult to observe the period, velocity and length 

 of ocean waves. In case of a ship anchored in the direction of propagation 

 of the waves, the period of the waves can be determined by means of a stop- 

 watch, checking the times at which the consecutive wave crests pass the point 

 of observation ; or one observes the passage of n waves. If the time required 

 for this was t x , the wave period is T = t x \n. The wave velocity is obtained 

 from the time required by the wave crest to cover the distance from the bow 

 to the stern of the ship. If this time is t 2 and the length of the ship /, the 

 velocity c will be ljt 2 . From c and T, the wave length can be calculated from 

 X == cT. If the waves are shorter than the ship {I < /), their length can be 

 marked out on the ship hull and measured (preferably by two observers). 



If the ship is moving, it is necessary to know and to take into account 



