36 Observations and Measurements of Ocean Waves 



for the log and the buoy, respectively, to line up with the stern of the ship 

 simultaneously on a wave crest. On account of the sag of the cord there is 

 a considerable error by obtaining too great wave lengths. 



It is much more difficult to measure correctly the wave height. If the waves 

 are of considerable height, so that the horizon is concealed by them from 

 the observer when the ship is in a wave trough, he can climb up high enough 

 in the shrouds in the middle of the ship to see the crest of the approaching 

 wave line up with the horizon and thus determine the wave height. In using 

 this visual method, attention should be paid to the fact that, at the time of 

 the observation, the ship must lie entirely on a flat keel in the wave trough, 

 otherwise the observation would be worthless. In a similar way, Wilkes 

 (1845, p. 135) determined the height of the ocean waves, looking from the 

 shrouds of a ship in a wave trough across two successive wave crests. Figure 21 



Fig. 21. Wave height measurement. (By Ch. Wilkes.) 



illustrates this procedure. It should be considered that, in consequence of 

 its buoyancy, the ship makes dipping motions. This causes considerable errors 

 in the observations. In the wave trough the ship dips less than at the top of 

 the wave crests. Especially in observing the wave height, these corrections 

 should be made by means of very fine aneroid barometers. Neumayer was 

 the first to refer to this method, which was then frequently used later. The 

 conversion factor from pressure variations to differences in height varies 

 slightly with atmospheric pressure: at a pressure of 10399 mbar(780 mmHg), 

 a height difference of 40-5 in. (103 m) corresponds to a pressure increase 

 of 0- 1 3 mbar (01 mm Hg), whereas at 986-6 mbar (740 mm Hg) the difference for 

 013 mbar (01 mm) is 42-5 in. (108 m). The error resulting from dipping 

 oscillations can be estimated at ±40 in. (1 m) at the least. Furthermore, 

 there are errors arising from the elastic after-effect of the aneroid and from 

 short period variations of air pressure which have a different origin. Only 

 very sensitive aneroids and great care in measurements will give useful results. 

 If the wave heights are too small to be determined by the above-mentioned 

 methods, they can be estimated from high ships, provided one can watch 

 the exterior of the marked hull. In the open sea it is easy to carry out such 

 measurements from a small boat. 



In former times it was left to the observer to decide which waves he was 

 going to measure, and observation instructions were rather vague. Thus, 

 the results of wave measurements were entirely inhomogeneous and could 

 hardly be compared. A procedure to standardize wave observations was 

 recently agreed upon. The aspect of a travelling wave system, as well as the 



