Chapter 10 — vVATCH ROUTINES 



weather observations. However the data must 

 be intelligently interpreted when received from 

 other sources. For a detailed description of 

 the principles and procedures of weather radar 

 observations, refer to FMH No. 7, Weather 

 Radar Observations. 



OCEANOGRAPHIC OBSERVATIONS 



Accurate sea condition observations are 

 necessary if the forecaster is to be expected 

 to provide accurate sea condition forecasts for 

 operational use. 



When a breeze comes up, the sea surface 

 will instantaneously become covered with tiny 

 ripples which form more or less regular arcs 

 of long radius. They increase rapidly in height 

 until they attain a maximum steepness where 

 the pointed crests take on a smooth glassy ap- 

 pearance, indicating small breaking processes. 

 As the wind continues to blow over the sea, it 

 drags over the surface producing increasingly 

 larger waves. The wind acts in a manner simi- 

 lar to a paddle rhythmically stroking the water. 

 There is, however, one difference. The wind is 

 constantly making small changes in direction 

 and speed. The result is that the wind is acting 

 like many different paddles stroking the sea in 

 different directions and at different speeds. 

 This produces many different wave trains in 

 the sea, all with different directions, all with 

 different periods, and all at the same given 

 point in the ocean. Figure 10-4 illustrates this 

 concept. 



If all the different wave trains generated are 

 considered to be sine waves with different 

 heights, directions, and periods, the sum of 

 the heights of the sine waves at a given instant 

 is the mechanism that produces the irregular 

 appearance of the sea. The various waves which 

 comprise the sea surface are commonly re- 

 ferred to as either sea waves or swell waves. 



Sea Waves 



Sea waves are defined as the waves gener- 

 ated by local winds. If locally developed winds 

 persist for sufficient time, they will cause 

 sea waves to develop whose dimensions vary 

 directly with the wind velocity. Waves can con- 

 tinue to grow only as long as they receive more 

 energy from the wind than they lose due to tur- 

 bulent mixing of the breaking sea. 



It is important to keep in mind that sea 

 waves are locally generated, and do not in- 

 clude "swell waves" for observational purposes. 

 Swell waves are those waves moving into the 

 area from some storm outside the local area. 



There are a number of differences between 

 sea waves and swell waves. An illustration of 

 each type is shown in figures 10-5 and 10-6. 



Description of Sea Conditions 



Wave height is the height difference between 

 the wave trough and its crest. Wave period is 

 the time interval (in seconds) in which a wave 

 repeats itself exactly; that is, the time between 

 consecutive troughs, crests, or any specific 

 corresponding points. The sea cannot be prop- 

 erly described by a single height and period 

 because no single height and period is truly 

 representative. There are many different heights 

 and periods in the sea, and to describe it, it 

 is more correct to give several heights and 

 several periods which give a better idea of what 

 is actually happening in the sea. For this reason 

 the following terms are needed to properly 

 describe the sea condition: 



1. Average Wave Height. — The average height 

 of ALL the waves. 



2. Significant Wave Height. — The average 

 height of the highest one-third of the waves. 



3. One-Tenth Highest Waves. — The average 

 height of the highest one-tenth of the waves. 



4. Average Period. The average period of 

 all the waves. 



5. Lower Limit of Periods. The lowest 

 (shortest) period found in a particular sea state. 



6. Upper Limit of Periods. The highest (or 

 longest) period found in a particular sea state. 



Sea Condition Measurements 



There have been a number of new develop- 

 ments regarding instruments for measuring tem- 

 perature, pressure, density, etc, of the sea; 

 however, to describe the physical characteristics 

 of the sea surface as related to height, period, 

 and direction of the sea, the observer must 

 continue to use his experience while comparing 

 the sea surface to some existing reference ob- 

 ject. This means utilizing reference points such 



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