usually produce seas with small wave heights, small wave lengths, and short periods; higher winds usually produce  waves  with  higher  heights,  longer  wave lengths, and longer periods. When the winds over the water produce sea waves, the wave crests are generally aligned perpendicular to the direction the wind is blowing. The continuing force of the wind on the waves distorts the ideal sine wave pattern, forming sharper crests (fig. 1-33). The waves move in the direction the wind  is  blowing,  with  wave  crests  and  troughs perpendicular  to  the  wind  direction. In a given sea condition, many different size waves are present. Observers determine significant wave height, or the average wave height of the highest 1/3 of all the waves present. Ideally, the heights of 50 to 100 waves should be recorded on a piece of paper, then the highest 1/3 of the recorded heights should be averaged to  obtain  significant  wave  height  for  the  seas.  In practice, taking the average height of the "most well defined" waves approximates the significant wave height.  Attempt  to  observe  50  or  so  waves  as  a minimum, and then average the height of the "best" 16 or 17 waves. The average significant wave period in an area of sea waves gives analysts and forecasters a better idea of the total wave energy present in the area than does the observation   of   the   significant   wave   height. Observations of the average significant period should be made by timing the passage of "well defined" wave crests past a fixed point, such as a buoy, clump of seaweed,  wood  block,  or  square  piece  of  cardboard mentioned earlier, and then dividing to find the average. The observer should attempt to time the passage of the same "significant" wave crests that were used in the determination of average significant wave height. If, for example, you timed the passage of 17 "well defined" waves out of 50 waves of various size passing the cardboard square (before you lost sight of the square) in 120 seconds, the average wave period is 120/17, or 7 seconds. The important factor in determining both the average height and the average period for sea waves is that only the highest 1/3 of the waves, the significant waves, are evaluated. A direction is always determined for sea waves, and the direction found should be in general agreement with the wind direction. If the sea wave direction does not agree within plus/minus 20° of the wind direction, recheck both sea and wind direction. The sea direction is usually not recorded or reported, since it is assumed that the sea direction is nearly the same as the recorded wind  direction. SWELL  WAVES Swell waves are seas that have moved out and away from the area in which they were formed. Because of their different wave lengths and wave speeds, waves move outward from the windy areas where they formed, and separate into groups of waves with distinct wave periods.   Since the winds are no longer pushing on the waves, they take on a more typical sine wave pattern with generally equally rounded crests and troughs, and thus are smooth and regular in appearance. Typically, when only one group of swell waves is present, the wave heights and the wave periods are fairly uniform. Determinations for the average swell wave period, the average swell wave height, and the wave direction may be easily made. When determining Figure 1-33.—Typical sea wave pattern. Note the sharper crests and the irregular wave pattern caused by the superposition of many different wave length/wave height patterns. 1-48