the most accurate wave height estimations. This is understandable since an observer on a carrier catwalk is some 60 feet above the water line, and the waves look small  from  that  height.  Estimation  may  be  improved  by observing wave height from the hangar deck, which is only some 30 feet above the water. Until the observer becomes very experienced in observing wave heights, reference  objects  should  be  used  to  judge  the  wave height. Good reference objects are ships-in-company or small  boats  operating  alongside.  Waves  may  be compared to the heights of the freeboard along the sides of the ships, or to the size of the small boats. "Load line" markings in feet may be visible on the sides of ships and will assist in wave height determination. Be careful not to observe the waves near the bow of a ship, since the bow-wave is caused by the ship and is not a true representation of the actual wave heights. The best reference object is something of known size. Some shipboard weather offices keep square 1-, 2- , or 3-foot pieces of cardboard on hand to throw over the side of the ship and use as a reference. Other ships use 1-foot-square pieces of scrap wood as a reference. (Cardboard  is  preferable,  since  it  will  soon  become soggy, breakdown, and sink in the water, and it is biodegradable. It is also readily available.) Using a block of wood or cardboard as a reference is known as the chip or block method. Wave Length Wave length is the horizontal distance from one wave crest to the next wave crest, or the distance from one wave trough to the next wave trough. Although difficult to measure at sea, this parameter may be measured on aerial photographs and is directly related to wave period by the approximation L = 5.12T² , where L is the wavelength in feet and T is the wave period in seconds.  Wave  lengths  are  not  directly  observed  or reported by observers. Wave Period Wave period is the time, usually measured in seconds, that it takes for a complete wave cycle (crest to crest or trough to trough) to pass a given fixed point. Wave period is dependent upon the speed of movement of the wave across the surface. The speed of movement varies with wave length, with shorter wave-length waves moving slower and longer wave-length waves moving  faster.  This  relationship  is  approximated  by C = 1.34 Ö L, where C is the wave speed (knots) and L is the  wave  length.  Many  calculations  dealing  with  waves use the wave frequency instead of the wave period as a basis for the argument. The wave frequency is the number of wave cycles passing a fixed point in 1 second, and it is inversely proportional to wave period. Conversions between wave frequency ( f ) and wave period(T) are made by the formula f= 1/T or T = 1/f. Wave  Direction Wave direction is the direction, in true degrees of azimuth, that the majority of the waves in a group are coming  from.  Wave  direction  is  best  determined  during an observation by sighting along the wave crests and troughs and either adding 90° to or subtracting it from the direction obtained, as shown in figure 1-32. Use the gyroscope repeater on one of the pelorus columns (chapter 2, fig. 2-29) to sight along the wave crests or troughs. Add or subtract 90° to/from the true bearing thus obtained to determine the wave direction. The observer may also sight directly into the oncoming waves, perpendicular to the crests and troughs, to obtain wave  direction. SEA  WAVES Sea waves, often referred to as "seas," are waves generated by the wind in the local area. Light winds Figure 1-32.—Method for obtaining wave direction. 1-47