Ch 08.qxd 8/25/04 11:27 AM Page 8-1 OPERATIONS IN OPEN SEAS Swell Face —The side of the swell toward the observer. The back is the side away from the observer.  Open sea operations are very risky and should be avoided if possible. If an open sea landing cannot be avoided, a thorough reconnaissance and evaluation of the conditions must be performed to ensure safety. The sea usually heaves in a complicated crisscross pattern of swells of various magnitudes, overlaid by whatever chop the wind is producing. A relatively smooth spot may be found where the cross swells are less turbulent. Both a high and a low reconnaissance are necessary for accurate evaluation of the swell systems, winds, and surface conditions. Swell Length —The horizontal distance between suc- cessive crests. Swell Period —The time interval between the passage of two successive crests at the same spot in the water, measured in seconds. Swell Velocity —The velocity with which the swell advances in relation to a fixed reference point, meas- ured in knots. (There is little movement of water in the horizontal direction. Each water particle transmits energy to its neighbor, resulting primarily in a vertical motion, similar to the motion observed when shaking out a carpet.) Chop —A roughened condition of the water surface caused by local winds. It is characterized by its irregu- larity, short distance between crests, and whitecaps. Downswell —Motion in the same direction the swell is moving. Upswell —Motion opposite the direction the swell is moving. If the swell is moving from north to south, a seaplane going from south to north is moving upswell. Wind is the primary cause of ocean waves and there is a direct relationship between speed of the wind and the state of the sea in the immediate vicinity. Windspeed forecasts can help the pilot anticipate sea conditions. Conversely, the condition of the sea can be useful in determining the speed of the wind. Figure 8-1 on the next page illustrates the  Beaufort wind scale with the corresponding  sea state condition number . When performing open sea operations, it is impor- tant to know and understand some basic ocean terms. A thorough knowledge of these definitions allows the pilot to receive and understand sea condition reports from other aircraft, surface vessels, and weather services.  Fetch —An area where wind is generating waves on the water surface. Also the distance the waves have been driven by the wind blowing in a constant direc- tion without obstruction. Sea —Waves generated by the existing winds in the area. These wind waves are typically a chaotic mix of heights, periods, and wavelengths. Sometimes the term refers to the condition of the surface resulting from both wind waves and swells. Swell —Waves that persist outside the fetch or in the absence of the force that generated them. The waves have a uniform and orderly appearance characterized by smooth, regularly spaced wave crests.  Primary Swell —The swell system having the greatest height from trough to crest. Secondary Swells —Swell systems of less height than the primary swell. Swell Direction —The direction from which a swell is moving. This direction is not necessarily the result of the wind present at the scene. The swell encountered may be moving into or across the local wind. A swell tends to maintain its original direction for as long as it continues in deep water, regardless of changes in wind direction. While the height of the waves is important, it is often less of a consideration than the wavelength, or the dis- tance between swells. Closely spaced swells can be very violent, and can destroy a seaplane even though the wave height is relatively small. On the other hand, the same seaplane might be able to handle much higher waves if the swells are several thousand feet apart. The relationship between the swell length and the height of SEA STATE EVALUATION DEFINITIONS 8-1