CHAPTER 6 SEA SURFACE FORECASTING The task of forecasting various elements of the sea surface is the responsibility of senior Aerographer’s Mates filling a variety of billets. Aboard carriers, sea condition forecasts for flight operations,  refueling,  or  underway  replenishment  must be provided on a routine basis. Staffs of larger facilities will generally provide surf forecasts for amphibious operations, while at air stations that support search and rescue  (SAR)  units,  you  maybe  called  upon  to  provide forecasts  of  sea  conditions  and  surface  currents. Therefore, it is important that you be familiar with these elements  and  be  able  to  provide  forecasts  as  necessary. In   this   chapter,   we   will   discuss   sea   surface characteristics and the forecasting of sea waves, swell waves, surf, and surface currents. Now let’s consider sea  surface  characteristics. SEA SURFACE CHARACTERISTICS LEARNING OBJECTIVES: Describe the basic principles of ocean waves. List and describe the properties that all waves have in common. Define additional terms used in sea surface forecasting.  Explain  wave  spectrum  in  terms  of wave frequency, energy, and wind speed. To accurately forecast sea conditions it is necessary to understand the process of wave development, the action that takes place as the energy moves, and to have an  understanding  of  the  various  properties  of  waves. In this section, we will discuss this background information  and  the  terminology  used.  A  complete understanding of these terms is necessary to produce the most  usable  and  accurate  sea  condition  forecast. BASIC PRINCIPLES OF OCEAN WAVES Ocean  waves  are  advancing  crests  and  troughs  of water  propagated  by  the  force  of  the  wind.  When  winds start to blow, the frictional effect of the wind on the water creates ripples that form more or less regular arcs of long radii. As the wind continues to blow, the ripples increase in height and become waves. A wave is visible evidence of energy moving in an undulating motion through a medium, such as water. As the energy moves through the water, there is little mass motion of the water in the direction of travel of the wave. This can best be illustrated by tying one end of a rope to a pole or other stationary object. When the free end of the rope is whipped up and down, a series of waves moves along the rope toward the stationary end. There is no mass motion of the rope toward the stationary end, only the energy traveling through the medium, in this case  the  rope. A sine wave is a true rhythmic progression. The curve   along   the   centerline   can   be   inverted   and superimposed  upon  the  curve  below  the  centerline.  The amplitude of the crest is equal to the amplitude of the trough, and the height is twice that of the amplitude. Sine  waves  are  a  theoretical  concept  seldom  observed in  reality.  They  are  used  primarily  in  theoretical groundwork so that other properties of sine waves may be applied to other types of waves such as ocean waves. Principles   of   other   types   of   waves   are   modified according to the extent of deviation of their properties from those of sine waves. Waves that have been created by the local wind are known as sea waves. These waves are still under the influence of the local wind and are still in the generating area. They are composed of an infinite number of sine waves superimposed on each other, and for this reason they have a large spectrum, or range of frequencies. Sea  waves  are  very  irregular  in  appearance.  This irregularity  applies  to  almost  all  their  properties.  The reason  for  this  is  twofold:  first,  the  wind  in  the generating area (fetch) is irregular both in direction and speed; second, the many different frequencies of waves generated have different speeds. Figure 6-1 is a typical illustration of sea waves. The waves found in this aerial photograph  are  irregular  in  direction,  wave  length,  and speed. As the waves leave the generating area (fetch) and no  longer  come  under  the  influence  of  the  generating winds  they  become  swell waves.  Because  swell  waves are no longer receiving energy from the wind, their spectrum of frequencies is smaller than that of sea waves. Swell waves are also smoother and more regular 6-1