Figure 6-7.-Measuremeuts of angles for angular spreading.2. Extend the top and bottom edge of the fetchoutward parallel to the main direction of the wind. Thisis shown as dashed lines in figure 6-7.3. Draw lines from the top and bottom edges of thefetch to the forecast point.4. The angles to the forecast point are designatedTheta 3 (q3) and Theta 4 (q4). Theta 3 is measured fromthe top edge of the fetch and Theta 4 from the bottomedge.5. Any angle that lies above the dashed line isnegative while any angle that lies below the dashed lineis positive.After the angles Theta 3 and 4 have been measuredthey are converted to percentages of the swell that willreach the forecast point. This conversion is made byentering sea and swell graph 7, figure 6-8, with thepositive or negative angles and reading thecorresponding percentages directly. The percentagesare then subtracted ignoring the plus or minus to find theangular spreading.OBJECTIVE METHOD FORFORECASTING SWELL WAVESA number of terms used in dealing with forecastingsea waves will be used again in this process; however,a number of new terms will be introduced. Table 6-3lists most of these terms with their associated symboland definition.As with objective forecasting of sea waves there area number of different methods for forecasting swellwaves. Some of the methods are too technical or timeconsuming to be of practical use.When ship operations are conducted outside a fetcharea it becomes necessary to forecast swell conditionsat that location. Prior to computing swell conditions theheight and period of the significant waves departing thefetch area must be determined. For more details referto Sea and Swell Forecasting, NAVEDTRA 40560.FORECASTING SURFLEARNING OBJECTIVES: Explain thegeneration of surf and describe the two changesthat occur upon entering intermediate water.Recognize the characteristics of the three typesof breakers. Define the terms associated withsurf. Describe an objective method for surfforecasting and the calculations of the modifiedsurf index.Thus far we have discussed the generation of seawaves, their transformation to swell waves, some of thechanges that occur as they move, and objective methodsof forecasting both waves.The Navy is greatly involved in amphibiousoperations, which requires the forecasting of another seasurface phenomena: surf. Senior Aerographer’s Mateswill occasionally be called upon to provide forecasts foramphibious operations, and accurate and timelyforecasts can greatly decrease the chance of personnelinjury or equipment damage. Therefore, it is importantthat forecasters have a thorough understanding of thecharacteristics of surf and a knowledge of surfforecasting techniques.GENERATION OF SURFThe breaking of waves in either single or multiplelines along the beach or over some submerged bank orreef is referred to as surf.6-11

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