Determining how long the wind has blown isrelatively simple when the wind speed has bee n constantfor the entire duration. If this does not occur, arepresentative duration must be selected.SLOWLY VARYING WIND.— Suppose the windhas been blowing for 24 hours, with velocities of 10knots for 6 hours, 15 knots for 12 hours, and 20 knotsfor 6 hours. The duration is 24 hours but the speed valueis in question. The most consistent solution is to usethree durations with the corresponding wind speeds andwork up three successive states.MORE RAPID VARIATIONS.— Suppose thewind blows for 12 hours and during that time it increasesin velocity from 10 to 20 knots. Studies and experiencehave shown that in cases of variable winds a single valuemay be assigned for wind speed if the change has beenrelatively small. The following rules can be appliedunder these conditions:. Average the speeds when the change is gradualor increasing or decreasing. Apply the average to theentire duration.. Use the last wind speed when the speed changesin the first few hours, then remains constant. Apply thatspeed to the entire duration.OBJECTIVE METHODS FORFORECASTING SEA WAVESThere are a number of different methods forforecasting sea waves. Some of the methods are tootechnical or time consuming to be of practical use ofAerographer’s Mates.A relationship between wave velocity (c), wavelength (L), and period (T) maybe indicated using theequation C = 3.03 T. The length in feet of a deep-waterwave (L) may be computed using the equation L = 5.12T. The period of a wave in seconds (T) may becalculated using the equation T = 0.33 C, where (C) isthe wave velocity.Sea state forecasts are divided into four categories:significant wave height (HID), average wave height(HAvG), one-tenth average wave height (HIJ1o),andhigh wave (Hw).For more information, refer to the practical trainingpublication Sea and Swell Forecasting, NAVEDTRA40560, published by the Naval Oceanographic Office.This publication presents a method for forecasting seawaves, and a brief summary follows.In order to prepare an accurate sea state forecast onemust frost determine wind speed over the fetch (U),length of the fetch (F), and the length of time the windspeed (u) has remained unchanged within the fetch (u).These parameters are determined using currentand/or previous surface charts. Using these parametersand the tables in NAVEDTRA 40560, an accurate seastate forecast may be obtained.FORECASTING SWELL WAVESLEARNING OBJECTIVES: Explain swellwave generation and recognize the twofundamental modifications that sea wavesundergo as they leave the fetch area. Define theterms associated with swell waves, and explainthe five rules used to determine how much ofthe swell will reach the forecast point. Preparean objective swell wave forecast.In the preceding portion of this chapter, we havediscussed the principles of sea waves and methods offorecasting them. With sea wave forecasting we areconsidering the point that we are forecasting to be withinthe generating area, with the wind still blowing. This,however, will not be the problem in the majority of theforecasts that will be required. Normally the forecastpoint will be outside the fetch area; therefore, it will benecessary to determine what effect the distance traveledis going to have on the waves. In this section we willdiscuss the basic principles of swell waves as well as anobjective method of determining what changes will takeplace in the spectrum of waves as they traverse from thegenerating area to the forecast point.GENERATION OF SWELL WAVESAfter a sea state has been generated in a fetch, thereare many different wave trains present with differentperiods, and most of them are moving out of the fetchin slightly different directions. Because of thesedifferent periods and slight differences in direction, thepropagation of swell waves follows two fundamentalprocesses. These processes are dispersion and angularspreading.DispersionAn accepted fact about wave travel is that the waveswith longer periods move faster than waves with shorter6-9

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