Once a fetch has been determined and the wind speed has been found, the next step is to determine the duration of that wind over the fetch. It is highly unlikely that the wind will begin and end at one of the 6-hour map times. Therefore an accurate value must be interpolated. Inmost cases a simple interpolation of the successive maps will be sufficient to locate the bounds of the wind field in space and time.

Determining how long the wind has blown is relatively simple when the wind speed has bee n constant for the entire duration. If this does not occur, a representative duration must be selected.

SLOWLY VARYING WIND. Suppose the wind has been blowing for 24 hours, with velocities of 10 knots for 6 hours, 15 knots for 12 hours, and 20 knots for 6 hours. The duration is 24 hours but the speed value is in question. The most consistent solution is to use three durations with the corresponding wind speeds and work up three successive states.

MORE RAPID VARIATIONS. Suppose the wind blows for 12 hours and during that time it increases in velocity from 10 to 20 knots. Studies and experience have shown that in cases of variable winds a single value may be assigned for wind speed if the change has been relatively small. The following rules can be applied under these conditions:

. Average the speeds when the change is gradual or increasing or decreasing. Apply the average to the entire duration.

. Use the last wind speed when the speed changes in the first few hours, then remains constant. Apply that speed to the entire duration.

OBJECTIVE METHODS FOR FORECASTING SEA WAVES

There are a number of different methods for forecasting sea waves. Some of the methods are too technical or time consuming to be of practical use of Aerographers Mates.

A relationship between wave velocity (c), wave length (L), and period (T) maybe indicated using the equation C = 3.03 T. The length in feet of a deep-water wave (L) may be computed using the equation L = 5.12 T. The period of a wave in seconds (T) may be calculated using the equation T = 0.33 C, where (C) is the wave velocity.

Sea state forecasts are divided into four categories: significant wave height (HID), average wave height (HAvG), one-tenth average wave height (HIJ1o), and high wave (Hw).

For more information, refer to the practical training publication Sea and Swell Forecasting, NAVEDTRA 40560, published by the Naval Oceanographic Office. This publication presents a method for forecasting sea waves, and a brief summary follows.

In order to prepare an accurate sea state forecast one must frost determine wind speed over the fetch (U), length of the fetch (F), and the length of time the wind speed (u) has remained unchanged within the fetch (u). These parameters are determined using current and/or previous surface charts. Using these parameters and the tables in NAVEDTRA 40560, an accurate sea state forecast may be obtained.

LEARNING OBJECTIVES: Explain swell wave generation and recognize the two fundamental modifications that sea waves undergo as they leave the fetch area. Define the terms associated with swell waves, and explain the five rules used to determine how much of the swell will reach the forecast point. Prepare an objective swell wave forecast.

In the preceding portion of this chapter, we have discussed the principles of sea waves and methods of forecasting them. With sea wave forecasting we are considering the point that we are forecasting to be within the generating area, with the wind still blowing. This, however, will not be the problem in the majority of the forecasts that will be required. Normally the forecast point will be outside the fetch area; therefore, it will be necessary to determine what effect the distance traveled is going to have on the waves. In this section we will discuss the basic principles of swell waves as well as an objective method of determining what changes will take place in the spectrum of waves as they traverse from the generating area to the forecast point.

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