Definitions of Other Terms Other definitions that the Aerographer’s Mate should be familiar are as follows: Deep water. Water that is greater in depth than one-half the wave length. Shallow water. Water that is less in depth than one-half the wave length. Fetch (F). An area of the sea surface over which a wind with a constant direction and speed is blowing, and generating sea waves. The fetch length is measured in nautical miles and has definite boundaries. Duration time (t). The time that the wind has been in contact with the waves within a fetch. Fully developed state of the sea. The state the sea reaches when the wind has imparted the maximum energy to the waves. Nonfully developed state of the sea. The state of the sea reached when the fetch or duration time has limited the amount of energy imparted to the waves by the wind. Steady state. The state reached when the fetch length has limited the growth of the waves. Once a steady state has been reached, the frequency range produced will not change regardless of the wind. Wind field. A term that refers to the fetch dimensions, wind duration, and wind speed, collectively. Effective duration time. The duration time that has been modified to account for the waves already present in the fetch or to account for waves generated by a rapidly changing wind. E value. E is equal to the sum of the squares of the individual amplitudes of the individual sine waves that make up the actual waves. Since it is proportional to the total energy accumulated in these waves, it is used to describe the energy present in them and in several formulas involving wave energy. Co-cumulative spectra. The co-cumulative spectra are graphs in which the total accumulated energy is plotted against frequency for a given wind speed. The co-cumulative spectra have been devised for two situations: a fetch limited wind and a duration time limited wind. Upper limit of frequencies (fu). The upper limit of frequencies represents the lowest valued frequencies produced by a fetch or that are present at a forecast point. This term gets its name from the fact that the period associated with this frequency is the period with the highest value. The waves associated with this frequency are the largest waves. Lower limit of frequencies (fL,). The lower limit of frequencies represents the highest value frequencies produced by a fetch or that are present at a forecast point. This term gets its name from the fact that the period associated with this frequency is the period with the lowest value. The waves associated with this frequency are the smallest waves. Filter area. That area between the fetch and the forecast point through which swell waves propagate. This area is so termed because it filters the frequencies and permits only certain ones to arrive at a forecast point at a forecast time. Significant frequency range. The significant frequency range is the range of frequencies between the upper limit of frequencies and the lower limit of frequencies. The term significant range is used because those low-valued frequencies whose E values are less than 5 percent of the total E value and those high-valued frequencies whose E values are less than 3 percent of the total E value are eliminated because of insignificance.
The significant range of frequencies is used to determine the range of periods present at the forecast point. Propagation. Propagation as applied to ocean waves refers to the movement of the swell through the area between the fetch and the forecast point. Dispersion. The spreading out effect caused by the different group speeds of the spectral frequencies in the original disturbance at the source. Dispersion can be understood by thinking of the different speeds of the different frequencies.
The faster wave groups will get ahead of the slower ones; the total area covered is thereby extended. The effect applies to swell only. Angular spreading. Angular spreading results from waves traveling radially outward from the generating area rather than in straight lines or banks because of different wind direction in the fetch.
Although all waves are subject to angular spreading, the effect of such spreading is compensated for only with swell waves because the spreading effect is negligible for sea waves still in the generating area. Angular spreading dissipates energy.
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