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ners can improve the odds in their favor by avoiding weather conditions known to give rise to extreme waves and by avoiding those parts of the oceans where their incidence is greatest. Marine weather charts provide sea state conditions (surface wind speed and direction, significant wave heights, swell period and direction, isobar lines, atmospheric pressure highs and lows, locations and movements of developing storms or gales). Refer back to Figure 9 for an example of a wind-wave forecast. Upper air (500-millibar) charts show the direction and strength of the upper air winds that have an important effect on surface conditions. Winds at the 500-millibar level generally blow from west to east. By examining the “troughs and ridges” of upper-air isobars, forecasters can anticipate where surface lows and highs will occur. Other charts show tropical cyclone danger areas, sea surface temperature, and satellite imagery.12 Weather analyses are issued once or twice per day, and forecasts typically are issued for 24, 48, and 96 hours, although other periods also may be used.
Weather charts and text reports are available to ships at sea via shortwave (single sideband) radio or satellite links. Today, even small cruising yachts can access these reports. For example, on Dreams, using a laptop computer, a terminal node controller (a fancy name for a radio-frequency modem, a device that encodes or decodes text or graphics files transmitted by radio), and a high-frequency shortwave radio, I can download the latest marine weather charts. With this capability, a vessel can keep track of weather in its vicinity as well as distant weather trends.
At present, the ability to forecast extreme waves does not exist. Researchers are working to identify sea state conditions that can be correlated with the occurrence of extreme waves. If the work is successful, it may be possible to provide warnings to vessels to avoid areas where such waves could occur. In particular, a parameter called the Benjamin-Feir instability index may correlate with sea conditions where extreme waves are more likely. European weather forecasters are experimenting with this approach.13 There are global ocean maps that indicate prevailing significant wave heights for each month of the year. This information, combined with knowledge of major currents such as the Agulhas or Gulf Stream, provides an indication of areas where extreme waves are more likely to occur.