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 Another waveform mentioned in forecasts and warnings is the swell. 
  Swells are wind waves that have moved out of the area where the waves have formed. If the wind blows constantly from one direction over a large distance (fetch) the waves become organised. At this stage the waves are still wind waves or sea. 
  The generating wind can then drop and the waves undergo changes. The crests become rounder and the wavelength increases. The waves are now swell. These swell waves may travel many thousands of kilometres from where they were formed as wind waves. The further swell travels from its source the longer the wavelength and the swell period (time it takes for crest to crest to pass a point) becomes. 

  Swell waves can travel enormous distances retaining their energy. In 2005 a storm in the Gulf of Alaska sent swell waves southwards through the Pacific Ocean to the shores of Antarctica; over 13,000 kilometres. This storm and the swell waves it caused is responsible for the breaking up of iceberg B-15A. The berg, which had an area of over 2,000 square kilometres broke into half a dozen pieces as a result the Alaskan swell. The waves took six days to travel to the Antarctic shore. 
  Around the Australian coastline wave buoys are used to monitor and predict long period swells arriving onto the coast. The speed that a deepwater swell travels at is approximately three times its wave period. A wave with a 10 second period will travel at 30 knots; a fifteen second period will travel at 45 knots. Swell waves also travel in sets. The wave sets travel at approximately one half of the speed of the individual waves. However, once in shallow water the individual wave and wave set become the same. 
  Long period swells travel faster and have a greater ‘weight’ of water than a shorter period swell of the same height. This causes problems on beaches and for rock fishermen. Simply put, the speed of the wave and the amount of water ‘in’ the long period wave causes the wave to flood onto shore. Long period waves can be deceptive for their height. In NSW around ten rock fishermen are killed each year as a result of dangerous waves. On 29 and 30 January 2005, five people died in NSW as a result of a dangerous long period swell event. 
  The swells that affect the east coast of Australia are usually the result of low pressure systems in the Pacific. 
  The southwestern Tasmanian coast experiences some of the consistently largest swells in the world. This is due to the deep low pressure systems and the prevailing strong westerly winds through the southern Indian Ocean. 
  The swell from winter storms that effect Tasmania travel across the Pacific and generates the summer surf in Hawaii and California. 
  Occasionally, in Bureau forecasts, the swell is described as confused. This is the result of swells of different direction generated by different systems meeting. Where the two meet the swell becomes confused. 
  In Bureau forecasts swell heights are in metres and the direction is the direction the swells are coming from. 
  Smaller vessels in open ocean swells can generally bob up and down ‘like a cork’. Larger vessel can become suspended between two swells or may sit on top of an individual wave. In these conditions the unsupported weight can cause structural damage to the vessel. 

  Seas are the waves that are caused by the wind that is blowing at the time. Swells are the waves that have been generated elsewhere and may have travelled thousands of kilometres. 
  Both type of waves are included in Bureau of Meteorology forecasts and warnings. 
  However, when the mariner heads into the open sea their vessels are affected by a combination of the sea and swell; the sea state. 
  There is a mathematical formula to work out the significant combined wave height (sea state) a mariner is likely to encounter from a Bureau forecast. 
  From a forecast issued in April 2005: 

CENTRAL WEST COAST, Low Rocky Pt to Sandy Cape: A Gale Warning is current. Friday until midnight: West to southwest winds 30 to 40 knots, possibly 45 knots in the far south, gradually easing to 20 to 30 knots later. Seas 4 to 6 metres decreasing. West to southwest swell increasing to 4 to 6 metres.  


Take the highest wave heights (sea and swell) mentioned and square them both, add the two resulting figures together and take the square root of that figure. Or (6 x 6) + (6 x 6) = 36 + 36 = 72, Square Root of 72 = 8.5 
  The combined significant wave height from the above forecast is 8.5 metres; maximum possible wave from the above forecast 17 metres. 
  Using the graph to calculate the combined significant wave height. Significant = 8.5 metres, maximum = 17 metres. 
  Above is the graph from the Bureau’s waverider buoy that is located in the Central West Coast forecast area for the date of the forecast. The graph shows that the significant wave height at the buoy reached 7 metres and the maximum wave height peaked at just over 14 metres. 
  The buoy samples the waves at a single point in the forecast area that covers over 14,000 square kilometres. The readings from the buoy were close to the levels calculated, however, it is likely that waves up to the levels calculated occurred in other parts of the forecast area. 


*Malcolm Riley is the Public and Marine Officer for the Bureau of Meteorology in Hobart. He has worked in all States with the exception of QLD and is a Master V. He gives education courses on Marine Meteorology. 

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