Waves

The Bureau of Meteorology provides forecasts of wave (sea and swell) heights in metres.

Wave heights describe the average height of the highest third of the waves (defined as the significant wave height – see diagram below). It is measured by the height difference between the wave crest and the preceding wave trough.

The Bureau of Meteorology does not forecast maximum wave heights in routine forecasts. Statistically it is estimated that about one in every 2000 to 3000 waves (three to four times a day) will be approximately twice the height of the total wave height.

Wave height graphic and explanation.

 

Wave, sea and swell terms

King or rogue waves are waves greater than twice the total wave height. These very large waves occur when ocean currents run opposite to the prevailing sea and swell, and waves overrun each other. This generates steep and dangerous seas. Mariners should prepare for a rogue wave encounter.

Sea state refers to the combination of sea and swell waves. Sea state descriptions are provided in the tables below.

Sea waves are generated by the local prevailing wind. Their height depends on the length of time the wind has been blowing, the fetch (the distance the wind has blown over the water), and the water depth. They may also be referred as seas in marine text forecasts and wind waves in map displays.

Significant wave height is the statistical basis for all wave heights presented in text forecasts and map displays. Wave heights are variable over time. The statistical definition is calculated as the average height of the highest one-third of the waves experienced over time.

Swell waves are the regular, longer period waves generated by distant weather systems. They may travel over thousands of kilometres. There may be several sets of swell waves travelling in different directions, causing crossing swells and a confused sea state. Crossing swells may make boat handling more difficult and pose heightened risk on ocean bars. See more information about the second swell. There may be swell present even if the wind is calm and there are no sea waves. There are two methods for identifying multiple swells.

  • Energy-based identification: The Bureau's AUSWAVE wave model identifies individual swells based on their energy. The AUSWAVE model outputs a primary and secondary swell to refer to the height and direction of the swell with the highest (and second highest) energy component.
  • Direction-based identification: The Bureau's MetEye map displays swell forecasts that are assigned either Swell1 or Swell2 according to a pre-determined direction convention for the Australian coastline. This means Swell1 may not always represent the highest swell conditions.

Total wave heightis the combined height of the sea and the swell that mariners experience on open water. It may also be referred as the combined sea and swell or significant wave height. This lookup table can be used to calculate the total wave height from individual values of seas and swell heights.

  • The probable maximum wave height can be up to twice the total wave height.

Wave length is the average distance between crests (or troughs) of waves.

Wave period and swell period is the average time between crests (or troughs) of waves. The larger the time difference, the greater the amount of energy associated with the waves or swells.

See also the definitions in the Bureau Glossary and explanations in Weather Words.

 

Wave explanations

Description Height (metres) Effect WMO Sea State code
Calm (glassy) 0 No waves breaking on beach 0
Calm (rippled) 0 - 0.1 No waves breaking on beach 1
Smooth 0.1 - 0.5 Slight waves breaking on beach 2
Slight 0.5 - 1.25 Waves rock buoys and small craft 3
Moderate 1.25 - 2.5 Sea becoming furrowed 4
Rough 2.5 - 4 Sea deeply furrowed  5
Very rough 4-6 Sea much disturbed with rollers having steep fronts 6
High 6-9 Sea much disturbed with rollers having steep fronts (damage to foreshore) 7
Very high 9-14 Towering seas 8
Phenomenal over 14 Precipitous seas (experienced only in cyclones) 9

 

Swell explanations

Description Wave Length Period Wave Height
Low swell of short or average length 0 - 200 m Less than 11 sec  0-2 m 
Long, low swell over 200 m Greater than 11 sec 0-2 m
Short swell of moderate height 0-100 m Less than 8 sec 2-4 m
Average swell of moderate height 100-200 m Greater than 8 sec, < 11 sec 2-4 m
Long swell of moderate height over 200 m Greater than 11 sec 2-4 m
Short heavy swell 0-100 m Less than 8 sec over 4 m
Average length heavy swell 100-200 m Greater than 8 sec, < 11 sec over 4 m
Long heavy swell over 200 m Greater than 11 sec over 4 m
 
 

Reference

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