Waves and swell

Learn about types of waves, wave height and other features, and how we describe sea conditions

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How waves are generated

The main way that waves are generated is through wind blowing across the surface of an ocean, sea or lake. This can generate 2 types of waves – wind waves (or sea waves) and swell.

The size of these waves is determined by 3 factors:

  • wind speed (strength) – the intensity of the wind blowing over the water surface
  • duration – the length of time the wind blows
  • fetch – the distance over which the wind blows.

Some waves are not generated by the wind. For example, tsunami waves are caused by an earthquake or other large-scale ocean disturbance.

Video: Understanding waves

Explains how chop and swell are generated and can lead to dangerous waves.
Waves are created when strong winds from a weather system blow over the ocean's surface. In the immediate area, this wind action can generate sea waves (or chop).

The fetch is the distance over which the wind blows.

Sustained wind builds energy forming longer, deeper swell waves. They can travel thousands of kilometres before dying away.

Dangerous waves develop with stronger winds, a longer fetch, and more time to build. Dangerous waves can put your safety at risk and cause damage to coastlines and property.

Know your weather. Know your risk.

Wind waves or sea waves

Wind waves are also called sea waves or chop.

These waves are generated by local wind, around the time they are observed. There is a short time between the wave crests – that is, a short wave period.

Swell

Swell is also a wave. Swell waves have travelled far from the area where they were caused. They are generated by sustained wind in distant weather systems, such as tropical cyclones and low pressure systems like east coast lows.

Swell can travel thousands of kilometres before the waves fade. There can be swell even if the local wind is calm and there is no chop.

Multiple swells

Swell generally travels in one direction but there can be a second swell at the same time, travelling in a different direction.

To understand this, imagine 2 children throwing large rocks into either end of a pond. The waves travel from where each rock hits the water to the other side of the pond. The line of waves moving outwards is called a wave train.

The separate wave trains interact with each other where their paths cross. This creates higher peaks where the peaks of the wave trains intersect, and lower troughs where the two troughs meet. Even after the last rock has been thrown, the waves continue to exist and interact for some time.

This is how separate weather systems can generate different wave trains that travel to your location. If the wind associated with each weather system has been blowing from the same direction and for long enough, the waves become organised. The wind might stop but the waves it created continue moving outwards as a swell.

In this way, swell wave trains generated by separate weather systems can cross paths somewhere in the middle. These swell waves were generated separately, but can exist within the same area. Wind waves can also occur when there is swell.

King or rogue waves

King or rogue waves happen when ocean currents run opposite to the prevailing sea and swell, and waves overrun each other. This generates steep and dangerous seas.

These large waves can be greater than twice the total wave height.

Wave height

The height of a wave is measured by the difference between the wave crest and the wave trough that came before the crest.

We give our forecasts of sea waves (wind waves) and swell wave heights in metres. On the water, you will experience waves that vary in height from one wave to the next.

Significant wave height is the average height of the highest third of the waves – whether these are sea waves, swell waves or the combined sea and swell.

Total wave height is the combined height of the sea waves and the swell. It's also called the combined sea and swell.

We do not forecast maximum wave heights in routine forecasts. About one in every 2000–3000 waves (3–4 times a day) will be about twice the total wave height. Be prepared for a wave of this height before going out on the water.

Calculating total wave height – combined sea and swell

Total wave height is not calculated by simply combining the individual sea wave and swell heights. For example, a forecast 1.5 m sea and forecast 1.5 m swell results in a total wave height of 2.1 m.

You can calculate it using this formula:

Total wave height = [(Wind wave height)2 + (Swell wave height)2+ (Secondary swell wave height)2]1/2

Wave length, sets and period

Wave length

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

Wave period

Wave or swell period is the average time between the crests or troughs of the waves.

The more time between waves, the greater the amount of energy they have. Long period swells are generally not a problem while they are in deep water but as they approach shallow water, they lose speed while getting steeper and higher.

Wave sets

Over time and distance, waves that move at nearly the same speed keep pace with one another and organise to form a group. This is called a 'set' of waves.

The time between each set varies, as does the number of waves in each set – it can be 3 to more than 15 large waves in a row. Smaller waves are usually in the lead, larger waves in the middle, and smaller waves again towards the end of the set.

Sets of waves can come in every few minutes, or every half hour or more, depending on how far the waves have travelled. Usually the longer the distance they've travelled, the longer the 'lull' between sets.

A long time between sets can give the ocean a deceptively calm appearance before the next set arrives. Waves breaking on the beach or overtopping rocks can then take people by surprise.

Describing wave and swell conditions

Sea wave and swell conditions together are called the 'sea state'. The World Meteorological Organization (WMO) has a code to describe these conditions – it's called the Sea State Code.

Sea wave conditions

DescriptionHeight (m)EffectCode
Calm – glassy0No waves breaking on beach0
Calm – rippled0–0.1No waves breaking on beach1
Smooth0.1–0.5Slight waves breaking on beach2
Slight0.5–1.25Waves rock buoys and small craft3
Moderate1.25–2.5Sea becoming furrowed4
Rough2.5–4Sea deeply furrowed5
Very rough4–6Sea much disturbed with rollers having steep fronts6
High6–9Sea much disturbed with rollers having steep fronts,
damage to foreshore
7
Very high9–14Towering seas8
PhenomenalMore than 14Precipitous seas, experienced only in cyclones9

 

Swell conditions

DescriptionWave length (m)Wave period (seconds)Wave height (m)
Low swell of short or average length0–200Less than 11 0–2
Long, low swellmore than 200More than 110–2
Short swell of moderate height0–100Less than 82–4
Average swell of moderate height100–200More than 8, less than 112–4
Long swell of moderate heightmore than 200More than 112–4
Short heavy swell0–100Less than 8more than 4
Average length heavy swell100–200More than 8, less than 11more than 4
Long heavy swellmore than 200More than 11more than 4

 

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