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
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
Description | Height (m) | Effect | 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 | More than 14 | Precipitous seas, experienced only in cyclones | 9 |
Swell conditions
Description | Wave length (m) | Wave period (seconds) | Wave height (m) |
---|---|---|---|
Low swell of short or average length | 0–200 | Less than 11 | 0–2 |
Long, low swell | more than 200 | More than 11 | 0–2 |
Short swell of moderate height | 0–100 | Less than 8 | 2–4 |
Average swell of moderate height | 100–200 | More than 8, less than 11 | 2–4 |
Long swell of moderate height | more than 200 | More than 11 | 2–4 |
Short heavy swell | 0–100 | Less than 8 | more than 4 |
Average length heavy swell | 100–200 | More than 8, less than 11 | more than 4 |
Long heavy swell | more than 200 | More than 11 | more than 4 |