The science of waves

Surfers dream of the perfect wave, while dinghy sailors can do without them. But what water users share in common is that waves can make or break a day. So, let’s find out more about the science behind waves.
Seascape shot of waves breaking on the shore at Aldeburgh

Photo: RNLI/Nathan Williams

Ocean waves are generated by the wind. ‘When wind blows across the water surface, the friction between the wind and the surface ruffles the surface of the sea,’ explains Paul Russell, Professor of Coastal Dynamics, University of Plymouth. ‘Little waves, known as capillary waves, are created. If the wind continues to blow on these little undulations, they get bigger. 

There are three factors you need for big waves to be generated: you need a strong wind, blowing for a long time, over a long distance.’ The requirement for a long fetch (the distance over which the wind blows) and no obstacles, is why the biggest waves are generated in the open ocean, be it the Atlantic or the Pacific Ocean.

Waves can travel long distances and so may end up thousands of miles from the original storm that formed them. Scientific studies have tracked waves that were started by storms off New Zealand, all the way to the coast of California.

Professor Paul Russell researches wave behaviour

Photo: Professor Paul Russell

Professor Paul Russell researches wave behaviour

The formation of swell

 ‘There are two fundamentally different sorts of waves in the sea,’ explains Paul. ‘There are wind-driven waves, which are the ones generated, pushed along and grown by the wind. And then once they leave that storm area, they organise themselves into “swell waves”.’ So wave is the general term, and swell is a specific term meaning a kind of wave generated by distant winds.

The swell waves propagate away from the generating area and travel until they reach shallower water. So even if it’s a calm, still day, your local beach might have big swell waves which have come from a distant storm.

‘Waves travel about 20 or 30 miles an hour in the open ocean,’ adds Paul. ‘So big waves generated by storms far out in the ocean can take 2 or 3 days to get to the UK, by which time the conditions in the UK might be calm and sunny. And this is one of the most misunderstood things about waves – you can still have big waves at the coast even when local winds are calm.’

Types of waves

When waves come into shallow water, they break. The wave reaches a point where the crest is travelling faster than the trough, so the wave becomes unstable and breaks. There are two types of breaker: a spilling breaker and a plunging breaker.

‘A spilling breaker is a gentle breaking wave and occurs on flat, gently sloping beaches, such as Polzeath in Cornwall,’ says Paul, who’s also a former European surfing champion. ‘Spilling breakers topple over gently and spill shorewards, often accompanied by an onshore directed wind. These waves are popular with learner surfers.’

The converse is the plunging breaker, which breaks in a more violent way, when the transition from deep water to shallow is greater. 

‘The plunging breaker curls over at the top and forms a tube that surfers can ride in,’ Paul explains. ‘To get a good plunging wave, you need a steeper slope for the wave to break on, and an offshore wind to hold the wave up and delay it breaking. On UK beaches, that often occurs at low tide because beyond the low water mark beaches tend to drop off and there’ll be a steeper slope.’

You can see this effect at the popular surfing beaches of Croyde in North Devon and Fistral in North Cornwall. And if you’re a windsurfer or kitesurfer, you need wind and waves, or wind-driven waves.

RNLI Lifeguard Jonathan Waugh surfing

Photo: Jonathan Waugh

Local variations

Various factors on individual beaches will affect waves. If there’s an onshore wind, waves that might have been good for surfing can be messed up, whereas an offshore wind can improve waves. As the wind blows from the land towards the sea, it helps to hold waves up, keeping them ‘clean’. The shape of the beach and the sea bottom topography also affects waves as they bend and break.

'In the summer, smaller waves bring in sand, creating sandbanks that waves break around,' says Paul. 'It's ironic that this is good for surfing, but the rip currents that develop between the sandbanks are also hazardous for bathers and learner surfers.'

RNLI Lifeguard Jonathan Waugh

Photo: Jonathan Waugh

Jonathan Waugh, an RNLI Lifeguard on Porthmeor Beach in St Ives, Cornwall

Forecasting waves

In the past, forecasting waves was down to deciphering weather charts, tide conditions, wind conditions and having good local knowledge. Now, there are forecasting apps just a click away. These include magicseaweed.com, windy.com, windguru.cz.

‘On an open ocean scale, the science of predicting the size and timing of waves is very good,’ says Paul. ‘But how those waves interact with the coastline, such as bending round headlands, makes conditions on specific beaches harder to forecast.’

The perfect wave

Jonathan Waugh, an RNLI Lifeguard on Porthmear Beach in St Ives, Cornwall, has been a surfer for over 20 years.

‘I’m still looking for the perfect wave, but there are great waves all over the planet. Indonesia is a consistently good surf spot - you’ll get waves every day. Australia, Costa Rica and South Africa have renowned surf spots, as do Portugal, France and Spain. I haven’t been to Hawaii, but there’s a spot called the Pipeline, which has the most perfect barrelling wave in the world. The swell comes out of deep water and hits a very shallow short reef, so the swell breaks suddenly, producing an incredibly critical and powerful wave.’

Mythbuster: Rogue waves

A ‘rogue’ wave occurs when waves that are travelling in different directions come together and combine their heights into one wave. This sudden increase in size can be hazardous for water users out at sea. Closer to shore, waves typically arrive in ‘sets’ (a group of a few large waves one after the other) every few minutes, with a few minutes of smaller waves in between. These are not ‘rogue’ waves. But a particularly large ‘set’ is often responsible for washing anglers from rocks and coastal walkers into the sea. Our advice is to stay clear of the edge, especially in large wave conditions, when the water level can raise by several metres with each set.

Stay safe

  • In storm conditions exercise extreme caution near the coast and avoid exposed areas.
  • If you’re in trouble in the water, and you have a board, always stay with it.
  • If you're boating, always wear a lifejacket or a buoyancy aid.
  • A lifeguard patrolled surfing beach will usually have black and white chequered flags designated for surfing. Stay within the lifeguarded area and read safety signage.
  • If you’re caught in a rip current, stay calm and swim or paddle across the current to escape it.
  • Carry a means to call for help. If you or someone else is in danger call 999 and ask for the Coastguard.

What to do if you're caught in a rip

  • Stay calm – don't panic.
  • If you can stand, wade. Don't try to swim.
  • If you have an inflatable or board, keep hold of it to help you float.
  • Raise your hand and shout for help loudly.
  • Don't swim directly against the rip or you'll get exhausted.
  • Swim parallel to the beach until free of the rip, then make for shore.
  • If you can't swim, float. Lean back in the water and extend your arms and legs.
  • Looking for more safety advice? Check out our activity pages.

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