Who, What, Why: How do you measure a wave?

Reports in the past 24 hours suggested that the UK had recorded its biggest ever wave, at Penzance in Cornwall. It later turned out to be an anomaly. But how do you measure the height of a wave, asks Tom Heyden.

Waves off the coast of the UK are typically measured by "Waverider buoys", which float along the surface of the water, loosely tethered to the sea bed. Lodged inside them is a highly sophisticated "accelerometer", which records the rate at which the buoy rises and falls with the water. The accelerometer - which also measures frequency and direction - then integrates the information to calculate the displacement over a few seconds - giving a height reading.

Initial reports suggested that a wave as high as 75ft (22m) had been recorded. But even if it had been true, it wouldn't have been one specific wave, as one might imagine it. "[Waverider buoys] don't aspire to measure individual waves, they aspire to take a sensible average of three-wave parcels," says Dr Kevin Horsburgh, head of marine physics and ocean climate at the National Oceanography Centre (NOC). And wave height alone doesn't really tell you the "magnificence" of it, he says. "What impresses people about waves is their steepness," he adds, but that differs depending on how far apart the two peaks are. In other words, a height of 75ft (23m) over a 33ft (10m) distance would be more impressive than one over 98ft (30m). Individual waves can be recorded from ships, he says, which take accelerometers on board. That's how some of Horsburgh's NOC colleagues recorded the biggest UK wave of 95ft (29.1m) in the Atlantic Ocean in 2000. But when it comes to the Waverider buoys, accuracy is best reached over time. "If you use different sized buoys you actually get different readings, so it's a very ongoing area of research to find the most reliable and consistent wave measurement."

But this isn't the only potential record breaker to have hit the news in the past few days. UK surfer Andrew Cotton claims to have broken the world big wave surfing record by riding an 80ft (24m) wave off the Portuguese coast. And these waves are measured differently. Providing the video footage is good enough, which is how big surf waves are typically analysed, scientists take measurements from the wave's highest peak to its trough - the bit in front "where the curve is as flat as possible", says Dave Reed, director of the UK Pro Surf Association.

It sounds pretty straightforward. But waves don't behave uniformly, and much depends on the geography of the sea bed. Some waves "pitch ahead of the curve" - typically breaking over the head of the surfer - while others start breaking behind the surfer, as with Cotton. As such, it's often "extremely difficult" to decide where to take the measurements from. There's an added subjective element when it comes the world of surfing, says Reed, as it's about "making" a wave as much as merely recording its size. And Reed says it's not easy to tell with Cotton because of all the white water. There's a lot of controversy about how to judge "completion", he says.

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