What is a rogue wave?

In December 2022, a massive wave struck a cruise ship traveling to Antarctica. It shattered windows and flooded rooms, killing one person and injuring another four. News reports called it a rogue wave—a wave of unusual size that seemed to come out of nowhere.

Measured amplitude graph showing the Draupner wave with the spike in the middle. (Public domain)

Mariners have reported such unexpected, massive waves for hundreds of years. For most of that time, people thought it was mere folklore. But on New Year’s Day in 1995, one of these waves was detected by a scientific instrument. A laser was attached to the Draupner gas platform in the North Sea, midway between Scotland and Norway. The laser measured the height of the waves. Most were big at the time, due to a weather system that had just moved through. The tallest were about 12 meters (39 feet) high. But one measured a whopping 25.6 meters (84 feet) from trough to crest. Luckily, the wave didn’t damage the platform. But the measurement confirmed, for the first time, that rogue waves exist.

But what, exactly, is a rogue wave? When scientists talk about wave height, they tend to focus on the tallest one-third of the waves in an area. They call the average of these the “significant wave height.” No matter the weather conditions, waves vary in size. It’s those taller, significant waves that have the most energy and are most likely to cause damage. They’re also the best ones for surfing. For a scientist to call a wave rogue it has to be at least twice as high as the significant wave height in the area. The most extreme rogue wave recorded was three times taller. But unless instruments are taking measurements, it’s hard to say if a wave qualifies as rogue, especially in stormy seas.

What causes some waves to grow higher than others?

Rogue waves aren’t the only waves of unusual size. There are many accounts of large, unexpected waves washing over seawalls and up onto beaches. These can injure people and damage structures along the shore. What causes some waves to grow to unusual heights?

Scientists measure waves in terms of their height from crest to trough. They also measure the wavelength, which is the distance from one crest to the next, and the period, the time between wave crests as a wave passes. Waves move at different speeds, depending on their wavelength, their period, the water depth, and any underlying water motion.

Faster-moving waves can catch up to slower ones; if the peaks and troughs align, they can reinforce each other. Changes in wave speed can also cause waves to change direction. If they bend away from each other, they create areas of low wave heights. If they focus toward each other, wave heights increase. These processes can create waves that wash far up the beach, soaking towels and other items that seemed to be in a safely water-free zone.

What causes a rogue wave?

But not all waves add up in this way. Some exchange energy when they interact. This causes the wave heights to multiply, rather than simply add together. It’s this kind of interaction that creates rogue waves. Although the exact cause is not yet understood, rogue waves are the subject of serious investigation. Scientists hope it won’t be long before we have a better sense of when and where they will strike.