Rogue waves: from a sailor's nightmare to scientific acceptance

What is a rogue wave?

The technical definition of a rogue wave, also known as an extreme storm wave by ocean scientists, is a wave that is 2.2 times taller than the average height of the tallest third of surrounding waves, measured from trough to crest.

This could be a wave only a couple of feet taller than the gentle waves on a mildly windy summer day, to the fabled walls of water 100 feet tall that stalked anxious mariners crossing the great seas and oceans. It is common for rogue waves to be very steep-sided relative to the waves around them and for their troughs to be especially deep. 

They are also characterized by their unpredictability, often emerging from an unexpected direction. They typically don't align with prevailing winds and ocean currents, which has likely led to their reputation for "sneaking" up on unsuspecting vessels.

How do rogue waves form?

We don't know everything about how a rogue wave forms, but there are some processes known to produce a rogue wave.

One such process is constructive interference. Waves move at different speeds and have different amplitudes, and sometimes one wave can overtake another. When these waves pass through one another, their crests, troughs, and lengths can combine to reinforce each other.

If you think of a wave's crest as a 1 and its trough as a -1, adding two crests together can produce a 2, while two troughs combine to form a -2. This process is also cumulative, so if several waves combine in such a way, they can quickly grow to enormous heights.

Despite their height, rogue waves formed from constructive interference tend to be fairly short-lived, usually returning to a normal height after several minutes.

Another way a rogue wave can form is when wave energy is focused, usually during a storm. When the storm produces waves that go against the prevailing ocean current, the wave frequency shortens.

This allows waves to join together to produce towering rogue waves that can be much longer-lasting.

Where do rogue waves occur?

A rogue wave is an open-water phenomenon usually occurring in seas and oceans rather than lakes and rivers, but rogue waves can occur on any body of water where gravity waves are possible.

And since a rogue wave is a physical/mathematical phenomenon and not specifically a maritime one, you can actually get a rogue wave in any medium where a wave of some kind is possible, including quantum mechanics.

Can a rogue wave flip a cruise ship?

It's not very likely, but it's not impossible either, it really depends on how big a cruise ship we're talking about. A large modern cruise ship is really big, about as tall as a 16-story building.

The average height for a modern large cruise ship is almost 200 feet tall, so even the largest rogue wave isn't going to over-top a cruise ship, but it can still knock it pretty hard, as the video above demonstrates.

The other major thing to remember is that modern cruise ships are incredibly heavy. The largest cruise ship in operation, the Wonder of the Seas (operated by Royal Caribbean International), was launched in January 2022 and weighs 236,857 gross tons.

The force of a 100-foot tall rogue wave is incredible, but not nearly enough to flip over nearly a quarter of a million tons of boat. It will still do some damage though.

If you're talking about smaller cruise ships, then a rogue wave could absolutely flip one of those over if it hits a ship's broadside,e which they could very well do, seeing as rogue waves are known for not moving with the prevailing wave direction, which is the direction a ship is likely to be moving as well. 

What is the biggest rogue wave ever recorded?

There are a few ways we can define the biggest rogue wave ever recorded.

If we are talking about sheer height, the largest rogue wave was actually the one that proved its existence in the first place.

The so-called Draupner wave was an 83.6-foot (25.5 meters) wave that crashed into the Draupner oil platform on New Year's Day, 1995, just off the coast of Norway in the North Sea. The wave was recorded by a laser detector on the platform.

While tall waves in the North Sea are not unusual, this wave was twice as tall as any ever measured in that part of the ocean and towered over the surrounding waves as it passed, which were "only" about 40-feet tall.

Until the Draupner wave was recorded, scientists were still debating whether rogue waves were just the exaggeration of traumatized sailors or a genuine physical phenomenon. The Draupner wave put any doubts to rest.

More recently, the most extreme rogue wave ever recorded was spotted off the coast of British Columbia in November 2020 by a wave-measurement buoy, measuring about 58 feet (17.6 meters) tall.

What makes the Ucluelet wave, as it's known, so special is that it was nearly three times larger than the surrounding waves.

"Proportionally, the Ucluelet wave is likely the most extreme rogue wave ever recorded," said physicist Johannes Gemmrich from the University of Victoria. "Only a few rogue waves in high sea states have been observed directly, and nothing of this magnitude."

Can rogue waves occur in calm seas?

Rogue waves aren't exclusive to stormy seas, especially those produced by constructive interference.

While we usually think of the super-tall rogue waves like the Draupner wave, the only thing a rogue wave needs to be is just over twice the height of the tallest third of the surrounding waves in the area.

If the average wave height on a lake is four inches tall, then a wave that is only a foot tall would be comparable to the Ucluelet wave in terms of extremity, even though its absolute height is barely noticeable.

This is one of the things that makes rogue waves tough to identify, and why only the tallest ever really seem to stand out in the record as well as in the memories of surviving sailors.

Rogue wave vs tsunami

It's also important to distinguish a rogue wave from a tsunami, which is a very different phenomenon, and not just because of the processes that form them.

A rogue wave is thought to be the product of smaller converging waves that reinforce each other and so grow in height.

A tsunami is the rapid physical displacement of water, usually produced by sudden movements on the seafloor from an earthquake or an underwater volcano.

A characteristic of tsunamis is that there is no peak or trough the way rogue waves have, but rather a sudden rise in sea level well above what is normal. This is similar to the way we experience sea and ocean tides, which leads to a tsunami's other name, the tidal wave.

And since there is no trough behind the leading edge of the tidal wave, it simply brings the sea or ocean with it onto land, inundating the area, often for miles inland.

Needless to say, neither tsunamis nor rogue waves are forces to be messed with, though both are fascinating phenomena that we can't help but study in spite of their terrifying power.