Why do waves seem to break or move very slowly from a distance?

March 8, 2021 | Surfing
Ocean: waves that break far away from our eyes seem to crash very slowly | Photo: Shutterstock

Have you ever stood on a boardwalk staring at the sea and somehow witnessed large waves breaking slowly in front of your eyes?

It's a common phenomenon, and it is particularly visible when powerful swells hit the coastline or during stormy days.

You're standing in front of the surf, and the big waves seem to crash very slowly at a distance.

Like in a slow-motion replay, the wave grows in size, starts to bend, and then slowly crumbles forward.

A similar event can occur when you're flying, minutes before landing, and you look down at the coastline from the airplane window.

You can see and recognize waves and white foam from above, but they look like they're stationary and not moving at all.

So, why do large waves seem to break and move very slowly and, at the same place but on another day, smaller waves break quickly?

Waves: they travel at different speeds | Photo: Shutterstock

Visual Perception

It all has to do with several visual perception variables and how our eyes and brain interpret optical inputs and movement.

In the beach example, in most cases, what happens is that those big waves are crashing further out the back compared to the smaller waves that we often see breaking near the shore.

So, our eyes perceive big waves breaking slowly because they're far away and take longer to cross our line of vision than those close by.

From up above, they seem like they're actually frozen or still, and the interval between waves - wave period - seems super long, if not inexistent.

The first thing we have to understand is that when we're far from land - for instance, inside an airplane at 10,000 feet (1.9 miles or 3 kilometers) - your eyes have no stationary references.

As a result, you can't exactly determine the "real" motion of the waves.

But there's more - relative velocity plays a trick on us.

Relative velocity: why do waves seem to break or move very slowly from a distance? | Photo: Hawaiian Airlines

Relative Velocity

When we're on terra firma, we estimate the wave velocity with respect to the shoreline.

However, when we're inside an airplane, we tend to get a bigger picture and gauge the velocity of the waves based on the whitewater lines and other wave crests moving at the same speed.

So, because we get that bigger picture and compare the several visible wave lines, there's no apparent movement.

Well, in reality, there is movement.

But our brain is fooled to believe that those waves should be moving faster and can't figure out why.

We have to take into consideration that a centimeter of ocean or land observed from the sky equals dozens of yards or meters of ground or water.

Therefore, it's not hard to imagine why a wave looks like it is stationary, given the fact that, from up above, a regular wave travels at an average speed of 10 feet (3 meters) per second.

Our eyes can't detect a 30-meter move from a long distance.

It's also important to stress that while we're flying, we only paying attention to the waves for a short amount of time, resulting in a small displacement of waves.

And when we're about to land - at around 1,000 feet (300 meters) from the ground - the phenomenon might be even more mesmerizing.

The ground is moving fast from left to right (or vice versa), and the waves in the back appear to move slowly.

Why? Let's do simple math.

The whitewater lines take around one second to travel 10 feet; the airplane is flying at 600 miles per hour (965 kilometers per hour), which means that it travels at 880 feet per second.

So, we've got a huge discrepancy between speeds - 10 feet per second (waves) versus 880 feet per second (airplane).

Motion Is Relative

Science tells us that all motion is relative.

When something is moving, there is not a correct answer to how fast it is going or even if it is moving at all.

It all depends on the frame of reference of the observer. The frame of reference is the view of the person or object observing the motion.

The frame of reference may be static or moving.

From a person standing on the beach, the waves are moving.

But from the person's frame of reference on an airplane near the ground, it's the houses and shoreline that appear to be moving.

According to the laws of physics, there's no way to distinguish between an object at rest and an object moving at a constant velocity.

As a result, we can't answer the question: "How fast is something moving?" It's all relative, and the eyes can easily deceive the observer.