Marine Weather: Ocean Waves & Swells
There are plenty of factors that can affect your fave ocean-going activities, but ocean waves might be number one. Depending on sea conditions, waves can turn a peaceful trip into a dangerous situation in a flash. But do you actually know how waves work? Where they come from? WTF they even are?
The more you know about the open seas, the more confident you’ll be in navigating them, so we’re doing our part to clue you in on all the elements that affect marine weather.
(If you slept through your college oceanography class, this one’s for you.) First up, we’re exploring the wonderful world of waves, from what they are to how we measure them.
Welcome to wave 101.
What is a wave?
To start, let’s bust the biggest myth about waves. An ocean wave is not actually made from moving water. It’s made from energy moving through water particles. (We’ll break this down in detail later.) But just remember that without an energy source, there are no waves. Luckily, because the ocean is constantly drawing energy from different sources, it is constantly full of waves, even when they’re pretty small.
Waves are simply the transfer of energy through water, not water from one place to another.
Where does wave energy come from?
All waves are born from an energy source. When it comes to ocean waves, our planet provides several energy sources (many of which also make for great disaster movies). The most exciting sources of ocean wave energy include: Earthquakes, volcanoes, and landslides.
But the vast majority of waves are caused by something more common:
Most of the waves you see on the open sea or crashing on the shore are wind waves. And their path to your local beach is much more exciting than you might think.
How wind makes waves
Our world’s oceans are moved by winds. But where do winds start? With the sun.
The Earth’s surface is uneven; think of the many land masses, mountains, valleys, seas, etc. all over it. Because of this, the sun heats the surface and atmosphere unevenly. As the air heats more in certain areas, heat particules rise and cold air rushes in to fill the space those particles occupied. That rush of cold air is where wind movement comes from.
Now back to waves. Winds are constantly blowing across the ocean. Some winds are lighter; some are more intense (e.g., a storm). But they all contain a certain amount of energy.
How much wave energy does the world have?
Estimates suggest that just 0.2% of the ocean’s untapped energy could power the entire world. Seriously.
When wind rubs against the surface of water, the wind’s air molecules transfer that energy to the ocean’s water molecules, setting those water molecules in motion in the direction the wind was blowing.
The wave is created when those water molecules bump into other water molecules, transferring energy and creating a domino effect. But remember that waves are made from energy moving through water; the water itself barely moves. Those individual water molecules simply move in a small clockwise circle in the same place. (This repetitive motion is known as oscillation.) During their rotation, they come into contact with fellow molecules and continue the transfer of wave energy.
Here’s what it looks like. (Notice that the oscillation of water molecules in deeper water is smaller.)
So that’s what’s happening on a molecular level, but what does that look like to our eyes?
Let’s start with an ocean storm. Most waves are made by storm winds, which are strong and therefore have a concentrated amount of energy. The wind meets the ocean surface and begins the transfer of energy. This makes the ocean waters turn choppy, forming peaks and whitecaps (these are small, foamy waves called capillary waves) that move in all sorts of directions. This is called a confused sea. The choppier the water, the more the wind can grab onto it and turn those peaks into higher whitecaps.
The size of whitecaps depends on:
Wind duration: The length of time the wind blew for.
Wind speed: How strongly it blew.
Fetch: How much surface area it blew over.
Some storms blow across a large area, but the wind is not very strong. Some storms have very strong winds, but they only cover a small area. This, of course, affects the size of the waves formed.
The stronger the winds, the more the waves increase in height, length, and speed. When the speed of the wave matches the speed of the wind, no more energy can be transferred. At that point, the wave is as big as it will get. When this happens, we have a fully developed sea.
The anatomy of a wave
Here’s what that fully formed wave looks like.
Crest: The top of the wave.
Trough: The lowest part of the wave.
Wave height: The vertical height between the top of the crest and the bottom of the next trough.
Wave length: The horizontal distance between crests or troughs.
Wave speed: The speed of a swell.
Amplitude: One-half the wave height or the distance from either the crest or the trough to the still-water line.
Wave period: The time it takes for a complete wave to pass a stationary point.
Wave frequency: The number of waves that pass a stationary point in a given time period.
Direction of Propagation: The direction a wave is traveling in.
Still-water line: The level of the sea surface if it were undisturbed.
Wave steepness: The ratio of height to length.
How waves travel
Now, as waves are formed, they begin to move away from the storm center, in the direction the wind that formed them was blowing. At this point, they are all moving at different speeds and in different directions, away from the storm. This is called wave dispersion.
Once these waves get outside the storm zone, they lose a little bit of energy as they travel. (They still move faster than local winds, though.) As they slow, they smooth out some. Soon, they collide with other waves that are going at their similar speed and direction, forming a wave train. This uniform group of waves is what we call swell.
When you see a wave pattern in the ocean, you are looking at swells traveling. Depending on their power and size, swells can travel thousands of miles. It all depends on the wave length and the depth of water it’s traveling through. The wave length determines the size of the orbits of water molecules within a wave. The water depth determines the shape of the orbits (as outlined in the diagram above). The longer the wavelength, the faster the wave energy will move.
In 2005, scientists tracked a powerful swell that traveled all the way from the Gulf of Alaska to Antarctica—8,000 miles.
The most common analogy for this process is a pebble thrown into a pond. When the pebble enters, the disturbance triggers the transfer of wave energy, which we see as water ripples that propel away from the center in concentric rings. Each ring travels at a different speed, which is why we see the concentric ring pattern.
When waves break
After a long journey across the ocean, swell waves break when they hit a coastline. In the ocean, where water is deeper, waves can travel uninterrupted. But when they hit shallower waters, they slow down because they have come into contact with the sea floor. The friction of hitting the sea floor makes the bottom of the wave drag, yet the top still moves forward. As the bottom slows, the waves behind it are still traveling quickly. When those hit the slower wave, it all turns into a water traffic jam. The extra energy moving through those waves has nowhere to go, so it moves vertically, increasing the first wave’s amplitude. This increase in amplitude is called shoaling.
This process actually changes the orbit of the water molecules from those perfect circles to ellipticals, which ultimately tip because the wave has becomes so tall it can’t sustain itself. This is when it crests (aka breaks) and washes up the beach. Gravity then pulls the broken wave back into the water.
Many factors affect how a wave breaks. The wave height is dictated by the wave length and the beach slope. And, of course, it only happens if the swell heads in the direction of the coast. Even then, the angle at which is approaches affects how the waves break.
How we measure waves
When you’re out at sea, waves can make or break your day (break—get it?). During a storm, waves get taller and closer together, making dangerous conditions for any mariner. That’s why heavy weather monitoring is so important. Luckily, technology has given us more tools to measure—and predict—wave conditions. Some methods rely on technology and some rely on old-school physical observation, but the aggregation of this data is what most weather services use to produce weather reports.
The magnitude of a wave is measured by three* variables:
The vertical height between the top of the crest and the bottom of the next trough.
The horizontal distance between crests or troughs.
The time that elapses between the passing of successive crests or troughs.
*Wave steepness might also be tracked.
To track wave patterns, researchers rely on ever-improving technology to get more accurate readings, specifically with the use of:
Weather buoys in the ocean track different weather data, including wind measurements, but they can also be used for wave measurement. Using an “accelerometer,” which records the rate at which the buoy rises and falls in the water, they can measure frequency and direction, then use that information to give a height reading, based on averages. Buoys are particularly useful to measure wave steepness.
The more technology available, the more accurately we can track oceanic movement and apply weather models to predict outcomes.
In 2015, researchers from the University of Miami Rosenstiel School of Marine & Atmospheric Science used a single satellite image to directly measure the speed of a wave located 80 meters below the ocean's surface.
Information derived from ships and buoys is compiled to determine Significant Wave Height in weather reports.
Significant wave height (SWH):
Based on the work of oceanographer Walter Munk, this mathematical measurement is the average wave height of the highest 1/3 of waves (again, measured trough to crest) measured over a period.
When you are looking at the ocean, you are seeing a wave spectrum (a group of waves at all different heights and speed). The SWH calculation considers the averages of those waves. When plotted on a graph, it breaks down like this:
By interpreting SWH calculations, you can get the statistical likelihood of the waves you’ll encounter out on the sea.
While wave height is useful, most mariners want to know the condition of the sea state (waves caused by local winds) and the swell (waves caused by winds or storms outside of the local area). For centuries, sailors relied on subjective data to determine sea conditions. This made for inconsistent predictions. Now, there are two common methods to determine conditions:
Beaufort Sea Scale
Developed in 1805 by Sir Francis Beaufort, a U.K. Royal Navy officer, the Beaufort Wind Scale is a method of measurement that includes observations of wind speed, wave height, and sea conditions. It has been refined over the last two centuries, and its current iteration is largely accepted. While not completely objective, it provides a rubric to interpret sea conditions, based on a scale of 0-12.
While the scale does denote approximate wave heights, skilled mariners understand that the impact waves have on vessels vary greatly depending on wave period, water depth, and other factors.
Douglas Sea Scale
The Beaufort Scale is most popular, but the Douglas Sea Scale is also useful because it accounts for both sea state and sea swell (again, waves that may be caused by weather conditions outside local range). Developed in 1917 by English Admiral H.P. Douglas, it relies on observation of the sea surface, presented on a scale of 0-9.
Together, these methods of measurement can provide an approximate picture of projected sea conditions. That said, any mariners venturing out should pursue the most recent, accurate weather condition information available.
Staying safe at sea
The more you know about weather conditions, the better off you’ll be. We’re determined to help you get educated, so keep an eye out for the next installment in our series: Ocean Winds. Until then, stay safe and sail on.