The A-Z of Waves

Tony Butt

by on

Updated 13d ago

Waves are everywhere. They form a part of every single aspect of our lives, every day. The sound hitting our ears, light hitting our eyes, the heat from the Sun, the changing weather patterns, the signals spewed out by our phones and the ones received by our televisions; even the motion of our cars in a traffic jam. All are waves of some sort.

As surfers, we use waves in a very particular way. Energy reaching the Earth as electromagnetic waves from the Sun, after a long series of links involving many other types of waves, eventually reaches us, in the form of clean, rideable swell. We then extract a minuscule part of that energy to push us along for a few seconds. Many of us have dedicated our lives to the quest for more and more of that liquid energy.

Not categorised; hellish waves that'll smash you into the death zone, like Nazare.

What follows is a list of waves and a brief description of each one. Of course, there are many more; but to avoid writing a whole encyclopaedia I’ve just stuck to the ones most relevant to us: those with something to do with the ocean or the atmosphere, or at least something to do with science or nature.

Airy wave: A theoretical approximation to a wave, named after English Astronomer Sir George Biddell Airy (1801-1892). Airy wave theory is the least mathematically complex and the most common wave theory in use today.

Bore: The scientific name for a broken wave. Here, the wave does actually transport water as well as energy. The water above is flowing over the water below, causing a shearing action, hence all that turbulence and white water.

Bore surfing is a thing.

Capillary waves: Tiny waves that appear at the beginning of the process of wave generation by the wind. They usually have a wavelength of less than 10 centimetres, and their restoring force (the force that tries to bring the water surface down again after something has lifted it up) is the surface tension of the water.

Dune: Dunes and sand waves are formed by the wind blowing over the surface of the sand in a similar way to ocean waves. The size and shape of the dunes are determined by the characteristics of the sand grains and the velocity of the wind.

Electromagnetic waves: These really are fundamental to the existence of the Universe and life itself. They include radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. They contain electric and magnetic components oscillating perpendicular to each other (one goes up and down and the other from side to side).

Electromagnetic waves have some pretty bizarre properties. For example, they can behave as waves and they can behave as particles at the same time. They don’t need a medium to propagate in and they always travel at the same speed relative to any observer no matter how fast the observer is travelling. This last property has the weirdest implications of all. It means that the speed with which time flows for you depends on what you’re up to. If your mate is up to something different, then time might flow differently for him.

Freak wave: A single wave much bigger than the average wave height, suddenly appearing in the open ocean. Waves of over 30 metres have been sighted in the open ocean, even when the average wave height is well under half that. Scientists are still struggling to find a suitable explanation.

Read more about freak waves HERE.

Gravity wave: A surface water wave whose restoring force (the force that tries to bring the water surface down again after something has lifted it up) is gravity. Wind-generated waves (those we surf) and tsunamis are examples of gravity waves. Not to be confused with gravitational waves – curvatures of space-time found mainly around the outside of binary star systems. Maybe one day we’ll be able to ride them as well.

Harmonic: A wave whose frequency is an exact multiple of another wave which is called the fundamental. The mathematical technique of Fourier analysis allows a wave of any irregular shape to be broken up into a number of sinewaves. This greatly simplifies any analysis of the wave.

Internal wave: A wave that propagates along the boundary of two water masses of different densities, below the surface of the sea. These waves are typically of much longer wavelengths than normal wind-driven surface waves.

Jump, hydraulic: A hydraulic jump is a sudden increase in water depth when the flow speed and shallowness of the water reach a critical limit. The water cannot flow any faster and the depth cannot continue to be that shallow at the same time, so a kind of shock wave is created. They can be seen everywhere, from rivers to kitchen sinks. In fact, they can even be surfed; tidal bores, for example, are a type of hydraulic jump.

Kelvin wave: A very long (thousands of kilometres) wave which propagates around ocean basins from east to west.

Light waves: The fact that light is a form of electromagnetic radiation was first deduced by James Clark Maxwell (1831-1879). Visible light makes up the very small section of electromagnetic radiation with wavelengths between 0.0004 and 0.0007 millimetres.

Megatsunami: Also called a landslide tsunami, this is a giant wave caused by the collapse of millions of tonnes of rock from a volcanic island into the sea. It was brought to the public eye a few years ago in a famous BBC Horizon documentary called Megastunami: Wave of Destruction.

Tsunamis and megatsunamis are pure destructive force.

Nonlinear wave: One theory of how freak waves are formed in the deep ocean is based upon a ‘non-linear’ superposition of other waves of different wavelengths, whereby the resulting height is more than just sum of all the heights of the waves that go to make it up.

Ocean surface wave: This is the type wave we spend our lives chasing. To be specific, the wave we are interested in is one that propagates over the surface of the water, is generated by the wind, and whose restoring force is gravity; in other words, the wind-generated water surface gravity wave.

That’s a mouthful, but it needs to be distinguished from, say, a tsunami or tidal bore, which are also water surface gravity waves but not generated by the wind. Or, say, an underwater sound wave which is a water-wave but definitely not much use to us.

Pressure wave: A type of wave where the direction of oscillation of the medium is in the same plane as the direction of energy transfer. In other words, the oscillation is back and forth, not up and down or from side to side. Sound waves are of this type.

Quasi-biennial oscillation: This is where the winds in the upper atmosphere above the equatorial zones completely change direction about every two years. This oscillation has been hypothesised to be important for predicting hurricanes and monsoons.

Rayleigh wave: Normally used for seismic surface waves whereby the surface oscillates up and down while the energy is propagated across the surface. Strictly speaking, the term can mean any wave with these characteristics, including a surface water wave.

Standing wave: A wave that just oscillates about the same point, with no net transfer of energy. It can happen when a wave tries to propagate in one direction but the medium itself is moving in the other direction, similar to someone on a running machine.

But it can also happen when a series of waves hits a boundary and is reflected; the waves then interfere with their own ‘mirror images’ and produce standing waves, typically in harbours and bathtubs.

Tsunami: A surface water wave produced by some kind of episodic seismic event like an underwater earthquake. Tsunamis are much longer and travel much faster than normal wind-generated waves.

A robust warning system is now in place for Tsunamis across the globe but they can still catch locales off guard.

A robust warning system is now in place for Tsunamis across the globe but they can still catch locales off guard.

Underwater sound wave: Sound in water is a pressure wave (longitudinal wave) just like it is in air. The difference is that, in water, it travels about five times faster than it does in air. It also travels great distances with very little attenuation. Whale calls, for example, have been heard thousands of kilometres away from where the whale sent them.

Vibrating string: The basis behind musical instruments like the guitar or piano. When a taught string is plucked, a wave is generated which travels to the ends of the string, and is reflected back to produce a standing wave.

The movement of the string transmits energy to the atmosphere to produce sound waves. The wavelength of the wave in the string is directly related to the pitch of the sound.

Windsea: Relates to waves ‘inside the storm’ where the wind is still blowing over the surface of the sea and imparting its energy on the water. Contrast this with swell, where the waves have propagated away from the storm, and are no longer receiving energy from the atmosphere.

X-wave: A theoretical wave that can travel faster than the speed of light. Therefore, it can travel backwards in time.

Apparently it can travel faster than light because no information is actually transmitted with the wave. If, one day, an X-wave is found that actually transmits information, we would all win the lottery. On the down side, wave-prediction websites would go out of business.

Y-wave: The wave in the atrial and venous pulse curves reflecting rapid filling of the ventricles just after the atrioventricular valves open. Sounds pretty important, whatever it is.

Zenneck wave: A type of electromagnetic wave which travels along the surface of the Earth, trapped at the interface between the ground and the air. This wave, discovered in 1909 by the German engineer Johann Zenneck, is very important for telecommunications.

Cover shot of Portugal by Maikel Kersbergen