Wind wave facts for kids

A man standing next to large ocean waves at Porto Covo, Portugal

Wind waves are ripples and larger waves that form on the surface of water bodies like oceans, seas, and lakes. They happen when wind blows over the water. The distance the wind blows over the water in one direction is called the fetch. Ocean waves can travel thousands of kilometers before they reach land. Wind waves on Earth can be tiny ripples or huge waves over 30 m (100 ft) high. Their size depends on how fast the wind blows, how long it blows, the fetch, and how deep the water is.

When wind waves are directly created and affected by local wind, they are called a wind sea. After these waves move out of the area where the wind created them, they are called swells. Swells can travel for thousands of kilometers. For example, waves made by strong winds south of Tasmania can travel across the Pacific Ocean to southern California, creating great conditions for surfing. Swells are wind-generated waves that are not much affected by the local wind where they are now. They were made somewhere else, sometimes a long time ago. Wind waves in the ocean are also called ocean surface waves. They are mostly gravity waves, meaning gravity is the main force that pulls them back down after they rise.

Wind waves can be a bit random. Each wave might be different in height, how long it lasts, and its shape, making them hard to predict perfectly. Scientists use special models to predict the main features of wind waves, like their height and how they move.

Even though we usually think of waves in Earth's oceans, there might also be wind-driven waves on Titan, a moon of Saturn, which has seas made of liquid hydrocarbons. Waves in water bodies can also be caused by other things, both on the surface and underwater.

How Wind Waves Form

Aspects of a water wave

Wave formation

Water particle motion of a deep water wave

The parts of an ocean surface wave: 1. Wave Crest (the top). 2. Falling wave. 3. Trough (the bottom). 4. Rising wave.

NOAA ship Delaware II in bad weather on Georges Bank

Most of the large waves you see breaking at a beach come from winds far away. Five main things affect how wind waves form:

1. Wind speed: The wind must be moving faster than the wave's top for energy to transfer to the wave.
2. Fetch: This is the clear distance of open water over which the wind blows without changing direction much.
3. Width: How wide the area is that the fetch affects.
4. Wind duration: How long the wind has been blowing over the water.
5. Water depth: How deep the water is.

All these factors work together to decide how big the waves will be and how the water moves within them.

The main measurements for waves are:

• Wave height: The vertical distance from the bottom (trough) to the top (crest) of a wave.
• Wavelength: The distance from one wave crest to the next.
• Wave period: The time it takes for two wave crests to pass a fixed point.
• Wave direction: The direction the waves are moving, mostly set by the wind direction.

A "fully developed sea" means the waves have reached the biggest size they can for a certain wind speed, how long it blows, and the fetch. If the wind keeps blowing, the waves won't get bigger; instead, their tops might break, forming "whitecaps." Waves in one area usually have different heights. For weather reports and science, the "significant wave height" is used. This is the average height of the tallest one-third of waves over a certain time. It's also what a trained observer, like someone on a ship, would guess the wave height to be just by looking.

Waves start forming on flat water when the wind blows. The wind's uneven pressure creates tiny ripples. As the wind keeps blowing, these ripples grow bigger. The bigger the waves get, the more the wind can push them, making them grow faster.

For example, imagine a calm sea with no waves. If a strong wind suddenly starts blowing steadily:

1. The wind creates small, random pressure changes on the water surface. This makes tiny ripples, only a few centimeters long.
2. The wind keeps pushing on these ripples, making them grow. As they get bigger, the wind can transfer more energy to them, making them grow even faster.
3. Over time, these waves interact with each other, creating even longer waves. Eventually, the waves can move faster than the wind blowing across them.

Types of Wind Waves

Surf on a rocky irregular bottom at Porto Covo, west coast of Portugal

Wind waves develop into three main types over time:

• Capillary waves: These are tiny ripples, mostly controlled by the water's surface tension. They appear on smooth water when the wind blows, but disappear quickly if the wind stops.
• Gravity waves: These are larger waves, controlled by gravity and how the water moves.
  • Seas: These are gravity waves created locally by the wind. They are often irregular.
• Swells: These are gravity waves that have traveled far from where the wind first created them. They tend to be more organized and regular.

Individual "rogue waves" can also happen. These are much taller than other waves around them. For example, the Draupner wave was 25 m (82 ft) high, which was more than twice the average height of the tallest waves in that area.

It's important to know that wind waves are different from other types of water waves:

• Tides: These are caused by the Moon and Sun's gravitational pull.
• Tsunamis: These are caused by underwater earthquakes or landslides.
• Waves from underwater explosions or falling meteorites.

All these other types of waves have much longer wavelengths than wind waves.

The biggest wind waves ever recorded are not rogue waves, but very large waves in extreme stormy conditions. For instance, waves 29.1 m (95 ft) high were seen on the RRS Discovery ship during a storm. The biggest wave recorded by a buoy (a floating device) was 32.3 m (106 ft) high during Typhoon Krosa near Taiwan in 2007.

Wave Characteristics

Classification of ocean waves based on their period

Ocean waves can be grouped by what creates them, how much that force keeps affecting them, how much gravity pulls them down, and their wavelength or period. For example, seismic sea waves (tsunamis) have a period of about 20 minutes and travel very fast. Wind waves in deep water have a period of up to about 20 seconds.

The speed of all ocean waves is controlled by gravity, their wavelength, and the water depth. Most features of ocean waves depend on how their wavelength relates to the water depth. The wavelength determines how big the circles of water molecules are within a wave. However, water depth changes the shape of these circles.

Water molecules move in perfect circles only when a wave is in deep water. A wave is considered a "deep-water wave" if the water is deeper than half its wavelength. This is because the wave's energy doesn't reach the bottom at that depth.

When waves move into shallow water (where the depth is less than 1/20 of their original wavelength), the water molecules' paths become flattened into ellipses. These are called "shallow-water waves." "Transitional waves" are in between, in water deeper than 1/20 but shallower than half their original wavelength.

Generally, the longer the wavelength, the faster the wave's energy moves through the water. The relationship between a wave's speed (C), wavelength (L), and period (T) is:

For deep-water waves, the speed can also be estimated by:

Here, C is in meters per second and L is in meters. This shows that longer waves travel faster.

For shallow-water waves, the speed is described by a different equation:

Here, C is in meters per second, g is gravity, and d is the water depth in meters. A wave's period stays the same no matter the water depth. But as deep-water waves enter shallow areas and "feel" the bottom, they slow down. Their crests get closer together, and their wavelength becomes shorter.

Shoaling and Refraction

Waves create ripple marks in beaches.

As waves travel from deep to shallow water, their shape changes. The wave height increases, but their speed and length decrease. This process is called shoaling.

Wave refraction happens when waves interact with the seabed. The parts of the wave in shallower water slow down more than the parts in deeper water. This causes the wave crests to bend and realign, becoming more parallel to the depth lines. This bending can change the wave's direction quite a bit. Because the wave energy gets focused in certain areas, the wave height increases.

Breaking Waves

Large wave breaking

Giant ocean wave

Some waves "break." A breaking wave happens when its base can no longer support its top, causing it to collapse. This usually occurs when a wave moves into shallow water or when two wave systems meet and combine forces. If a wave becomes too steep, it will break.

In deep water, a wave breaks when its steepness (the ratio of its height to its wavelength) is more than about 0.17. In shallow water, a wave breaks when its height is more than 0.8 times the water depth. Waves can also break if the wind is strong enough to blow the top off the wave.

In shallow water, the bottom of the wave slows down because of friction with the seabed. The top of the wave keeps moving faster, making the front of the wave steeper and the back flatter. This can make the wave form a "barrel" or "tube" as it collapses.

Surfers and lifeguards identify three main types of breaking waves:

1. Spilling, or rolling: These are the safest waves for surfing. They happen on flatter shorelines. The wave's base slows down gradually, and the top doesn't move much faster than the bottom.
2. Plunging, or dumping: These waves break suddenly and can push swimmers down with great force. Experienced surfers prefer these. They often form where the seafloor rises suddenly, like a reef or sandbar. The top of the wave rises and curls over, forming a "barrel" or "tube."
3. Surging: These waves might not actually break as they reach the shore because the water under them is very deep. They form on steep shorelines. These waves can knock swimmers over and pull them back into deeper water.

If the shoreline is almost straight up and down, waves don't break but instead bounce back. This creates interesting patterns where incoming and reflected waves meet.

How Waves Work (Physics)

Stokes drift in shallow water waves

Stokes drift in a deeper water wave

Water particle orbits under a surface gravity wave in a wave tank.

Wind waves are mechanical waves that travel along the surface between water and air. Gravity is the force that pulls them back down. When wind blows, it creates pressure and friction on the water surface, transferring energy from the air to the water and forming waves.

In simple waves in deep water, water particles near the surface move in circles. They move forward at the top of the wave and backward at the bottom. This makes the water surface look like a special curve called a trochoid, with sharper curves at the top. So, wind waves are a mix of waves that move up and down (transversal) and waves that move back and forth (longitudinal).

When waves move in shallow water (where the depth is less than half the wavelength), the water particles' paths are squashed into ellipses.

In reality, water particles don't make perfect closed circles. After each wave crest passes, the particles move slightly from their original spot. This is called Stokes drift.

As you go deeper below the water surface, the circular motion of the water particles gets smaller. At a depth equal to half the wavelength, the movement is less than 5% of what it is at the surface.

The speed of a surface gravity wave in deep water is approximately:

Here, is in meters per second, and (wavelength) is in meters. This means that waves with different wavelengths travel at different speeds. The longest waves from a storm will arrive at the coast first.

If the wavelength is very long compared to the water depth, the wave speed can be approximated by:

Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): c_\text{shallow} = \sqrt{gd}

Here, g is gravity and d is the water depth.

For very short waves, surface tension becomes important. These are called gravity-capillary waves.

When many waves are present, which is always true in nature, they form groups. In deep water, these groups travel at half the speed of the individual waves. If you watch a single wave in a group, you'll see it appear at the back, grow, and then disappear at the front.

As water depth decreases near the coast, the wave height changes due to wave shoaling and refraction. As the wave height increases, the wave might become unstable and break, creating surf.

The energy from wind waves can be captured by wave energy devices. The energy in regular waves depends on the water's density, gravity, and the wave height.

Wave Models and Forecasts

The image shows the global distribution of wind speed and wave height as observed by NASA's TOPEX/Poseidon's dual-frequency radar altimeter from October 3 to October 12, 1992.

People who surf are very interested in wave forecasts. Many websites provide predictions of surf quality for the coming days and weeks. These wave models use information from larger weather models that predict winds and pressures over oceans, seas, and lakes.

Wind wave models are also important for planning shore protection and beach nourishment projects. They help estimate the effect of wind waves, which is key for managing coastal areas.

You can predict a wind-generated wave based on two things: the wind speed 10 meters above sea level and how long the wind has been blowing. If the wind blows for a long time, the waves can become "fully developed." Then, you can predict the significant wave height and the main wave frequency for a certain fetch length.

Seismic Signals from Ocean Waves

Ocean water waves can create seismic waves that travel hundreds of kilometers into the land. These seismic signals usually have a period of 6 to 2 seconds. Scientists first noticed and understood these recordings around 1900.

There are two types of seismic "ocean waves":

• Primary waves: These are made in shallow waters when water waves directly interact with the land. They have the same period as the water waves (10 to 16 seconds).
• Secondary waves: These are more powerful. They are created when ocean waves of the same period travel in opposite directions and meet, forming standing waves. These standing waves create pressure changes that don't get weaker with depth.

In Spanish: Ola para niños