Wind wave

A wind wave, or wind-generated water wave, is a type of wave that forms on the surface of bodies of water when the wind blows over them. The wind pushes against the water, making ripples and larger waves. The distance the wind blows over the water, called the fetch, affects how big the waves can get.

Waves in the ocean can travel very far — sometimes thousands of kilometers — before reaching land. These waves can be small ripples or very large, depending on the speed of the wind, how long it blows, the fetch, and the depth of the water. When waves are being made and changed by the wind right where they form, they are called a wind sea. Once they move away from the wind, they are called swells and can travel long distances across the ocean.

Wind waves are mainly gravity waves, which means gravity helps to bring them back down after they are lifted by the wind. Even though we can measure and predict some things about waves, they also have a lot of randomness — each wave can be a little different in size and shape. Scientists use special models to help understand and predict how waves will behave in different conditions. While most people think of waves in Earth's oceans, there might even be wind-driven waves on the hydrocarbon seas of Titan, one of Saturn's moons. Waves can also be created by other things, like boats, animals, waterfalls, landslides, earthquakes, bubbles, and impacts.

Formation

Most big waves you see at the beach start far away, created by wind blowing over the ocean. Five main things affect how waves form:

1. Wind speed – the wind needs to move faster than the wave for it to give energy to the wave.
2. The fetch – this is the open water distance the wind blows over without changing direction.
3. The width of the water area affected by the wind.
4. How long the wind blows over the water.
5. The depth of the water.

These factors work together to decide the size and shape of the waves. Important wave features include wave height (how tall the wave is), wave length (how far apart wave peaks are), and wave period (how often wave peaks pass by a spot).

When wind blows over flat water, it creates tiny ripples at first. These ripples grow bigger because of the wind and their own interactions, eventually forming the large waves we see at the beach.

Types

There are three main types of wind waves. Capillary waves, or ripples, are small waves caused mainly by surface tension. Gravity waves, often called seas, are larger waves formed by the wind and last longer. Swells are waves that have moved away from where they started and spread out.

Sometimes, very large and unexpected waves called rogue waves can appear in the ocean. These are much taller than the other waves around them. They are different from tides, which are caused by the Moon and Sun, or tsunamis, which are caused by underwater earthquakes.

Spectrum

Ocean waves can be grouped by what causes them and how they move. Some waves, like those from earthquakes, come every 20 minutes and move very fast. Waves made by wind, called deep-water waves, usually come about every 20 seconds.

The speed of ocean waves depends on gravity, their length, and how deep the water is. In deep water, waves move in circular paths. In shallow water near the shore, waves slow down and their paths flatten out. Longer waves generally move faster than shorter ones.

Scientists use special models to describe how waves behave in the ocean. These models help us understand wave height and direction, which is important for predicting rough seas.

Shoaling and refraction

Main articles: Wave shoaling and Water wave refraction

When waves move from deep water to shallow water, they change shape. They grow taller, move slower, and get shorter. This change is called shoaling.

Refraction happens when waves meet the sea floor. This slows the waves down and changes their direction. Parts of the wave in deeper water move faster than parts in shallow water, causing the wave to bend. This bending can make waves taller in some places. Similar changes can also happen when waves meet ocean currents.

Breaking

See also: Surf wave, Breaking wave, and Iribarren number

Some waves go through a process called "breaking." This happens when a wave can no longer hold up its top and falls down. Waves break when they reach shallow water or when two waves meet and combine.

Surfers know three main types of breaking waves. Spilling waves are gentle and safe, often found near flat shorelines. Plunging waves break quickly and can be fun for experienced surfers. Surging waves often happen on steep shores and can push swimmers into deeper water.

Physics of waves

See also: Airy wave theory

Wind waves are waves that move across the surface of water when the wind blows. They are called surface gravity waves because gravity helps keep them together.

When wind blows over water, it pushes the surface. This gives energy to the water and makes waves.

Water particles near the surface move in circles with the waves. As the water gets deeper, these circles get smaller.

The speed of a wave depends on its length. Longer waves move faster. After a big storm, the first waves to reach the shore are often the longest ones.

Models

Surfers often check wave forecasts to find the best time for surfing. These forecasts come from wind wave models. They use information from weather models to guess how winds will change the waves on oceans, seas, and lakes.

Wind wave models also help scientists see how building near the shore, like shore protection and beach nourishment, might change the waves. This helps us take better care of the littoral areas.

Seismic signals

Main article: Microseism

Ocean water waves make seismic waves that we can see all over the world on seismographs. There are two main types of these seismic waves. The stronger one is called secondary microseism. It is made by the pressure from ocean waves on the ocean floor. The weaker one is called primary microseism. It happens above shallower parts of the ocean. Scientists have studied these waves for over a hundred years. They help us learn about big changes in ocean waves over long times.