Dictionary Definition

A ridge or swell moving through or along the surface of a large body of water.

Structure

Explanation

When wind ruffles the water, it creates turbulence that makes the energy lift the water particles. This creates a wave, which is made of many simple parts (described below). One thing to remember about a wave is that it moves the water in more of an up-and-down movement than a forward movement. It is the wave pattern, not the water, that moves forward. To put it another way, waves move energy, not water.

• Crest: The highest point of the wave. When the energy forms the wave, it creates a watery ridge called the crest.
• Trough: The lowest point of the wave, this is the separation between the crests.
• Height: The vertical distance between the crest and the trough.
• Amplitude: The distance between the crest and the average sea level. Wave Length: The distance from one crest to the next.
• Period: The time it takes a wave to pass from one crest to the next.
• Fetch: The area of the sea over which the wind creates waves.
• Surf Zone: The zone between the outermost breakers and the shoreline.

Experiment #1: Seeing the Wave

What You'll Need:

• SlinkyŌ toy
• friend

Directions:

1. Lay the Slinky on the floor and stretch it between you and your friend.
2. Gently move one end back and forth several times.
3. Remove the foil from the freezer, fold it, and place it on top of the bottle.
4. Change the speed of the back and forth movement by increasing and decreasing the distance the Slinky is moved.

What's Happening?

The slinky moves up and down like a wave. The wave height increases with an increase in the distance that the end is moved. Waves that move up and down are called transverse waves. The highest part is the crest, and the lowest part is the trough (see above). This Slinky is a flat demonstration of what waves look like and how they move. Water molecules, like the rings in the Slinky, move up and down not forward. Only the energy of each wave moves forward. The wave moves forward through the Slinky, but the Slinky stays in basically the same place.

Life Cycle

1. A wave is born when the wind blowing across the water's surface creates ripples that eventually turn into turbulence. If wind strength, duration, and fetch are great enough, a sea develops. This all happens in the fetch, the birthplace of waves. Once the choppy sea leaves the fetch area, the wave patterns organize themselves into lines of swells that radiate downwind from the area where they developed.
2. The energy that waves transmit as they travel through the sea causes particles near the surface to rotate in circular orbits. The orbits get smaller when the depth of the water increases.
3. As the waves approach the shore, they are influenced by the seabed and change character. Now they are slower and their length shortens.
4. When they get shallow enough, the waves break because the shallow water no longer allows the complete internal rotation of the water particles.
5. The momentum of the wave pushes the water toward the shore, expending the last of its energy.

Types

Believe it or not, the waves you see crashing on the beach aren't the only type there are. There are several ways to classify waves, so look below for some descriptions of these different waves.

• Surface Waves: These are the waves most people talk, the ones that break on the surface and we see.
• Internal Waves: Some waves exist beneath the surface, as well. They are created by tides, the interaction of surface waves, and the direct effects of very strong winds. These usually move very slow. The longest in the world stretch for hundreds of miles, and are found in the Sulu Sea near the Philippines.
• Stationary Waves: Also known as standing waves, or seiche, these waves are caused by storms or other disturbances in the atmosphere or the water. These waves look kind of like a pan of water that has been tilted and then uprighted. They are found in enclosed or semienclosed bodies of water such as lakes or bays.
• Tsunami: This "tidal wave" is caused by earthquakes and other seismic disturbances. Look below at the Tsunami section for more information.
• Shallow-Water Wave: The wave's length is more than twenty times the depth of the water. This type of wave's speed is controlled by the water depth. The deeper the water, the faster the wave moves.
• Deepwater Wave: The wave's length is less than four times the depth of the water. This type of wave travels at a speed independent of its distance to the seafloor.
• Shore Breakers: When waves approach the shore, the crests turn so they're nearly parallel to the shore. By the time the waves reach water that is as deep as the waves are high, they topple over and break.

Wind

Wind powers the wave by blowing its energy and making ripples in the water that turn into waves. The wind can be very strong, and stronger winds create greater waves by putting more energy into the water. Learn more about how wind works by reading the Wind section of the Glossary. Or, explore how wind works on water by doing the experiment below.

Experiment #2

What You'll Need:

• large, shallow pan
• drinking straw

Directions:

1. Fill the pan half full with water.
2. Hold one end of the straw close to the surface of the water.
3. Blow air across the water's surface gently.
4. Now blow a little harder.

What's Happening?

The energy of the moving air is transferred to the surface of the water and waves are formed. The harder you blow, the higher the waves are because more power is blown onto the water. When you blow harder, the air moves faster and has more energy. The energy from wind is transferred into waves when it hits the surface of water, as well. The energized water is pushed upward to form a wave.

Tsunamis

These waves are violent and gigantic. They can travel through the ocean with the speed of a jetliner and rise to heights of thirty feet (nine meters) or more, crashing onto shore and rushing far inland. In the last century, tsunamis have killed more than 50,000 people.

What triggers a tsunami? Seismic activity, such as an earthquake, or a sudden undersea movement, such as a landslide or volcanic eruption, are the most common triggers. After being triggered, the tsunami heads to shore and pounds it for days. When a tsunami is triggered by an earthquake, the land rises on one side of the fault line and all the water above it rises with it to form a high and a low point (the crest and trough) in the water.

A tsunami can travel at great speeds, usually speeding at more than 500 miles (800 kilometers) peer hour in the open ocean. In the very deepest point, they increase to nearly 600 miles (960 kilometers) per hour. So, they can cross the Pacific Ocean in less than a day. The tsunami only slows down when it reaches the coast, but that's when it becomes dangerous. Near to shore, the tsunami hits shallow water and the friction slows the water in the front of the wave. But the back of the wave may still be in deep water and traveling fast, so the waves come up. This creates a "pileup" where the rest of the wave behind the leading edge bunch up with nowhere to go. When it finally hits the shore, a tsunami is only at about thirty miles (48 kilometers) per hour. But the amount of water it dumps on the coast makes it powerful and devastating. Often, the tsunami that hits the coast creates more damage than the preceding earthquake.

Power

We use water to get a lot of our energy. By using water, we can create enough energy. However, building large dams can create social and environmental problems. So scientists are now looking to the sea as an energy source. Few power plants use the tides of the sea right now, but the movement of waves has a lot of power that could be useful. Some wave-power generators are based on land, but scientists are looking to develop methods that workout at sea. This idea, of using waves for power, is still being developed.

Measurement

It is hard to measure a wave when you're standing on a ship. The largest wave ever recorded was in February 1933 when the navy tanker USS Ramapo was in the Pacific Ocean and it hit a storm with winds of 68 knots (78 miles or 126 kilometers per hour). An officer standing on the bridge saw the wave rise to a height even with a part at the back of the ship called the crow's nest. Since the ship was in the trough of the wave, calculations measured this wave at 112 feet (34 meters).

It is easier to measure the height of waves from the shore. Standing at the water's edge, note the highest and lowest position of the waterline for three or four waves, and determine the midpoint. Drive a tall stick in the sand at that point. Go back a short way from the water's edge and look at different points of the stick to find one where the crests of the highest waves line up with the horizon. The distance between this point and the point where the stick meets the surface represents the height of the crest above the mean sea level. The total wave height is almost twice this number.

The wave's period can also be measured from shore. Find an offshore object (such as a rock or a pier pile) that is stationary and note the exact time that two successive wave crests pass this object. The period is the difference between the two times. If the first came by at 10:15 and the second at 10:16, the period is one minute.

You can also approximate the proximity of the fetch and whether the winds are strong or weak. Use the table below.

This system works well if the waves come from one dominant storm system. But oceanographers use a more complicated process to measure the periods of multiple-wave trains.

Forecasting

Wave conditions are determined by winds-- present and past, local and distant. Therefore, understanding wind conditions helps predict the nature of the waves. In the 1940s, a method was developed where forecasters consult a set of weather maps and locate all the important storm fetches. Then they compute the dimensions of the expected waves using wind speed, storm duration, fetch length, distance to the fetch, and other features.

Wave forecasting has developed to become more accurate using satellite monitoring. We can now predict the height and period of the waves, as well as the range of their variations. Some satellites predict rough seas so oceanographers can warn people of dangerous conditions. The meteorologists can use the same information to predict the arrival of storms over land.