Waves and Tides
The surface of the ocean is in constant motion. Waves of all sizes are always moving across the waters. Where do these waves come from and what determines how they behave?
With the exception of tsunamis and tidal waves , all ocean waves get their energy from the wind. Wind that is blowing across the water starts tiny ripples in motion. As the wind continues, the waves get larger. The size of the waves depends on three things: wind speed, the length of time that the wind continues to blow, and the distance over which the wind is blowing. If any of these increase, the waves will get larger. A 20 mile an hour wind makes bigger waves than a 10 mile an hour wind. A wind that blows steadily for 12 hours creates larger waves than one that blows for 6 hours. A wind that blows across a distance of 100 miles causes bigger waves than one that only blows across 50 miles.
Parts of a Wave/ Wave Behavior
Every wave has a highest and lowest point. The high point of a wave is called the crest. The low point is called the trough You can’t have a crest without a trough. It takes both to make a complete wave. The distance between two crests is called the wavelength. The height of a wave from trough to crest is called the wave’s amplitude. The time it takes for a complete wave to pass by is called the period.
Although waves move through water, the water itself hardly moves at all. Waves carry the wind’s energy through the water but the water particles stay pretty much in place. They just bob up and down in small circles as the waves pass by. This is fortunate because ,otherwise, huge amounts of water would pile up on the shore. On the open ocean, large waves can travel for thousands of miles in long smooth swells but as the waves approach the shoreline their behavior changes. These waves are called surf or breakers . At a certain depth, a wave begins to be affected by the ocean bottom and slows down and piles up into a breaker. When waves break along the shore, they can create very powerful currents . One is called an undertow. People in the water caught in an undertow may feel they are being pulled out to sea. Another current is called a rip current. Rip currents travel farther than an undertow.
Tsunamis are huge waves that are created by undersea earthquakes and volcanic eruptions. Earthquakes can cause a sudden shift in the earth’s crust beneath the sea. The motion sends out powerful and rapidly moving waves in all directions. They can travel through the ocean at speeds of 500 miles an hour. Tsunami means “harbor wave” in Japanese. On the open ocean, tsunamis are not very noticeable because their wavelengths are so long. The water rises only a few feet as the tsunami passes by. But when they approach the shore, they slow down and pile up into monstrous waves. Tsunamis are most common in the Pacific Ocean because more earthquakes happen there. Tsunamis can be extremely destructive.
The earth and moon attract each other through gravity. The moon’s gravitational force pulls on the ocean’s water creating a wave we call the tide. At the same time, on the opposite side of the earth another wave forms.
As the earth rotates or spins on its axis, the earth’s crust moves underneath the tidal bulges. Each of the tides travels completely around the globe in about a day.
The sun’s gravity also causes tidal motion but because the sun is much farther away, it affects the tide less strongly. When the sun and moon are aligned with each other during the full and new moon, their gravitational forces work together causing stronger than usual tides. When we see a half moon, their gravities pull in different directions causing weaker tides. If the earth were completely covered with water the tides would move smoothly and evenly every 12 hours and 25 minutes. However, on earth the height and timing of the tides depends on the shape of the shoreline. In some places there is only a small difference between high and low tides. In others, the flow is much greater between high and low tide.