Chapter 16.2 Waves and Tides

Waves Ocean waves are energy that travels. Waves will transfer energy from a storm out at sea several thousand kilometers. Sometimes waves are low and gentle but other times they are powerful.

Wave Characteristics Most ocean waves gain energy and motion from the wind. A breeze less than 3 km/h will produce small waves. The tops of a wave is called a crest. The bottom of a wave is called a trough. Halfway between the crest and trough is the stillwater level. The distance between the crest and trough is the wave height. The distance between 2 crest or 2 troughs is the wavelength. The wave period is the time it takes for 1 full wave to pass a fixed point.

The height, length, and period are dependent on 3 factors: wind speed, length of time the wind has blown, and the fetch. Fetch is the distance that the wind has traveled across open water. As more energy is transferred from the wind to the water the greater the height and steepness of the wave.

Wave Motion Waves can travel far distances. As a wave travels, the water particles pass the energy along by moving in a circle. This is called orbital motion. Floating objects show this by moving up and down but also slightly forward and backward. Circular orbital motion allows energy to move forward while water particles that move the wave move in a circle.

Breaking Waves When a wave approaches the shore, the water becomes shallower. The bottom of the wave slows and causes the top to become higher until it reaches a point where the wave is too steep and the front collapses. A surf is turbulent water created by breaking waves.

Tides Tides are regular changes in the elevation of the ocean surface. This has to do with gravity and the moon. The closer parts of the Earth’s surface are to the moon the greater the pull of gravity. Ocean tides are made from the differences in the gravitational attraction on different parts of the Earth’s surface by the moon and, a much lesser extent, the sun.

The Cause of Tides The moon rotates around the Earth every 29 ½ days. The gravitational pull is the strongest on the side of Earth closest to the moon. This causes Earth to be stretched slightly. The solid Earth is not really affect but water on the Earth will feel the pull. The ocean will flow toward the moon, creating a tidal bulge, or high tide. On the side farthest from the moon also creates a tidal bulge. As the Earth rotates it changes where the bulge is. This affects the tide.

Tidal Cycle When the new and full moon are present we feel the suns gravitational pull more so there are higher high tides, and lower low tides during this time. The tidal range is the different in height between successive high and low tides. Spring tides have the largest tidal range. Neap tides are produced at first and third quarter moons because the moon and sun are at right angles.

Tidal Patterns The shape of the coastline, the water depth, and the ocean basin affects tides. There are 3 main tidal patterns world wide: diurnal tides, semidiurnal tides, and mixed tides. Diurnal – a single high tide and single low tide each tidal day. Semidiurnal – 2 high tides and 2 low tides each tidal day. Mixed – close to semidiurnal but there is a difference in water heights. There are usually 2 high and 2 low tides but the high tides are different heights and the low tides are different heights.