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Tides Tides are long waves that move through the oceans in response to the forces exerted by the moon and sun . Tides originate near the middles of oceans and progress toward the coastlines. Tides.
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Tides Tides are long waves that move through the oceans in response to the forces exerted by the moon and sun. Tides originate near the middles of oceans and progress toward the coastlines .
Tides When the highest part, or crest of the wave reaches a particular location, high tide occurs; low tide corresponds to the lowest part of the wave, or its trough. The difference in height between the high tide and the low tide is called the tidal range. High tide Low tide
Causes of the Tides • Tides are created by the imbalance between two forces: • Gravitational force of the Moon and Sun on Earth • Centripetal (center-seeking) force required to keep bodies in nearly circular orbits • The sun is further from the Earth than the moon., the sun’s tide-generating force is about half that of the moon.
Gravitational forces on Earth due to the Moon • Force decreases with increasing distance• Force is directed toward the Moon’s center of mass Centripetal forces on Earth• Moon’s Force is the same everywhere on Earth• Force is directed perpendicular to Earth’s center everywhere on Earth
Centripetal Force
Tidal bulges Tide-generating forces produce 2 bulges:• Away from Moon on side of Earth opposite the Moon • Toward Moon on side of Earth facing Moon Earth rotates into and out of tidal bulges, creating high and low tides
The Lunar day • Tidal bulges follow the Moon as it rotates around Earth • A Lunar day is 50 minutes longer than a solar day because the Moon is moving in its orbit around Earth Figure 9-7
Spring tide Neap tide • The Monthly Tidal Cycle • About every 7 days, Earth alternates between: • Spring tide Alignment of Earth-Moon-Sun System in line • • Large tidal range • 2. Neap tide • Alignment of Earth-Moon-Sun System at right angles • • Small tidal range
Tidal patterns • Diurnal One high and one low tide daily. Gulf of Mexico • Semidiurnal Two high and two low tides of about the same height daily. East coast of US. • Mixed Characteristics of both diurnal and semidiurnal with successive high and/or low tides having significantly different heights. Pacific coast of US.
Tidal patterns in the U.S. Figure 9-15
Tidal Bores The incoming tide produces a wave that flows into a river or bay. Tidal bores occur in about 100 rivers throughout the world. Three of the better known bores are the Qiantang Bore in China. This bore reaches heights of 15 ft. The Pororoca, in the Amazon River, forms waves 12 ft. high. The Severn Bore in England is a popular one to surf as shown below. Qiantang Bore Surfing the Severn Bore
Fig. 1 Where do the tides start? Tides start in the middles of the world’s oceans.
Tides in the ocean Amphidromic Points • Cotidal map shows tides rotate around amphidromic points. There are 140 amphidromic points in the world’s oceans. • More realistic pattern of tides in the ocean Figure 9-14
“If you don’t understand amphidromic points I’m gonna bite you !
Since we are becoming TIDAL BORES We will move on to waves
Waves A wave is the transmission of energy through matter.
Waves Properties of Ocean Waves - An ocean wave is an undulation of the sea surface Waves Period - The time it takes one wave to pass a given point Wave frequency - The number of waves that pass a given point in a given time
Speed= wavelength / Period S = L/T T=5s S= 100m/5s L= 100m S= 20m/s • Find the speed of a wave with a wavelength of 50m and a period of 6 seconds? • Find the wavelength if the speed is 200m/s and a period of 2 seconds.
Progressive Waves • Wind-generated waves are progressive waves because they travel across the sea surface. • Types: • Longitudinal • Transverse • Orbital
Longitudinal: when the matter moves back and forth in the same direction. • Transverse: The matter moves perpendicular as the wave. • Orbital: when the energy moves the fluid in a circular motion. • http://paws.kettering.edu/~drussell/Demos/waves/wavemotion.html
Disturbing forces: Cause waves • Restoring forces: resist them • What Causes Waves? 1. Wind 2. Gravitational attraction of sun and moon 3. Seismic activity
Most wind-generated waves eventually reach shorelines and dissipate any remaining energy.
Wave Motions Two basic motions associated with an ocean wave: • The forward movement of the wave form (not the water). • The orbital motion of water particles beneath the wave. It is wave energy, not water molecules, that moves across the sea surface. A floating seagull demonstrates that it is the wave form that travels, not the water. As the wave moves from left to right the gull, and the water in which it is resting, rotates in an imaginary circle, moving slightly to the left up the front of the approaching wave, then to the crest, then sliding to the right down the back of the wave.
Fetch is the distance over which the wind interacts with the water surface to create waves. The longer the fetch the bigger (higher) the waves are
Wave Motion with Depth • Motion of Water Particles Beneath Waves • Deep Water Waves • In deep water most waves • do not interact with the • sea bottom and are called • deep-water waves. • The orbits of the water • molecules are circular. • •Shallow water waves • Interact with the sea floor
Wave Refraction & Wave Diffraction Refraction Diffraction Occurs when waves approach a shore at an angle. As drag along the bottom and differential speed along the crest, waves refract. It is this action that allows surfers to surf. Occurs when waves pass an obstacle.
Breakers Shore breakers (surf) are collapsing waves. Breaking is determined by wave steepness. When the wave height/wave length ratio is about 1/7, waves begin to break.
Tsunamis Tsunamis are incorrectly called “tidal waves” because they have nothing to do with tides. They are caused by movement of the sea floor (earthquakes, landslides). As they move out from their source, the waves formed end up having very small heights (1 - 2 ft.), very long wave lengths (100+ miles long) and they move at high speed (hundreds of miles per hour). When they approach shallow water, the water “piles up” and a very high wave may form, and when it breaks, thousands of people living near the shore could be killed.
