Waves and Tides December 8, 2015December 8, 2015December 8, 2015

Parts of a Wave Crest Trough - the highest point of a wave - the lowest point of a wave Wave height - the distance from crest to trough Wavelength - the distance from crest to crest or trough to trough Wave Base – ½ the wavelength

There are three factors affecting wave height: 1. wind speed 2. distance over which the wind blows (fetch) 3. length of time the wind blows

Why waves crash on shore 1. wave base hits ocean floor 2. friction slows wave 3. wavelength decreases and wave height increases 4. gravity pulls the crest down

Tides Crest of this wave is actually a bulge of ocean water created by the moon’s gravity. Crest of this wave is actually a bulge of ocean water created by the moon’s gravity. E M The bulge on side opposite the moon is caused by inertia and centrifugal force.

Semi-Diurnal Tides Most shorelines experience 2 High Tides and 2 Low Tides per day – Semidiurnal tides Most shorelines experience 2 High Tides and 2 Low Tides per day – Semidiurnal tides Because the Earth is rotating on it axis, the times between tides are as follows: Because the Earth is rotating on it axis, the times between tides are as follows: –From High Low = 6 hours 12 min 30 sec –From High High = 12 hours 25 min Difference in sea level elevation between high tide and low tide in one location – tidal range Difference in sea level elevation between high tide and low tide in one location – tidal range

Diurnal Tides Because of the ocean bottom shape or the tilt of the moons revolution, some shorelines experience only 1 High Tide and 1 Low Tide per day – Diurnal tides ex) Gulf of Mexico Because of the ocean bottom shape or the tilt of the moons revolution, some shorelines experience only 1 High Tide and 1 Low Tide per day – Diurnal tides ex) Gulf of Mexico

The Sun and Tides The gravity of the Sun can intensify or diminish the moon’s tidal effects. Spring Tides occur during New Moon and Full Moon phases ~ 2 times/month –S–S–S–Sun, Earth, Moon are in a line, Sun adds to Moon’s gravity –H–H–H–High Tides are higher than normal –L–L–L–Low Tides are lower than normal

Neap Tides occur during First Quarter and Third Quarter phases ~ 2 times/month Neap Tides occur during First Quarter and Third Quarter phases ~ 2 times/month –Sun, Earth, Moon are at a right angle, Sun takes away from Moon’s gravity –High Tides are lower than normal –Low Tides are higher than normal

The Moon’s Orbit The orbit of the Moon is not perfectly circular – it is elliptical, therefore there is a point in the Moon’s orbit called The orbit of the Moon is not perfectly circular – it is elliptical, therefore there is a point in the Moon’s orbit called –Perigee when the Moon is closest to the Earth –Apogee when the Moon is farthest from the Earth MM E

Questions How would the tidal range be affected if spring tide occurs at perigee? tidal range would be greater (largest range of the month) At which moon position would the tidal range be the smallest? neap tide at apogee