Waves and TidesWaves and Tides

April 21, 2023April 21, 2023

Parts of a WaveParts of a WaveCrest

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 There are three factors affecting wave height:affecting wave height:

1. wind speed1. wind speed

2. distance over which the wind 2. distance over which the wind blows (fetch)blows (fetch)

3. length of time the wind blows3. length of time the wind blows

Why waves crash on shoreWhy waves crash on shore1. wave base hits ocean floor 2. friction slows wave3. wavelength decreases

and wave height

increases 4. gravity pulls the crest down

TidesTides

• Crest of this wave is actually a Crest of this wave is actually a bulgebulge of ocean water created by the of ocean water created by the moon’s gravitymoon’s gravity..

EM

The bulge on side opposite the moon is caused by inertia and centrifugal force.

Semi-Diurnal TidesSemi-Diurnal Tides• Most shorelines experience 2 High Tides Most shorelines experience 2 High Tides

and 2 Low Tides per day – and 2 Low Tides per day – Semidiurnal Semidiurnal tidestides

• Because the Earth is rotating on it axis, Because the Earth is rotating on it axis, the times between tides are as follows:the times between tides are as follows:– From HighFrom High Low = 6 hours 12 min 30 sec Low = 6 hours 12 min 30 sec– From HighFrom High High = 12 hours 25 min High = 12 hours 25 min

• Difference in sea level elevation between Difference in sea level elevation between high tide and low tide in one location – high tide and low tide in one location – tidal rangetidal range

Diurnal TidesDiurnal Tides

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

The Sun and TidesThe Sun and Tides

• The gravity of the Sun can intensify or The gravity of the Sun can intensify or diminish the moon’s tidal effects.diminish the moon’s tidal effects.

• Spring TidesSpring Tides occur during New Moon occur during New Moon and Full Moon phases ~ 2 times/monthand Full Moon phases ~ 2 times/month– Sun, Earth, Moon are in a line, Sun Sun, Earth, Moon are in a line, Sun adds adds

toto Moon’s gravity Moon’s gravity– High Tides are High Tides are higherhigher than normal than normal– Low Tides are Low Tides are lowerlower than normal than normal

The Sun and TidesThe Sun and Tides

• Neap TidesNeap Tides occur during First occur during First Quarter and Third Quarter phases ~ Quarter and Third Quarter phases ~ 2 times/month2 times/month– Sun, Earth, Moon are at a right angle, Sun, Earth, Moon are at a right angle,

Sun Sun takes away fromtakes away from Moon’s gravity Moon’s gravity– High Tides are High Tides are lowerlower than normal than normal– Low Tides are Low Tides are higherhigher than normal than normal

The Moon’s OrbitThe Moon’s Orbit• The The orbitorbit of the Moon is not perfectly of the Moon is not perfectly

circular – it is circular – it is ellipticalelliptical, therefore there is , therefore there is a point in the Moon’s orbit called a point in the Moon’s orbit called – PerigeePerigee when the Moon is closest to the when the Moon is closest to the

EarthEarth– ApogeeApogee when the Moon is farthest from the when the Moon is farthest from the

EarthEarth

M ME

QuestionsQuestions

• How would the tidal range be How would the tidal range be affected if spring tide occurs at affected if spring tide occurs at perigee?perigee?

• tidal range would be greater (largest tidal range would be greater (largest range of the month)range of the month)

• At which moon position would the At which moon position would the tidal range be the smallest? tidal range be the smallest?

• neap tide at apogeeneap tide at apogee