Waves and Tides

1
Waves and Tides

• November 14, 2009

2
Parts of a Wave
Crest
- the highest point of a wave
Wavelength
- the distance from crest to crest or trough to
trough
Wave height
- the distance from crest to trough
Trough
- the lowest point of a wave
Wave Base ½ the wavelength
3
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

4
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

5
Tides

• Crest of this wave is actually a bulge of ocean
  water created by the moons gravity.

E
M
The bulge on side opposite the moon is caused by
inertia and centrifugal force.
6
Semi-Diurnal 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
• 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

7
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

8
The Sun and Tides

• The gravity of the Sun can intensify or diminish
  the moons tidal effects.
• Spring Tides occur during New Moon and Full Moon
  phases 2 times/month
• Sun, Earth, Moon are in a line, Sun adds to
  Moons gravity
• High Tides are higher than normal
• Low Tides are lower than normal

9
(No Transcript)
10
The Sun and Tides

• 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 Moons gravity
• High Tides are lower than normal
• Low Tides are higher than normal

11
(No Transcript)
12
The Moons Orbit

• The orbit of the Moon is not perfectly circular
  it is elliptical, therefore there is a point in
  the Moons orbit called
• Perigee when the Moon is closest to the Earth
• Apogee when the Moon is farthest from the Earth

M
M
E
13
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