Tides

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Tides
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What are they?

• The rhythmic rising and falling of ocean surface
  levels.

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Why study them?

• Affect the coastline
• Affect the life of marine organisms in tidal
  areas
• Raise and lower sea level
• Affect lives of people
• Drive circulation in bays and estuaries

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What causes them?

1. Gravitational pull of sun and moon
2. Centrifugal force of earth as it spins

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1. Gravitational Pull

• Moon pulls water on earths surface wherever it
  faces earth causing a bulge.

2. Centrifugal force

• Opposite side of earth has less pull by the moon
  so it does not pull toward the moon.

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Result looks something like this
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• Because there are two bulges and two flats,
  most places on earth have 2 high tides and 2 low
  tides each day.

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But theres more

• One rotation of earth takes

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But theres more

• One rotation of earth takes24 hours

The moon advances a little on its own rotation
each day it takes a spot on earth 50 minutes to
catch up
So one tidal cycle high low high again takes
24 hours and 50 minutes
Tide charts predict when high and low tides will
occur
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How does the sun affect tides?

• Also has a gravitational pull on earth

But the sun is 400 times farther away so the pull
is not really noticed
Except
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Spring Tides

• New Moon

Pull from moon combined with pull from sun.
Result exceptionally high HIGH TIDES and
exceptionally low LOW TIDES
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Spring Tides

• Full Moon

Pull from moon on one side and pull from sun on
opposite. Result exceptionally high HIGH TIDES
and exceptionally low LOW TIDES
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Neap Tides
First quarter Third quarter
Range between high and low not significantly
great because sun and moons gravitational pull
cancel each other out
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Types of Tides
Diurnal One high and one low each day. (very
uncommon)

• Semidiurnal two high, two low each day
• (most common)

Mixed semidiurnal successive high tides of
different heights (most of U.S. west coast)