Tides
What are they?
• The rhythmic rising and falling of ocean surface levels.
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
What causes tides?
1. Gravitational pull of the sun and moon on the Earth
1. Gravitational Pull
• Moon pulls water on earth’s surface wherever it faces earth causing a bulge.
Result looks something like this:
most places on earth have 2 high tides and 2 low tides each day.
But there’s more…
• One rotation of earth takes…
But there’s more…
One rotation of earth takes…24 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
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…
Spring Tides – straight line
• New Moon
Earth
Pull from moon combined with pull from sun. Result: exceptionally (wicked) high HIGH TIDES and exceptionally (wicked) low LOW TIDES
Spring Tides
• Full Moon
Earth
Pull from moon on one side and pull from sun on opposite. Result: exceptionally high HIGH TIDES and exceptionally low LOW TIDES
Neap Tides – Earth, moon and sun at a right angle
Earth
First quarter
Third quarter
Small difference between high and low not significantly great because sun and moon’s gravitational pull “cancel” each other out
Types of Tides
Semidiurnal – two high, two low each day(most common)
Mixed semidiurnal – successive high tides of different heights (most of U.S. west coast)
Diurnal – One high and one low each day. (very uncommon)