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)