Tides

• A tide is a repeated cycle of sea level changes in the following stages:

• Over several hours the water rises or advances up a beach in the flood tide.

• The water reaches its highest level and stops at high tide. Because tidal currents cease this is also called slack water or slack tide. The tide reverses direction and is said to be turning.

• The sea level recedes or falls over several hours during the ebb tide.

• The level stops falling at low tide. This point is also described as slack or turning.

Tides and the Forces That Generate Them

What are the characteristics and causes of tides?

•Tides are caused by the gravitational force of the moon and sun and the motion of earth.

•The wavelength of tides can be half the circumference of earth. Tides are the longest of all waves.

•Tides are forced waves because they are never free

of the forces that cause them.

The changing tide produced at a given location is the result of the changing

positions of the Moon and Sun relative to the Earth

coupled with the effects of the Earth’s rotation and the local

shape of the seafloor

Top: The positions of the Sun, the moon and Earth during a spring tide.

Bottom: The positions of the Sun, the moon and Earth during a neap tide.

Sun and Moon Together

The Equilibrium Theory of Tides

The action of gravity and inertia on particles at five different locations on Earth. Forces are balanced only at point CE, the center of Earth. Note the bulges that are aligned with the moon as Earth spins on its axis. The key to understanding the equilibrium theory of tides is to see Earth turning

beneath these bulges.http://www.tubechop.com/watch/1821798

Tide Terminology• Full moons and new moons occur when the moon, the sun,

and Earth are aligned, centrifugal force and gravitational forces are maximized by this arrangement,

resulting in high tides – also called spring tides

• When the sun, moon, and Earth are at right angles to one another (first and last quarter), the tidal range is

decreased, producing neap tides.

High Tide: ocean reaches its highest elevation on

the ocean floor

Low Tide: ocean reaches its lowest elevation on

the ocean floor

• Tidal Range: difference in the water height from high tide to low tide.

Bay of Fundy (near Nova Scotia and Maine) -

largest tidal ranges in the world (40-50 feet)

http://www.youtube.com/watch?v=YiHP8EG_Wq4

Types of Tides

1. Semidiurnal Tide: tidal cycle of two high tides & two low tides each day, with the two high tides of nearly equal height; occurs on the east coast of the US, most of Europe and Africa

2. Mixed Semidiurnal Tides: tidal cycle of two high tides & two low tides each day, high tides are of differing heights each day; occurs throughout most of the coastal western US and Canada.

3. Diurnal Tides: one low tide & one high tide daily; occur in Antarctica, some parts of the Gulf of Mexico, the Caribbean, and the Pacific Ocean.

Tidal Tables: list the anticipated high and low tides –on the Internet on various NOAA and GA DNR sites.

Three Types of Tides

Tide curves for three common types of tides.

Tide Types and Locations

The worldwide distribution of the three tidal patterns.

Tide tables include data showing times of high and low tide.

The Tidal Day• The moon does not stay put, but rotates around the

earth at a rate of about 12° a day, or one rotation a month. The rotation is in the same direction as the earth’s spin, so by the time the earth has done one rotation, the moon has shifted 12° further, and it takes an extra 50 minutes for the moon to be in the same position relative to a point on the earth.

• Therefore, the tidal cycle is not 24 hours long, but 24 hours and 50 minutes. Because of this, high and low tides are about 50 minutes later every day.

Why is it important to know when high and low tides occur?

• Storms cause less damage during low tide.

• Ships have to wait in deep water to dock.

Floating Dock

Tides and Marine Organisms

Tides have a profound affect on coastal marine life. •Coastal life is sorted into zones and sub-zones, depending on the amount of emergence and submergencethe organisms can tolerate.

•Intertidal zone - area exposed at low tide, covered at high tide•Subtidal - always submerged

Vertical zonation in the intertidal zone on a rocky Atlantic and

Pacific shore.

Organisms at the top of the intertidal zone tolerate more air while

those at the bottom need to be covered with

water.

Biological Importance of Tides

• Transport nutrients and organisms between shore and deeper water• Life cycle signals – full moon

ex. grunion and horseshoe crab

Southern California

Atlantic/Gulf Coast