010 tides

• High water: a water level maximum ("high tide") • Low water: a water level minimum ("low tide") • Tidal range: the difference between high and low

tide• Spring Tide: full moon and new moon (14.77 days)• Neap Tide: 1st quarter and 3rd quarter (14.77 days)

Description of tides

Intertidal zoneHigh tide

Low tide

1.Gravitational pull of the moon and sun

2.Centripetal force of the rotating Earth

Tides are generated by:

Tides are generated by:• the gravitational pull of the moon and sun

- moon has 2x greater gravitational pull than the sun - sun is 10 million x more massive than the

moon and is 390 times farther away

Centripetal force

CENTRIPETAL

GRAVITATIONAL FORCE

GRAVITATIONAL & CENTRIPETAL

Why Two Tides?

• Tides are caused by the stretching of a planet.• Stretching is caused by a difference in forces on the

two sides of an object.• Since gravitational force depends on distance, there

is more gravitational force on the side of Earth closest to the Moon and less gravitational force on the side of Earth farther from the Moon.

Tidal PatternsSemidiurnal tides- two high and two low per day; Cape Cod, MA (high latitudes)Diurnal tides- one high and one low per day; Mobile, AL (low latitudes)Mixed pattern tides- Two high and two low tides per day BUT with successive high tide levels that are VERY DIFFERENT from each other; Hawaii (mid latitudes)

Type of tide depends on:• Position on the globe• Water depth• Contour- shape of ocean basins

Timing the Tides The moon revolves around Earth

much more slowly than Earth rotates.

High and low tides at that place happen 50 min. later each day.

How do Tides Vary?

A tidal range is the difference between levels of ocean water at high tide and low tide.

The combined forces of the sun and the moon on Earth produce different tidal ranges.

Hide Tides & Low Tides

• Second highest tidal range at Ancorage Alaska ~ 12 m

• Diurnal Tide: 24 hr 50 min cycle

• Semi Diurnal Tide: 12 hr 25 min cycle

• Mixed Tide: 12 hr 25 min cycle

Tidal Cycles

The Bay of Fundy: Site of the world’s largest tidal range

• Tidal energy is focused by shape and shallowness of bay

• Maximum spring tidal range in Minas Basin = 17 meters (56 feet)

Alma at High Tide

Alma at Low Tide

La Rance tidal power plant at St. Malo, France.

Corals exposed to air at extreme low tide

Bay of Fundy – tidegets huge (15 m) because of resonance

High tide

Low tide(6 hours later)

Earth’s Tides

• There are 2 high tides and 2 low tides per day.• The tides follow the Moon.

Tidal bore = a true tidal wave

• Wall of water that moves upriver

• Caused by an incoming high tide

• Occurs in some low-lying rivers

• Can be large enough to surf or raft

• St. Michel, N. coast of France

• ~16.8 m highest tidal range in Nova Scotia

Nova Scotia Tides

• Wolfville, NS (16 m tidal range)• Diurnal Tides (one high and one low every 12 hours

and 25 minutes

Why the Moon?

• The Sun’s gravitational pull on Earth is much larger than the Moon’s gravitational pull on Earth. So why do the tides follow the Moon and not the Sun?

Why the Moon?

• Since the Sun is much farther from Earth than the Moon, the difference in distance across Earth is much less significant for the Sun than the Moon, therefore the difference in gravitational force on the two sides of Earth is less for the Sun than for the Moon (even though the Sun’s force on Earth is more).

Why the Moon?

• The Sun does have a small effect on Earth’s tides, but the major effect is due to the Moon.

Different kind of tides

• Spring tides: tides that have the largest daily tidal range are spring tides.

• Neap tides: tides that have the smallest daily tidal range are called neap tides.

Earth-Moon-Sun positions and the monthly tidal cycle

Spring TideHighest high tide and lowest low tide

Neap TideModerate tidal range

Tides

• The rise and fall in sea level is called a tide.

• Caused by a giant wave.• One low-tide/high-tide cycle takes about

12 hrs and 25 min.• Tidal range is the difference in ocean

level between high-tide and low-tide

Gravitational Effect of the Sun• Spring Tides

–Earth, Moon, and Sun are lined up–High Tides are higher and Low Tides

are lower than normal

Gravitational Effect of the Sun• Neap Tides

–Earth, Moon, and Sun form right angles

–High Tides are lower and Low Tides are higher than normal

Why Tides Happen? The moon’s gravity pulls

on every particle on Earth However, the gravity

doesn’t pull on every particle with the same length of strength.

The moon’s gravitational pull on Earth decreases with the distance from the moon.

Therefore, the pull on different parts of the Earth is stronger than on other parts.

Sun and moon effects can be additive or not…

By the time the Earth has completed one revolution (one day), the moon has moved slightly, so the Earth needs to rotate a bit more to catch up with the moon…

The monthly tidal cycle(29½ days)

• About every 7 days, Earth alternates between:– Spring tide

• Alignment of Earth-Moon-Sun system (syzygy)• Lunar and solar bulges constructively interfere• Large tidal range

– Neap tide• Earth-Moon-Sun system at right angles (quadrature)• Lunar and solar bulges destructively interfere• Small tidal range