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NAU 102

Lesson 24

Agenda

Tides

•Definitions

•Importance

•Causes

•Tidal Cycles

•Reference Planes

Tides

Why do we care?

We don’t want to:

•Run aground.

•Hit the bridge.

•Snap the mooring lines.

Tide

The vertical rise and fall of the ocean level due to the gravitational and

centrifugal forces between the earth and the moon (and the sun).

Definitions

High tide (or high water) – highest water level reached in a rising tide

Low tide (or low water) – lowest water level reached in a falling tide

Stand – the brief period at high & low water when no change in water level can be detected.

Range – the difference in height between high tide and low tide.

Causes

Centrifugal Forces

Gravitational Forces

Causes

Centrifugal Force

A force that tends to impel a thing outward from the center of rotation.

Causes

Centrifugal Force

The Earth/Moon system revolves around its center of mass, the barycenter.

Earth

Barycenter

Moon

Causes

Centrifugal Force

The Earth/Moon system revolves around its center of mass, the barycenter.

Earth

Barycenter

Moon

Causes

Gravitational Force

Gravity forces the water to flow towards the Moon.

Earth

Moon

Causes

Gravitational Force

Gravity forces the water to flow towards the Moon.

Earth

Moon

Causes

Resultant Force

Earth

Moon

Causes

Resultant Force

Earth

Moon

Causes

If the Earth was a perfect sphere, completely covered with water:

Two High Tides & Two Low Tides

Every Lunar Day (24 hours 50 minutes)

Daily Rotation of Earth

Earth

Monthly Orbit of Moon

Earth

Combination of Motions

Earth

Orbit of the moon

The moon’s orbit is elliptical.

Its distance from Earth varies during the month.

Earth

apogeeperigee

Tides are 15%-20% higher than average at perigee

Effect of the Sun

The Sun affects the tides as well.

Due to distance, it has only 46% of the Moon’s effect.

Spring Tides

High tides are higher than

normal.

Low tides are lower than

normal.

When the sun, Earth and moon are in line:

Neap Tides

High tides are lower than

normal.

Low tides are higher than

normal.

When the sun, Earth and moon are 90° apart:

Tides

Spring tides and neap tides each happen twice each month.

Greatest range of tide occurs at perigean spring tides.

Local Tides

If the Earth was a perfect sphere

And, it was covered by a uniform amount of water

There would be 2 high tides and two low tides each day at every location.

The tide range would be about 12 inches at every location.

Local Tides

Local differences affect both the period of the tide and tide range.

Because land shape and water depth varies from place to place, the tides vary with

location.

In coastal areas, the range of tide can be as much as 50 feet!

Local Tides

Bay of Fundy

Nova Scotia

Tidal Cycles

Semidiurnal Tide

2 high and 2 low waters each tidal day, with relatively small differences in the respective

highs and lows.

Tides on the Atlantic coast of the US.

Semidiurnal Tides

Tidal Cycles

Diurnal Tide

A single high and single low water occur each tidal day.

The northern shore of the Gulf of Mexico, in the Java Sea, the Gulf of Tonkin, and a few

other places.

Diurnal Tides

Tidal Cycles

Mixed Tide

2 high and 2 low waters each day, with a large inequality in the high water heights, low water

heights or both.

Pacific coast of the US.

Mixed Tides

Reference Planes

But, charted depth isn’t necessarily the depth of the water at the moment.

How do we know how deep the water is?

Look on the chart.

It is an average of the depths taken at specific times, depending on the Sounding Datum.

Reference Planes

Mean Sea Level (MSL)

The average level of the ocean at a location.

Data is collected over a 18.6 year period called the Nodal Period.

Half the time the water is shallower than MSL.

Half the time the water is deeper than MSL.

Dangerous!

Reference Planes

Mean low water (MLW) - the average of all low tides at a given place.

Mean lower low water (MLLW) - the average lower low waters of each tidal day.

(The sounding datum used for U.S. waters)

Mean low water springs (MLWS) - the average level of the low waters at spring tides.

(Used for some British charts)

Reference Planes

Mean higher high water (MHHW) - the average higher higher waters of each tidal day.

Mean High Water (MHW) - the average of all high tides at a given place.

(The vertical datum used for U.S. waters)

Mean high water springs (MHWS) - the average level of the High waters at spring tides.

Tides

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