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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
Introduction to Navigation
Questions?
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