Tides Flashcards

1
Q

What is are tides

A

Tides are the short term periodic rise and fall of the world’s oceans

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2
Q

What are tides a result of

A

Tides are the short term periodic rise and fall of the world’s oceans

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3
Q

What is a semi-diurnal regime

A

two high tides and two low tides each day

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4
Q

What is a diurnal tidal regime

A

one high and one low

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5
Q

What is tidal range

A

The difference in height between high tide and low tide

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6
Q

The side of the Earth closest to the moon has the strongest _______
________ towards the moon whilst water on the other side of the Earth experiences a weaker __________ ______

A

gravitational attraction
gravitational force

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7
Q

What are tidal streams

A

Periodic horizontal reversing movements of the water accompanying the vertical rising and falling of the Tides

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8
Q

What are spring tides

A

When the sun, earth and moon are in line, the gravitational forces of the bodies reinforce one another causing a greater tidal bulge resulting in a greater tidal range.

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9
Q

What are neap tides

A

When the sun, earth and moon form a right angle, the gravitational forces are working at 90° to each other. This results in a smaller tidal bulge and a smaller tidal range.

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10
Q

What factors affect tides

A

Barometric pressure
Wind
Seiches
Storm surges (typically in North sea)
Negative surges
Seasonal changes in mean level

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11
Q

How does Barometric pressure affect tides

A

Predictions are for an average barometric pressure. A different barometric pressure for a period of time over a large area can change the predicted heights.
Low pressure will raise sea level
High pressure will depress it

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12
Q

How does the wind affect tides

A

Changes heights and times of tides
Varies depending on local topography
Generally a strong onshore wind will pile up the water raising sea levels in the direction it is blowing
Strong offshore winds will have the opposite effect

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13
Q

How do seiches affect the tide

A

Abrupt changes in meteorological conditions such as the passage of an intense depression or line squall cause oscillations in sea level. The period between waves can be from minutes to 2 hours and the height from a few centimetres to a metre or more.

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14
Q

How do storm surges affect the tide

A

Strong persistent winds will pile up the water in say the Northern part of the North Sea.
A change of wind direction to the South releases this water which travels South as a wave, (which may also be reflected back).
If the tides are Springs this can result in large increases of up to 2.4m above predicted tidal heights.

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15
Q

How can negative surges affect tides

A

Similar cause to storm surges makes the tidal level lower than predicted. Import for large vessels with small UKC.
Negative surges of over 0.6m occur about 15 times a year in the Southern North Sea. There is a warning service specifically for the Southern North Sea area.

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16
Q

How do seasonal changes in mean level affect tide

A

Monthly variations in mean sea level do not exactly repeat themselves from year to year so are listed part II for each year

17
Q

What is low water

A

Min water level reached in a tidal cycle

18
Q

What is high water

A

Max water level reached in a tidal cycle

19
Q

What is stad

A

The brief period at high water and low water when no change in water level can be detected

20
Q

What is a chart datum

A

Reference height of nautical charts and tidal predictions defined as a level so low that the tide will not frequently fall below it For most charts this is the level of Lowest Astronomical Tide

21
Q

What is a lowest astronomical tide

A

The lowest level predicted to occur under average metrological conditions and any combination of astronomical conditions.

Storm surges may make it lower on occasions

22
Q

What is a highest astronomical tide

A

The highest level predicted to occur under average metrological conditions and any combination of astronomical conditions.

23
Q

How is depth of water calculated

A

Charted sounding + the height of tide

24
Q
A