Tides Flashcards
What is are tides
Tides are the short term periodic rise and fall of the world’s oceans
What are tides a result of
Tides are the short term periodic rise and fall of the world’s oceans
What is a semi-diurnal regime
two high tides and two low tides each day
What is a diurnal tidal regime
one high and one low
What is tidal range
The difference in height between high tide and low tide
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 __________ ______
gravitational attraction
gravitational force
What are tidal streams
Periodic horizontal reversing movements of the water accompanying the vertical rising and falling of the Tides
What are spring tides
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.
What are neap tides
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.
What factors affect tides
Barometric pressure
Wind
Seiches
Storm surges (typically in North sea)
Negative surges
Seasonal changes in mean level
How does Barometric pressure affect tides
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
How does the wind affect tides
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
How do seiches affect the tide
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.
How do storm surges affect the tide
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.
How can negative surges affect tides
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.
How do seasonal changes in mean level affect tide
Monthly variations in mean sea level do not exactly repeat themselves from year to year so are listed part II for each year
What is low water
Min water level reached in a tidal cycle
What is high water
Max water level reached in a tidal cycle
What is stad
The brief period at high water and low water when no change in water level can be detected
What is a chart datum
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
What is a lowest astronomical tide
The lowest level predicted to occur under average metrological conditions and any combination of astronomical conditions.
Storm surges may make it lower on occasions
What is a highest astronomical tide
The highest level predicted to occur under average metrological conditions and any combination of astronomical conditions.
How is depth of water calculated
Charted sounding + the height of tide
