Tides Flashcards
Explain the Tidal Theory
The moon and the sun exert attractive forces upon the rotating Earth, thereby causing the water levels of the oceans to rise and fall periodically. The effect of the Moon is slightly more than double that of the Sun due to its proximity to the Earth. Either body causes a build up of water directly below it on the earth (sublunar) due to its slightly higher gravitational pull at that point, and a further build up of water diametrically opposite (antinodal) due to its slightly lesser attraction. This results in High Water occurring in these two locations on Earth, and Low Water at locations half way between. Since the Earth rotates once per day relative to the position of the Moon and Sun, this generally results in two High Tides and Two Low Tides per day.
Explain Spring Tides
Because the Sun and Moon move at different rates, they are sometimes acting in line (Conjunction and Opposition). These forces combine to produce very high High Waters and very low Low Waters.
Explain Neap Tides
Because the Sun and Moon move at different rates, they are sometimes acting at right angles to each other (Quadrature). Because the Moon’s force is greater than that of the Sun, High Waters occur on the line of the Moon’s position, while the Sun’s attractive force acts at right angles to it. This causes the High Waters to be lower than normal, and the Low Waters to be higher than normal.
What is the Range of Tide, and what is its significance during Spring and Neap tides?
Range of tide is the difference between any High Water and Low Water, it is greatest during Spring Tides and least at Neap Tides.
What significance does the phase of the Moon have on tides?
Because the Moon is only visible when light from the Sun strikes that part of it facing Earth, the phase of the Moon depends on its position relative to the sun. New Moon occurs at Conjunction and is associated with Spring Tides. Full Moon occurs in Opposition and is also associated with Spring Tides. Half Moon occurs at Quadrature (first or last quarter) and is associated with Neap Tides.
How often do we experience Spring and Neap tides?
We experience Spring tides every 14 days, and Neap tides every 7 days before and after Spring tides.
Explain the difference between Semi-Diurnal, Diurnal and Mixed Tides.
- Semi-Diurnal : Two High Waters and two Low Waters per day
- Diurnal: One High Water and One Low Water per day
- Mixed Tides: Land masses modify effect of tide and result in great variety of tidal patterns and ranges. Characterised by large differences between the heights of the High Waters, Low Waters or in both.
What is chart datum?
A depth of water that is so low that the tide will not frequently fall below it. It is used as a reference point from which other values are calculated and is often the same as the Lowest Astronomical Tide (LAT) on Admiralty charts.
What are charted depths?
Areas of the seabed which are lower than Chart Datum and would therefore be submerged by the stated amount if the height of tide was zero (also known as soundings).
What are drying heights?
Areas of the seabed (or land) which are higher than chart datum and would therefore be uncovered (dry) by the stated height if the height of tide was zero.
What are Mean High/Low Water Springs (MHWS/MLWS)?
MHWS is the average height of high water above chart datum during spring tides, and MLWS is the average height of low water above chart datum during spring tides. The difference between the two is called Spring Range.
What are Mean High/Low Water Neaps (MHWN/MLWN)?
MHWN is the average height of high water above chart datum during neap tides, and MLWN is the average height of low water above chart datum during neap tides. The difference between the two is called Neap Range.
What is height of tide?
The height of water above chart datum. Value can be calculated using tide tables.
What is the equation for finding actual depth?
Actual depth = charted depth + height of tide
What is Under Keel Clearance?
Is the depth of water between the vessels bottom and the seabed. UKC = Actual depth of water- Draft of vessel
What is charted height / elevation?
The height of an object. Often measured from MHWS to the highest point of the objects. Reference point will be given on chart.
What is charted vertical clearance?
The height of the lowest point of a bridge or obstruction. Normally measured from the Highest Astronomical Tide (HAT).
What is Highest/Lowest Astronomical Tide (HAT/LAT)
The highest and lowest levels which can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions.
What is a secondary port?
Ports for which tidal heights have to be determined using a correction from a specified standard port.
What other factors may influence the actual tide level?
Meteorological factors- Wind and atmospheric pressure, a strong onshore wind can pile up water against the coast, while an offshore wind will tend to lower the coastal water level. Winds blowing along a coast may set up long waves which will raise or lower the level depending upon the position of the crest and trough. When atmospheric pressure is greater than average, the sea level will tend to be lower and visa versa when the pressure is lower. These effects produce storm surges, sometimes resulting in coastal flooding. Negative surges (where tide level is lower than predicted) are of obvious relevance to the mariner.
