Time & Tide
you don't need a weatherman to know which way the wind blows - Dylan
How do tides work? its as simple as 1:2:3:3:2:1
Each Trainer Assessor has their own particular way to describe why twice a day, every day, the sea water level changes. This is a quick overview
The moon does it
OK so perhaps a little more information. The gravitational pull of the moon and, to a lessor extent, the sun causes the sea to surge and withdraw at different parts of the globe. As the moon spins around the earth it pulls the sea with it and as the water chases (or wobbles) its way after the moon the water level rises (comes in) or falls (goes out).
So knowing the lunar cycle is important?
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The moon's gravitational force pulls on sea creating "bulges" on opposite sides of the globe.
This means that water is higher when the moon is directly overhead.
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Correspondingly on the other side of the planet the earth is being pulled towards the moon also leaving a buldge that side too. So when your beach is experiencing high water so is the exact other side of the planet. so in plain english...... If you are on the beach and the moon is overhead, you will have a high tide.
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So why then are tides not regular - the moon is?
The Earth rotates 360o (degrees) in 24 hours, and 180o in 12 hours. The moon rotates 6o around the earth in the same 12 hours. The twin bulges and the moon's rotation mean that the cycle repeats about every 12 hours and 25 minutes.
So rule of thumb is that the tide is an hour later every day but over a few days this soon becomes inaccurate.
neap tide |
spring tide |
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Tidal differences When you read tide data it is important to know where the data is for as times change as you move around. So if you are in Dublin at the Poolbeg lighthouse and the high tide is at 12:00hrs the same high tide will be in Howth 5 minutes later. It is also important to make sure that the tide source you are using is both adjusted for the area you are in and for summertime or else you could find yourself with a lot less water than you had planned !
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Tidal Height & Ordinance Datum Tidal height is measured in metres so when you see 3.4 or 4.0 this relates to vertical height - but height above what? For this measurement we use what is called ordinance datum - a fixed point - in this case the mean sea level at Portmore Pier Malin Head between 1960 and 1969. Up to 1970 the measurement was taken from poolbeg lighthouse (the red one at the end of the south wall of Dublin harbour) It was calculated as the low water mark following the spring tide on 8/4/1937 These are also the measurements that are used to measure hills and mountains on Irish maps |
> Irish Tide Data
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Rule of Twelfths The rise and fall of sea water is regular and cyclical. Understanding it can make it easier to plan open water events. So imagine its low water now...... In a nutshell the rule states that in the first hour after low tide the water level will rise by one twelfth of the range, in the second hour two twelfths, and so on according to the sequence - 1:2:3:3:2:1 The same pattern is for water rising as well as falling. Think about the impact in the channel at HITW - for one hour either side of high tide the water moves slowest but three hours after it is three times faster - now you can see why the speed varies so much and how to plan for it . |
The lingo
| > Spring Tide | The largest possible tide caused when the sun moon and earth are in a straight line. Technically called syzygy. On a spring the water reaches is highest and lowest level |
| > Neap Tide | When the Moon is at first quarter or third quarter it is at a 90o angle to the sun. The sun the partially cancels out the pull of the moon causing a smaller tide |
| > semidiurnal tides | The type of tide we have, 2 high and 2 low per day. There are also diurnal (one high and one low per day) and mixed ones |
| > slack water | at the point of high or low water there is limited movement this time is called slack (and is the time we like to be in the water swimming) |
| > ebb | the period when tide is falling from high to low water the opposite of flood |
| > flood | the period when tide is rising from low to high water the opposite of ebb |
...............and finally if you want to sound like the experts it is as simple as always referring to a tide as rising or falling and never coming in or out !