Tides.....

We spent Friday afternoon's lesson learning about tides and how they form and there seemed to be lots of things that we needed to know about them. The extent of my knowledge of tides previous to Friday's lesson stretched as far as the fact that they are created by the moon. So, in an effort to try and consolidate what we got taught, I am going to try and summarise some of the key points....

The tide is the cyclic rise and fall of the ocean surface and it is caused by  the moon and the sun exerting force on the earth.  This gravitional force has the strength to alter both the depth of the ocean and the oscillating currents. The moon has the greatest influence on tides as it is significantly closer to us than the sun and so water is dragged to the point where the moon is directly ahead. However, the sun also has the gravitational pull to affect tides and this force complicates the lunar tide cycle as the alignment of the moon and the sun influences the amplitude of tides.

The location of the moon, in relation to the sun, determines whether or not we experience Spring Tides or Neap Tides. Spring Tides are the tides with the biggest difference between high and low tides. They occur twice a month (Day 1 and day 14) when the earth, moon and sun are aligned. This alignment puts an extra gravitational pull on the tidal bulge which results in an extra high tide.

Neap Tides are the tides with the smallest difference between high and low tides. Neap Tides occur when the moon and the sun are at right angles to each other (1st quarter = Day 7 and 3rd quarter = Day 21). A smaller tidal range is produced because the lunar high tide coincides with the solar low tide and they partly cancel each other out.

- It takes 28 days for the moon to orbit the earth and it takes roughly twelve and a half hours for a tidal cycle to occur.
- The time between high and low tides when the water is falling is called the ebb and when it is rising it is known as the flow.
- Most places experience two high tides and two low tides a day (semi-diurnal) but some only experience one high and one low tide a day (diurnal).
- Tides do have their benefits which include the fact that they have enabled for the development of ports, maintain the mudflats which migratory birds are dependent on, create currents and transport sediment - these are just some of them and there are plenty more!
- Some areas, like the Mediterranean Sea (experiences a tide of approx 11 inches), do not experience tides and this is because it is not a big enough body of water to feel the gravitational pull of the moon and it is restricted by land.
- However, other areas experience very large tidal ranges. The Bay of Fundy, Canada, has the highest tidal range in the world and one of the main reasons that they experience such high tides is because the water is funnelled through a narrowing channel.

The River Severn has a tidal range which is almost equal to that of the Bay of Fundy and at the time of the highest spring tides, the funnelling effect of the wide estuary, sends a wave several miles up the river against the current. This wave is known as a tidal bore and occurs when  the force of the incoming tide is strong enough to force the flow of the river back on itself, thus creating a wave. These tidal bores generally occur around the time of the spring and autumn equinoxes (when the moon is directly above the equator - usually late March abd late September).

The Qiantang River in China experiences possibly the largest tidal bores in the world (9 metres!) and is known locally as the Black Dragon.

Well I think this is the majority of the key points and I actually think that I have managed to consoldiate what I learnt quite well - I hope this will be useful for others of you out there. However, I still have a few questions and I would be very greatful if anyone could shed some light on them .......... so firstly, what determines whether or not an area experiences a semi-diurnal or a diurnal tide? And secondly, if the time between high tides is equal, how come low tides do not occur half way between them?