They result from the gravitational interaction between the Earth, the moon and to a lesser extent, the Sun. Different parts of the world experience different tidal regimes. Around the UK, there are mostly two high tides and two low tides each day: this is called a semi-diurnal regime.
Other parts of the world have a diurnal tidal regime with only one high tide and one low tide each day. The difference in height between high tide and low tide is called the tidal range.
Tides are due to the combined effects of gravitational attraction and the revolution of the Earth-moon system about its common centre of mass. At this point which lies within the solid Earth the gravitational attraction between Earth and moon exactly balances the forces required to maintain the moon's orbit.
Elsewhere the two forces are not in balance and give rise to the so-called tide generating force. The side of the Earth closest to the moon has the strongest gravitational attraction towards the moon whilst water on the other side of the Earth experiences a weaker gravitational force. A convenient concept is to think of the tide generating forces causing an ovoid of water, aligned with the position of the moon, enveloping the Earth although this is an over-simplification and such a bulge does not exist in nature.
According to this model, there are two bulges of water high tides divided by troughs of water low tides around the Earth. The schematic diagram above depicts the interaction between the Earth and the moon which explains the lunar tides.
The gravitational influence of the Sun on the Earth's surface manifests itself in a similar way giving rise to solar tides. The moon has the dominant effect. The tide generating force is proportional to the product of the mass of the two bodies but also inversely proportional to the cube of the distance between them. The moon therefore has the dominant effect: although its mass is much less than the Sun it is far closer to the Earth.
The tide generating force due to the Sun is 0. Spring tides occur when the lunar and solar semi-diurnal tides interfere constructively. Using the simplistic analogy of tidal bulges — this is when the lunar tidal bulge and the solar tidal bulge are superimposed upon one another. This occurs when the Sun and the moon are aligned in space at either new moon or full moon. Spring high tides are higher and spring low tides are lower than average. Neap tides occur when the moon is at its first or third quarter.
A very detailed, animated view of currents in the Solent and Southampton water calculated using a numerical model can be viewed on YouTube and predictions for specific days can be obtained from various commecial web sites by subscription.
A simulation of currents around the Isle of Wight and currents in the Solent between Cowes and Lepe by the research branch of ABP shows how they change during the period of a spring tide. NCI is not responsible for and cannot vouch for the accuracy or availability of the data sources and forecasts linked to from this web site. NCI watchkeepers provide the eyes and ears along the coast, monitoring radio channels and providing a listening watch in poor visibility. They are trained to deal with emergencies offering a variety of skills and experience, and full training by the NCI ensures that high standards are met.
Skip to main content. How to Find Us. Local Information Links. Solent Marine Information. Solent Marine VHF. How to genetically classify a weathering mineral? Why doesn't an injured spinal cord re-grow? Ask The Naked Scientists. Humans navigate inefficiently in cities. Hydrogen electrolyser wins Earthshot Prize. Talk the Talk. Whales eat and poop more than we thought. Is dark matter in lumps or like sand grains? Pollution affects babies even before birth. Regular winter swimming could boost health.
TEA Laser - How lasers work. Pause Play Stop Stream. Unmute Mute. Update Required To play the media you will need to either update your browser to a recent version or update your Flash plugin.
These characteristics mean that there is a significant tidal gradient in the Solent from 1. Near Calshot, the tidal regime and bathymetry of Christchurch Bay results in a double high water. Further to the east, around Spithead, changes in the relationship between the tides and bathymetry results in an extended rather than double high tide.
In Southampton Water, the tidal characteristics are exaggerated by internal resonance within the estuary, this results in a 'young flood stand' and a double high water period with little change in water level.
0コメント