The width of the shoreline strip that is affected by waves depends on the tidal range. A large tidal range means that a wide strip of land might be subjected to the force of the waves. If the tidal range is very small, all of the wave's energy will be concentrated in the same place.
The rise and fall of tides causes water to move in and out of estuaries, bays and harbours. This movement is called a tidal current. When the tide is rising, water flows from the ocean into the bay creating a flood current.
Since so much water is pulled into the two high tides, low tides form between the two high tides Figure below. As the Earth rotates beneath the Moon, a single spot will experience two high tides and two low tides every day.
The tidal range is the difference between the ocean level at high tide and the ocean at low tide Figure below. The tidal range in a location depends on a number of factors, including the slope of the seafloor. Water appears to move a greater distance on a gentle slope than on a steep slope.
So when the Sun and Moon are aligned, what do you expect the tides to look like? Waves are additive so when the gravitational pull of both bodies is in the same direction the high tides add and the low tides add Figure below.
Highs are higher and lows are lower than at other times through the month. These more extreme tides, with a greater tidal range, are called spring tides.
Spring tides occur when the tidal bulges from the Moon and Sun are aligned. The Moon is full in this image; in the bottom image the Moon would appear as a new Moon. Neap tides are tides that have the smallest tidal range, and they occur when the Earth, the Moon, and the Sun form a 90 o angle Figure below. How do the tides add up to create neap tides? At neap tides, the tidal range relatively small. Neap tides occur when the Earth, the Sun, and the Moon form a right angle; the Moon is in its first or third quarter.
High tides occur about twice a day, about every 12 hours and 25 minutes. Storm systems at sea and on land also shift large quantities of water around and affect the tides. Detailed forecasts are available for high and low tides in all sea ports, but are specific to local conditions. That many of the areas of the world with high ranges of tides are in the areas of Alaska, Canada, and northern Europe has created a misconception that the range of tide increases with increasing latitude as one moves farther from the equator and closer to the poles.
This is incorrect. Increased tidal ranges in these areas are created by the positions and configurations of the continents in the northern hemisphere. Many people have asked us about the tides; below are answers to a selection of their questions. Please note that the usual NERC disclaimer applies to any information given. Tides are the short term periodic rise and fall of the world's oceans.
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.
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