The periodic rise and fall of the waters in the oceans is called tides. Tides are the response to the gravitational pull of the sun, the rotation of the earth and the moon. The moon has the biggest influence on the earth’s tides, because it is closest but the sun also has a significant effect.
The patterns of the tides have three classifications.
The commonest tide pattern is semi-diurnal tides, which have two high and two low tides each tidal day. The consecutive high tides and low tides reach roughly the same height. The tidal day is the same as a lunar day which lasts 24 hours 50 minutes of a solar day. The Atlantic Ocean has semi-diurnal tides.
There is one high and one low tide a tidal day where there are diurnal tides. So the Gulf of Mexico, which has diurnal tides only has one low tide and one high tide a day.
The main characteristic of mixed tides is that they have consecutive high and low waters that have very different heights. Mixed tides are usually semi-diurnal with two high and two low tides each tidal day, they may become diurnal occasionally. The Pacific Basin has mixed tides, so it has one low tide followed by two high tides.
Causes of Tides
The gravitational pull of the sun and the moon affect the Earth and all the water on it, including the water in humans. The amount of attraction depends on mass and distance. As people have a very small mass and are close to the Earth, the sun and moon have a minute effect. The sun has an enormous mass but it is about 93 million miles from Earth, whilst the moon is about 400 times closer at 238,866 miles away. So the moon has twice the power over the oceans that the sun has.
Tides depend on how strong the gravitational pull is on different sides of the planet. High tide is occurs when the water nearest to the moon is pulled by it, whist on the other side of the world, 7926 miles away, the moon is having much less effect on the water and another high tide occurs. Halfway between the high tides are found the low tides. As the Earth rotates, these tides move with it as the distance from the moon changes.
The rotation of the Earth and the orbit of the moon round the Earth govern when the timing of the tides. It takes the Earth 24 hours to rotate around its axis, while the moon is moving 1/30th around its orbit in the same period. The Earth then takes about 50 minutes to “catch up” with the moon as it orbits, go that the time between two high tides and two low tides in about 24 hours and 50 minutes, a lunar day.
The greatest difference between the water levels at high and low tides occurs when the sun, moon and earth are in a straight line, known as syzygy. Twice a month, during the full and new moon and gives us spring tides. When the moon at its closest point to the Earth during its orbit, this is known as perigee and the tides are particularly low and high.
The half moon happens when the sun and moon are at right angles to each other. At this time, the gravitational pull of the sun and the moon fight each other and there is less of a difference between high and low waters. These tides are called neap tides.
There are other factors which affect the tides such as the depth of the water, the tilt of the Earth, the orbit the moon moves along as it goes round the earth and the ocean floor. So there aren’t two high and two low tides a day on some coasts.
The tidal range is the difference between the height of the surface of the water at high tide and the height of the surface water at low tide. Some tidal ranges, such as in Lake Superior, Michigan, are measured in inches whilst others are measured in feet or yards. The Canadian inlet in Nova Scotia, the Bay of Fundy, has the biggest tidal range on Earth at 50 feet. Large tidal ranges can cause boats anchored at high tide stranded on the beach when low tide comes.
Tidal current that move horizontally are generated by the rises and falls in sea level. Those that occur when the water level drops, as the tide goes out, are known as ebb currents whilst the rising tide causes flood current. When an estuary or bay is connected to the ocean the tidal currents are very strong so meaning that boats are forced to wait until the current flows in such a way that they can leave or enter the harbour.