Tides are a phenomenon that can be seen clearly from the shores of Earth’s many coasts. The tidal movement is the rise and fall of sea levels caused by the constant rotation of our planet.
Tides have been used by mankind for thousands of years, from building early coastal settlements to the creation of some of the world’s most important ports.
The periodical rise and fall of the sea level, once or twice a day, mainly due to the attraction of the sun and the moon, is called a tide.
Movement of water caused by meteorological effects (winds and atmospheric pressure changes) are called surges. Surges are not regular like tides.
The moon’s gravitational pull to a great extent and to a lesser extent the sun’s gravitational pull, are the major causes for the occurrence of tides.
Another factor is centrifugal force, which is the force that acts to counter balance the gravity.
Together, the gravitational pull and the centrifugal force are responsible for creating the two major tidal bulges on the earth.
When the tide is channelled between islands or into bays and estuaries they are called tidal currents.
Types of Tides:
There are many types of tides based on the period of time between high and low water. Once you understand the type of tide, you’re able to quickly determine what to expect, when to expect it, and where on the coast it is likely to occur. Tides based on Frequency
Semi-diurnal tide: The most common tidal pattern, featuring two high tides and two low tides each day. The successive high or low tides are approximately of the same height.
Diurnal tide: There is only one high tide and one low tide during each day. The successive high and low tides are approximately of the same height.
Mixed tide: Tides having variations in height are known as mixed tides. These tides generally occur along the west coast of North America and on many islands of the Pacific Ocean.
Tides based on the Sun, Moon and the Earth Positions
Tides are the rise and fall in the levels of the sea or lake on either side of the mean or average level of sea or lake, caused by the pull of sun on the oceans, on one side and on the other side by the pull of moon. Tides are due to the gravitational pulls of moon and sun.
Spring tides: The position of both the sun and the moon in relation to the earth has direct bearing on tide height. When the sun, the moon and the earth are in a straight line, the height of the tide will be higher. These are called spring tides and they occur twice a month, one on full moon period and another during new moon period.
Neap tides: Normally, there is a seven-day interval between the spring tides and neap tides. At this time the sun and moon are at right angles to each other and the forces of the sun and moon tend to counteract one another. The Moon’s attraction, though more than twice as strong as the sun’s, is diminished by the counteracting force of the sun’s gravitational pull.
Importance of Tides
The tides in the world are caused by the combined effects of gravitational forces exerted by the moon and the sun. The Earth, Moon and Sun are approximately in a straight line in space.
Between them, they create two tidal bulges on the Earth’s surface which causes high tide when one bulge coincides with the high tide time and low tide at the opposite side of the Earth when the other bulge coincides with that time.
Tidal heights are very important, especially harbours near rivers and within estuaries having shallow ‘bars’ at the entrance, which prevent ships and boats from entering into the harbour.
