What is Ocean Tides?
Tides are the rise and fall of sea level. They are caused by the gravity of the Sun and Moon as well as the rotation of the Earth.
Cycles of a Tide
The tides change as the Moon orbits the Earth and the position of the Sun changes. Throughout the day, the sea level is constantly rising or falling.
- The sea level is rising
- High tide is reached
- Sea level drops
- Low tide is reached
- Back to number 1
This cycle can happen once or twice a day depending on the position of the Moon region. Tides that occur once a day are called diurnal tides. Tides that occur twice a day are called semi-diurnal tides. Since the Earth rotates in the same direction as the Moon, the actual cycle is slightly longer than a day at 24 hours and 50 minutes.
Tides and the Moon
While the Sun and the Earth’s rotation both have some effect on the tides, the position of the Moon has the biggest influence on the tides. The Moon’s gravity causes high tides both on the side of the Earth just below the Moon (sublunar tides) and on the opposite side of the Earth (opposite). Low tides are found on the sides of the Earth 90 degrees relative to the Moon.
Tidal Currents
As sea levels rise or fall, water flows into or out of the ocean. This current causes currents called tidal currents.
Flood current – Flood currents occur when the sea level rises during high tide. Water flows towards the shore and out of the ocean.
Ebb current – An ebb occurs when the sea level drops during low tide. Water flows from the shore to the sea.
Slack water – At the exact time of high tide or low tide, there is no flow. This time is called slack.
Tidal Range
The tidal range is the sea level difference between low tide and high tide. The tidal range will vary at different locations depending on the position of the Sun and Moon as well as the topography of the coastline.
On the high seas, the tidal range is usually about 2 feet. However, the tidal range can be much higher near shore. The largest tidal beach is on the coast of the Bay of Fundy in Canada, where the tides can vary up to 40 feet from high tide to low tide.
Types of Tides
High – High tide is the point in the tidal cycle at which sea level rises the highest.
Low – Low tide is the point in the tidal cycle where the sea level is at its lowest.
Spring – High tide occurs when the Sun and Moon align to combine with the maximum tidal amplitude of the highest tide and the lowest high tide.
Neap – A neap tide is when the tidal range is smallest. This happens during the first and third quarters of the Moon.
Semidiurnal – A semidiurnal tidal cycle is a cycle in which there are two high tides and two low tides each day.
Diurnal – The diurnal cycle is a cycle in which there are only high tides and low tides in a day.
Interesting Facts about Tides
The same tidal forces that cause tides in the oceans affect the solid Earth, causing it to change shape inch by inch.
There are usually two high tides and two high tides each month.
During a semi-diurnal cycle, the high and low tides are about 6 hours and 12.5 minutes apart.
Local factors such as weather can also affect tides.
The energy of tidal forces can be harnessed to generate electricity using tidal turbines, fences or dams.