• Tides are very long-period waves that move through the oceans in response to the gravitational forces exerted by the moon and sun.
  • Tides originate in the oceans and progress toward the coastlines where they appear as the regular rise and fall of the sea surface.
  • When the highest part, or crest, of the wave reaches a particular location, high tide occurs; low tide corresponds to the lowest part of the wave, or its trough.
  • The difference in height between the high tide and the low tide is called the tidal range.
  • The time between the high tide and low tide, when the water level is fallin
    , is called the ebb.
  • The time between the low tide and high tide, when the tide is rising, is called the flow or flood


Formation of Tides:

  • The moon’s gravitational pullon the Earth and the Earth’s rotational force are the two main factors that cause high and low tides.
  • The side of the Earth closest to the Moon experiences the Moon’s pull the strongest, and this causes the seas to rise, creating high tides.
  • On the side facing away from the Moon, the rotational force of the Earth is stronger than the Moon’s gravitational pull.
  • The rotational force causes water to pile up as the water tries to resist that force, so high tides form on this side, too.
  • Elsewhere on the Earth, the ocean recedes, producing low tides.
  • The gravitational attraction of the Sun also plays a small role in the formation of tides.
  • Tides move around the Earth as bulges in the ocean.
  • The tidal bulges on wide continental shelves have greater height.
  • In the open ocean tidal currents are relatively weak.
  • When tidal bulges hit the mid-oceanic islands they become low.
  • The shape of bays and estuaries along a coastline can also magnify the intensity of tides.
  • Funnel-shaped baysgreatly change tidal magnitudes. Example: Bay of Fundy –– Highest tidal range.
  • The large continents on the planet, however, block the westward passage of the tidal bulges as the Earth rotates.
  • Tidal patterns differ greatly from ocean to ocean and from location to location.

Bore Tide:

  • A tidal bore (or simply bore in context, or also aegir, eagre, or eygre) is a tidal phenomenon in which the leading edge of the incoming tide forms a wave (or waves) of water that travels up a river or narrow bay against the direction of the river or bay’s current.

Types of Tides

Neap Tide:

  • When the Sun and Moon form a right angle, as when a half moon can be seen, their gravitational pulls fight each other and one can notice a smaller difference between high and low tides. These are called neap tides.

Spring Tide:

  • When the Moon, Earth, and Sun fall in a straight line, which is called as syzygy (siz-eh-gee), the greatest difference between high and low tide water levels can be observed.
  • These spring tides occur twice each month, during the full and new Moon.
  • If the Moon is at perigee, the closest it approaches Earth in its orbit, the tides are especially high and low.

Types of Tides

Rip Tide:

  • A rip current, commonly referred to simply as a rip, or by the misnomer rip tide, is a strong channel of water flowing seaward from near the shore, typically through the surf line.
  • Typical flow is at 0.5 meter-per-second (1–2 feet-per-second), and can be as fast as 2.5 meters-per-second (8 feet-per-second), which is faster than any human swimmer.
  • They can occur at any beach with breaking waves, including oceans, seas and even large lakes.

Types of Tides

Brown Tide:

  • Brown Tide is a bloom (excessive growth) of small marine algae (Aureococcus anophagefferens).
  • Although algae of many types are found in all natural freshwater and marine ecosystems, blooms of the Brown Tide organism literally turn the water deep brown, making it unappealing to swimmers and fishermen alike.
  • While not harmful to humans, the presence of the Brown Tide is a problem for bay scallops and eelgrass, and to a lesser degree other finfish and shellfish.
  • Brown Tide is unlike most other algal blooms because of its unusually high concentrations, the extent of area it covers and the length of time it persists.

Red Tide:

  • Harmful algal blooms, (HAB) occur when colonies of algae grow out of control while producing toxic or harmful effects on people, fish, shellfish, marine mammals and birds.
  • The human illnesses caused by HABs, though rare, can be debilitating or even fatal.
  • Many people call HABs ‘red tides,’ scientists prefer the term harmful algal bloom.
  • One of the best known HABs in the nation occurs nearly every summer along Florida’s Gulf Coast.

Semidiurnal Tide:

  • These are tides occurring twice a day.
  • This means a body of water with semi-diurnal tides, like the Atlantic Ocean, will have two high tides and two low tides in one day, much like the eastern seaboard of North America.

Diurnal Tide:

  • These tides occur once a day.
  • A body of water with diurnal tides, like the Gulf of Mexico, has only one high tide and one low tide in a 25-hour period.

Types of Tides

Mixed Tide:

  • Some bodies of water, including most of North America that’s in contact with the Pacific Basin, have mixed tides, where a single low tide follows two high tides.

Marine ecosystem

  • Tides affect marine ecosystems by influencing the kinds of plants and animals that thrive in what is known as the intertidal zone—the area between high and low tide.
  • Because the area is alternately covered and uncovered by the ocean throughout the day, plants and animals must be able to survive both underwater and out in the air and sunlight. They must also be able to withstand crashing waves.
  • For example, plants and animals that can anchor themselves to the rocks along a shoreline can survive the lashing from waves and the less violent movement of the changing tides.
  • Sand crabs not only burrow to survive, they actually follow the tides to maintain just the right depth in the wet sand.
  • Along many shorelines, tides form tide pools. These small pools of water are often left behind among the rocks at low tide. They can include a diverse population of tiny plants and animals that may serve as food for larger species.


  • Tidal heights are very important, especially harbours near rivers and within estuaries having shallow ‘bars’ [Marine Landforms]at the entrance, which prevent ships and boats from entering into the harbour.
  • High tides help in navigation. They raise the water level close to the shores. This helps the ships to arrive at the harbour more easily.
  • Tides generally help in making some of the rivers navigable for ocean-going vessels. London and Calcutta[Tidal Ports] have become important ports owing to the tidal nature of the mouths of the Thames and Hooghly respectively.


  • The high tides also help in fishing. Many more fish come closer to the shore during the high tide. This enables fishermen to get a plentiful catch.

Pollution mitigation

  • Tides are also helpful in desilting the sediments and in removing polluted water from river estuaries.

Power generation

  • Tides are used to generate electrical power (in Canada, France, Russia, and China).