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. Gravitational pull of sun and moon along with centrifugal force of earth (a force that acts to counterbalance the gravity) is responsible for the creation of tides. The gravitational pull and centrifugal force are responsible for creating the tidal bulges on the earth.
TYPES OF TIDES
Tides vary in their frequency, direction, and movement from place to place and also from time to time.
TIDES BASED ON FREQUENCY
- Semi-diurnal tide: The most common tidal pattern, featuring two high tides and two low tides each day.
- 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 POSITION
- Spring tides: These are the tides of largest range which occur when the Moon, the Sun, and the earth are in straight line. These tides occur twice in a month, one on full moon and another on new moon.
- Neap tides: When the sun and the moon are at right angles to each other the forces of sun and moon tend to counteract one another. At this time, the tidal phase is called as neap tide or fortnightly tides (lowest low tide and lowest high tide).
- When the moon’s orbit is closer to the earth (Perigee), the tidal range is greater than normal during this time. After two weeks, when the moon is farthest from earth (Apogee), the tidal range is less than the average.
- Similarly, when earth is closed to the sun (Perihelion), tidal range is much greater than the average and when earth is farthest from Sun (Aphelion), tidal range is less than the average.
IMPORTANCE OF TIDES
- Fishing: During ebb tide or high tide, fish come closer to the shore and help fishermen to have plentiful catch.
- Tidal energy: Tides are used in generation of electricity. The estimated power potentials for Tidal energy is 12,455 MW as per Ministry of New and Renewable Energy. A 50 MW tidal power plant was constructed at Gujarat.
- Navigation: Tides are used in navigation because the formation of tides depends on the position of earth, moon, and sun. They can be predicted in advance and help navigators to plan their journeys.
- Desilting: Tides help in desilting of rivers by removing the silt from water bodies. They also remove the polluted water from river estuaries.
