Tides are defined as slight oscillations of sea level that occur approximately twice a day and attain exaggerated proportion in marginal seas, straits and estuaries. The major cause of the tides is the gravitational pull of the moon and the sun. Though both the sun and the moon exert gravitational force on the earth to produce tides, the moon, by nature of its closeness to the earth has a greater control over the timing of the tidal rises and falls.
- As the moon travels in its orbit in the same direction as the earth’s rotation, a period of 24 hours, 50 minutes elapses between two successive occasions when the moon is vertically above a point.
- The highest level the water reaches is called a high tide and the lowest level is called a low tide.
- High and Low tides occur twice each during the period of 24 hours, 50 minutes, giving an interval of about 12.5 hours between successive high (or low) tides.
- When the Earth, the moon and the sun are in a straight line, the gravitational force is at its greatest because tide-producing forces of both sun and moon complement each other and they ‘pull’ together. This producing tides of unusually great range, called the spring tides.
- At these times, the high tides are very high and the low tides are very low.
- The gravitational forces of the Moon and the Sun both contribute to the tides.
- This occurs about twice a month: at new moon when the sun and the moon are in conjunction and at full moon when they are in opposition.
- When the earth, the moon and the sun are not in a straight line, but are at right angles to the earth, the gravitational force is less as the sun and the moon are not pulling together.
- This happens during phases of first and third quarter, i.e., at half moon, the sun’s tide-producing force tends to balance the tide-producing force of the moon, resulting in tides of unusually small range known as neap tides.
- Neap tides are especially weak tides. Neaps always occur about 7 days after spring tides.
Perigean and Apogean Tides
- Perigean and Apogean Tides When the moon is nearest to the earth in its orbit (at perigee), its tide-producing power is greater than average, resulting in perigean tides.These are 15-20 per cent greater than average.
- When the moon is farthest from the earth (in apogee), the tides are called apogean tides, which are about 15-20 per cent less than average.
- Coincidence of spring and perigean tides results in an abnormally great tidal range, while when neap and apogean tides coincide the range is abnormally small.
- River tides are experienced in the lower parts of many of the great rivers. These are known as tidal rivers, where either the coastal area has recently subsided or the ocean level has risen causing the lower part of the river to be drowned.
- Such water bodies are, actually, extensions of the sea itself, or estuaries. River tides are distinguishable from ocean tides by one characteristic: the interval between a low tide and the next high tide is shorter than the interval between a high tide and the next low tide.
- When a tidal wave meets a tidal river, or estuary, a tidal bore is formed: where the outgoing river currents are strong and the tidal river rather shallow and funnel-shaped, the rapidly rising high water advances upstream like a high ‘vertical wall, known as tidal bore.
- Bores occur at river mouths that face the direction of tidal surge and where there is a large tidal range. Rivers like the Amazon, Hooghly, Colorado, Tsientang, Elbe, Yangtze are characterised by tidal bores.
- The tidal changes in ocean level result in stream-like movements of water in and out of bays and tidal rivers known as tidal currents. Unusually strong tidal currents result where bays connect with the open ocean by narrow inlets.