tides

tides are shallow-water waves generated by the gravitational forces of the moon and sun acting on the oceans. The earth's rotation results in high tides occurring both on the side nearest to the moon and on the opposite side. When the moon and sun are most closely aligned, at full and new moon, their gravitational pulls work in concert to produce spring tides, the highest tides of the cycle. Neap tides, the lowest-ranging tides, occur at the first and third quarters of the moon's cycle when the sun and moon are unaligned. The earth rotates every 24 hours 50 minutes relative to the moon, so the two daily tides are often of different heights. The earth spins too quickly for the tide's wave to keep up with the maximum gravitational pull of the moon, so it lags behind. This lag acts as a form of friction, which is slowing the earth's rotation.

Tidal ranges are also affected by the shapes of ocean basins. The tidal wave (not to be confused with tsunamis, also colloquially known as tidal waves) circulates within a basin around an amphidromic point, which is a point at which the tidal range is zero. Islands also affect tides. In the Solent in southern Britain the tidal wave, which comes from the west, arrives earlier along the shorter route round the north of the Isle of Wight than along the longer route around the south and east of the island. The two peaks of high tide arrive separately generating high- and low-water stands; periods when the sea neither rises nor falls.

The tidal heights are also affected by weather conditions. When atmospheric pressure is low the water can rise higher than expected, and if in addition there are stronger winds blowing from the sea even more water can pile inshore. Such events are known as storm surges. During tropical storms in coastal regions, more people are drowned by the storm surge than by the direct effects of the wind's violence. In 1953 a 3-metre (10-ft) storm surge in the southern North Sea killed 300 people in Britain and 1,800 in Holland, and resulted in the building of the Thames Barrier and the Delta project across the Eastern Scheldt. See also tidal power.

M. V. Angel