Tides

Visit the Bay of Fundy in eastern Canada at low tide and you can walk on the sea floor among boats resting on mud; six hours later the same place may be under more than ten metres of water (check). Visit the Mediterranean and you may not notice the tide at all. In 1687 Isaac Newton explained the basic cause in his Principia: the pull of the Moon (and, to a lesser extent, the Sun). But it took another two centuries to understand why the tides differ so much from place to place, and the full story still depends on computer models. This chapter explains the cause, the rhythm and the local variation, with plenty of examples from the British coast.

The Moon and the Bulges

Gravity weakens with distance. The Moon pulls most strongly on the side of Earth nearest to it, less on the centre, and least on the far side. The difference in pull across Earth is called the tide-raising force. It stretches the ocean into two bulges: one facing the Moon, and one on the opposite side (check the usual explanation below).

Why 12 Hours 25 Minutes?

While Earth turns once in 24 hours, the Moon moves on in its orbit, so a place has to turn about 50 minutes further each day to face the Moon again. The lunar day is therefore about 24 hours 50 minutes, and the gap between successive high tides is half of that: about 12 hours 25 minutes. So tide times slide later by about 50 minutes each day, and a high tide at breakfast today becomes a high tide at about ten to seven the next morning, and so on.

Spring and Neap Tides

The Sun also raises tides, though its pull is about 46 per cent of the Moon's, since it is so much farther away (check). The two combine.

Springs have nothing to do with the season: the name comes from the idea of the water "springing up". The cycle repeats about every 14.8 days. Tides are larger still when a new or full Moon coincides with the Moon's closest approach to Earth, sometimes called a perigean spring tide, and combined with a storm surge these cause most coastal flooding (check).

Why the Range Varies So Much

Newton's picture predicts a rise of well under a metre in the open ocean. Real tides in some places are many times higher, because the ocean basins are not smooth: they have shapes of their own.

Types of Tide

• Semi-diurnal. Two highs and two lows of similar size each day. This is the usual pattern around the British Isles and the North Atlantic.
• Diurnal. One high and one low a day, as in parts of the Gulf of Mexico (check).
• Mixed. Two highs and lows of clearly different heights, as on much of the US Pacific coast.

The pattern comes from the shape of the basin and the Moon's changing position north or south of the equator.

Tidal Currents and Uses

As the water rises and falls, it also flows. These tidal currents can be fast in narrow channels: the Corryvreckan whirlpool, between two Scottish islands, is one of the best known (check). Tidal streams are predictable, which is attractive for energy, and the Pentland Firth and the Orkney waters host turbines and test sites. A tidal estuary can also produce a bore, a wave that runs up the river against the flow, such as on the Severn (check).

Exercises