Why the sea breathes

Twice a day around the UK the sea rises and falls, and the difference matters more to a small boat than almost anything else on the chart. Get the tide right and a drying harbour is a car park you can drive into; get it wrong and your keel meets it at the wrong time.

The pull comes from the moon and, to a lesser degree, the sun. When the two line up — at new moon and full moon — their pulls stack and the tide swings hard: the high water is HIGHER and the low water is LOWER than usual. These are springs. When the moon is at quarter phase it pulls at right angles to the sun, the effects partly cancel, and the swing is gentle: at neapsneapsThe fortnightly small tides after the half moons — the gentle half of the cycle.full glossary → the high water is LOWER and the low water is HIGHER than at springs. The cycle from springs to neaps and back takes about a fortnight.

Chart datum — the chart’s idea of empty

Every depth printed on a chart is measured from chart datum (CD) — a level chosen so low that the real tide almost never falls below it. So a charted depth of 2 m means: even on the worst low water you can reasonably expect, there is about 2 m there. Whatever the tide is doing sits on top of that.

Heights of tide from the tide tables are also measured above chart datum. That is the whole trick of tidal calculations: depth available = charted depth + height of tide. Two numbers, one sum.

Some features dry. A green-tinted bank with an underlined figure — say 2̲·1̲ — stands 2.1 m above chart datum and is exposed whenever the tide falls below that. Water over a drying bank = height of tide − drying height.

The levels you will meet

Almanacs quote four mean levels for each port: MHWS and MHWNMHWNMean High Water Neaps — the average height of high water at neap tides (lower than springs).full glossary → (mean high water at springs and neaps) and MLWN and MLWS (the matching low waters). They tell you instantly whether today’s tide is spring-ish or neap-ish: compare today’s range — HW height minus LW height — with the mean spring and neap ranges.

One more level matters: HAT, the highest astronomical tide. Overhead clearances — bridges, power cables — are charted from HAT, the most cautious case (the highest the tide can ever reach). If the chart says 16 m under a cable, you have at least 16 m even at the biggest tide of the century, and more whenever the tide is below HAT.

One caution: every level above is the ASTRONOMICAL tide — sun and moon only, in still air. The weather writes a correction on top. A deep low can lift the actual sea level above prediction, a strong high can hold it down — by up to half a metre or so — and a strong onshore wind piles water against the coast. So the almanacalmanacThe annual reference book of tides, ports, lights and radio details.full glossary → gives you the prediction; in a big blow, treat the figures as a guide and keep a margin.