Why tides happen: springs and neaps

Tide and tidal stream

Spend a day in almost any harbour around the British Isles and you will see the water climb the harbour wall, pause, and drain away again — twice. That regular vertical rise and fall of the sea is the tide. It matters to every navigator because the depth you actually have, which you met in Lesson 8, is the charted depth plus the height of the tide at that moment.

As the level changes, water also has to flow from place to place. That horizontal movement is the tidal stream. It can carry your boat sideways or slow you down, and Lesson 28 shows you how to find its direction and rate. In everyday talk people say “the tide is running”, but navigators keep the two words apart.

Here is one day of tide at Port Kerran, the standard port for our practice area, drawn from its tide table:

A handful of words describe everything on that picture:

• High water (HW) — the highest level reached in one rise; low water (LW) — the lowest.
• Range — the difference in height between a high water and the low water next to it. Here, 5.2 − 0.4 = 4.8 m.
• Flood and ebb — a rising tide is often called the flood and a falling tide the ebb. The same words are used for the streams that go with them, but don’t assume the stream turns exactly at high or low water: in many places it doesn’t.
• Stand — around high and low water the level changes very slowly for a while. This quiet spell is sometimes called the stand of the tide.

What makes the tide

The tide is caused mainly by the moon, with help from the sun. Gravity weakens with distance, so the moon pulls the side of the Earth nearest to it a little more strongly than it pulls the Earth’s centre, and the far side a little less strongly. It is this difference in pull across the Earth — the tide-raising force — that moves the oceans:

• On the side facing the moon, the water is pulled towards the moon more than the Earth is, so it heaps up into a bulge.
• On the far side, the water is pulled less than the Earth is, so it is left behind as a second bulge.

The sun does the same, but although it is enormous it is so far away that the difference in its pull across the Earth is smaller: its tide-raising effect is a little under half the moon’s.

Two bulges, one facing the moon and one facing away. As the Earth turns, each port passes through both.

Two tides a day, about 50 minutes later each day

The Earth turns once a day beneath the two bulges, so most ports around the UK get two high waters and two low waters a day. Tides like this are called semi-diurnal (twice-daily). Some other parts of the world get only one tide a day, but you will rarely meet that in UK waters.

The moon doesn’t stand still, though. It travels round its orbit in the same direction as the Earth turns, so after one full turn the Earth needs roughly another 50 minutes to bring a port back under the moon. The tidal day is therefore about 24 hours 50 minutes, high waters come roughly 12 hours 25 minutes apart, and each day’s high water is on average about 50 minutes later than the day before. You can see it in the Port Kerran tide table:

The times in the Port Kerran table are in UT (Universal Time). In June, clocks in the UK are on British Summer Time, one hour ahead; Lesson 24 shows how to convert. For working out how much later high water is each day, it makes no difference.

Springs and neaps

The sun’s two bulges and the moon’s two bulges add together, and how well they line up changes through the month:

• At new moon and full moon, the sun, Earth and moon are in line. The bulges add up, giving higher high waters and lower low waters — a big range. These are spring tides (springs). The name has nothing to do with the season.
• At first quarter and last quarter, the sun and moon pull at right angles. The sun’s bulges partly fill in the moon’s low waters, so high water is lower and low water higher — a small range. These are neap tides (neaps).

New moon to full moon is about half a lunar month, so springs come round about every 14¾ days — roughly once a fortnight — with neaps about a week after each springs. The oceans take time to respond, so in general the biggest tides arrive about one to two days after new or full moon, and the smallest a day or two after the quarters.

Not all springs are equal. The largest springs of the year usually come near the equinoxes, in March and September, when the sun is over the equator. Tides are also a little bigger when the moon is at its closest to the Earth in its orbit.

The Port Kerran fortnight

The tide table for Port Kerran covers 1–14 June 2027. Work out one range for each day and you can see the whole spring–neap cycle in it: big ranges at the start, shrinking to about 2.0 m around days 7 and 8, and growing back to springs by day 14.

Those mean ranges come from the average levels printed for each standard port. At Port Kerran, mean high water springs (MHWS) is 5.2 m and mean low water springs (MLWS) 0.4 m, giving 4.8 m; at neaps (MHWN 4.0 m, MLWN 1.6 m) it is 2.4 m. Lesson 24 explains these levels and chart datum properly.

Local effects and weather

The sun and moon set the rhythm, but they don’t decide what happens at a particular port. The real tide behaves as huge waves moving around each ocean basin, and the shape of the coastline and seabed changes it a great deal. Two places a few miles apart can have high water at quite different times and very different ranges, and a few places have unusual tides — parts of the Solent, for example, have a long stand or a double high water. That is why you always work from tide tables for the port, never from the moon’s phase.

The weather can also push the level away from the prediction. These are rough guides only:

• Pressure. Sea level changes by roughly 1 cm for every hectopascal (hPa) the pressure is away from average (about 1013 hPa). High pressure lowers the level; low pressure raises it. A deep low at 983 hPa could add something like 30 cm.
• Wind. Strong winds blowing onshore can pile water up and raise the level; strong offshore winds can lower it.
• Surges. A deep depression with strong winds can cause a storm surge, raising levels well above the prediction. A negative surge can lower them, which is more dangerous for a boat relying on depth.