TIDES · Lesson 1 of 6
How Tides Work
25 min · Beginner friendly
You’ll learn toExplain the lunar and solar causes of the tides · Distinguish spring tides from neap tides and their cycle · Define range, high water, low water and mean levels
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Twice a day around the British Isles the sea rises and falls by metres: a harbour that floats a fleet at lunchtime can be a plain of mud by teatime. The tide decides when you can leave, what you can cross and how much chain to lay — and it all starts with the Sun, the Moon and a spinning Earth.
Take a guess
Take a guess before you read: it is full Moon tonight. What should you expect of the tides this week?
Commit to an answer before reading on — a wrong guess here helps the right idea stick.
The Moon Does the Heavy Lifting
The main engine of the tide is the gravitational pull of the Moon. Water is free to move, so the ocean facing the Moon is drawn towards it — a bulge of slightly deeper water. There is a second bulge on the opposite side: the Moon pulls the solid Earth more strongly than the far-side water, which is, in effect, left behind.
Now the Earth spinning. As the planet rotates once a day, your bit of coastline sweeps through both bulges and both troughs between them. Result: roughly two high waters and two low waters every day. This twice-daily pattern is called a semi-diurnal tide, and it is what you get around almost all of the British Isles.
Knowledge check
High water was at 0900 this morning. Without a tide table, when should you expect the corresponding high water tomorrow?
The reason: while the Earth spins, the Moon moves on along its orbit. After one full rotation the Earth needs about another 50 minutes of spin to catch up, so the tidal day is about 24 hours 50 minutes — and every arrives roughly 50 minutes later than yesterday's.
The Sun Joins In: Springs and Neaps
Full Moon — Sun and Moon in line: spring tides, the biggest range of the fortnight.
The Sun's tide-raising pull is a little under half the Moon's, and what matters is how the two combine. In practice springs arrive a day or two after new or full Moon, because the oceans take time to respond. The line-up repeats twice per lunar orbit, giving a spring–neap cycle of about 14.8 days: springs and neaps are each about a week apart, so any week is either building towards springs or easing off towards neaps.
| Spring tides | Neap tides | |
|---|---|---|
| Moon phase | Just after new or full Moon | Just after first or last quarter |
| Range | Largest of the fortnight | Smallest of the fortnight |
| High water | Higher than average | Lower than average |
| Lower than average | Higher than average | |
| Tidal streams | Fastest | Slowest |
| How often | Roughly every 14–15 days | Roughly every 14–15 days |
Knowledge check
The Moon reaches first quarter today. A couple of days from now, what are the tides doing?
Work it out
Spring tides peak at Westhaven on Tuesday the 3rd. Roughly when should you expect the next neaps, and the next springs after that? Give each as a day of the month.
Just the number — 10 for the 10th.
Within 1 is fine.
The Language of Tides
- High water (HW)
- The highest level the tide reaches in one cycle, and the time at which it happens.
- Low water (LW)
- The lowest level the tide reaches in one cycle, and the time at which it happens.
- Range
- The between a high water and the following (or preceding) low water. Range = HW height minus LW height.
- Spring tides
- The tides with the largest range, occurring shortly after new and full Moon, roughly every two weeks.
- Neap tides
- The tides with the smallest range, occurring shortly after the Moon's quarters, in between springs.
- Semi-diurnal tide
- A tidal pattern with two high waters and two low waters per day — the normal pattern around the British Isles.
- Diurnal tide
- A pattern with only one high and one low water per day, found in some other parts of the world.
- Mean level
- A long-term average sea level. Averages of HW and LW at springs and neaps give the standard levels MHWS, MHWN, MLWN and MLWS, which you will meet in the next lesson.
Knowledge check
This morning's tide table for Westhaven shows HW 5.6 m and the following LW 0.8 m. What is the range of that tide?
Key points
- The Moon's gravity raises two bulges of water; the spinning Earth carries you through both, giving two HWs and two LWs a day (semi-diurnal).
- The tidal day is about 24 h 50 min, so tides run roughly 50 minutes later each day.
- Springs (large range) occur just after new and full Moon; neaps (small range) just after the quarters.
- Springs to neaps is about a week; the full spring–neap cycle is about 14.8 days.
- Range = HW height − LW height, and it tells you at a glance whether you are near springs or neaps.
- Tidal streams are fastest at springs and slowest at neaps.
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