What Are High Tide and Low Tide? How Tides Work
In short: tides are the regular rise and fall of sea level, caused mainly by the gravitational pull of the Moon and, to a lesser extent, the Sun, combined with the Earth's rotation. On most coasts there are two high tides and two low tides a day, and each one arrives about 50 minutes later than the day before.
The essentials
| Question | In short |
|---|---|
| What causes tides? | Mainly the Moon's gravitational pull on the oceans |
| What about the Sun? | It pulls too, but with a little under half the Moon's effect |
| When is the tide high? | Where a bulge of water forms: facing the Moon, and on the opposite side of the Earth |
| How often? | Two high tides and two low tides a day, about 12 h 25 min apart (semi-diurnal) |
| When are tides biggest? | Spring tides at new and full moon; weakest neap tides at the first and last quarter |
Ever been caught out by the sea creeping up the beach just as you settled down with your towel? Tides are easy to underestimate, but once you know how they work, you can plan your walks, swims and surf sessions with confidence. Here is a clear, jargon-free explanation of high tide, low tide and what drives them.
What are high tide and low tide?
A tide is the regular rise and fall of the sea's surface: high tide is the peak of the water level, and low tide is the point where it has dropped furthest.
The sea never stays still. For about six hours the water rises (the flood tide) until it reaches high tide, then it falls for about six hours (the ebb tide) until it reaches low tide. The difference in height between the two is called the tidal range.
On the beach, you can see the difference at a glance. At high tide the water covers part of the sand. At low tide the sea retreats and reveals a wider stretch of sand or rocks. On Atlantic coasts the water can move by several metres, which is why a beach that looked huge in the morning can be much smaller by the afternoon.
Why do tides rise and fall?
Tides come from the gravitational pull of the Moon and the Sun on the oceans, combined with the rotation of the Earth.
The Moon is the main driver. Because it is so close to us, its gravity pulls on the ocean, drawing the water towards it and creating a bulge of high water on the side of the Earth that faces the Moon.
The Sun pulls on the oceans in the same way, but it is much further away, so its effect is a little under half as strong as the Moon's.
A second bulge forms on the opposite side of the Earth, where inertial effects (often simplified as a centrifugal force) push the water outwards. So the sea is high wherever there is a bulge, and lower in between. The size of the ocean basin, the shape of the coastline and the weather also change how big a tide becomes on a particular beach.
Next time you look up at the Moon, remember that it has a say in your swim or your surf session.
Why are there two high tides a day?
The Earth turns on itself in about 24 hours, so each coast passes through both bulges every day: that gives two high tides and two low tides.
As the Earth rotates, every coastline travels through the two bulges of water: once facing the Moon, once on the opposite side. That is why most coasts have two high tides and two low tides a day, a pattern called semi-diurnal. The two highs are of similar height, and so are the two lows. This is the pattern along the Atlantic coasts of Europe, including France.
The Moon also moves along its orbit while the Earth turns, so a full lunar day lasts about 24 hours and 50 minutes. Two high tides are therefore about 12 hours 25 minutes apart, and each tide arrives roughly 50 minutes later than the day before.
All oceans have tides, but the pattern is not the same everywhere:
- Semi-diurnal: two high tides and two low tides of similar height (the Atlantic Ocean).
- Mixed: two high tides and two low tides, but of very different heights (the west coast of the United States).
- Diurnal: only one high tide and one low tide in 24 hours (rare).
To find out which pattern applies to your coast, check your national hydrographic service: SHOM in France, NOAA in the United States.
Spring tides and neap tides
Tides are strongest at new and full moon (spring tides) and weakest at the first and last quarter (neap tides).
The size of a tide changes through the month. When the Sun, the Earth and the Moon are lined up, at new moon and full moon, their pulls add together. High tides are higher and low tides are lower than average: these are spring tides. The name has nothing to do with the season; it comes from the idea of the tide "springing forth".
About a week later, the Moon and the Sun pull at right angles to each other, at the first and last quarter. Their effects partly cancel out, and the result is moderate tides called neap tides: high tides are a little lower and low tides a little higher than average.
In France, tide tables also give a tidal coefficient. A coefficient above 70 means a big spring tide, which is when the sea moves furthest and fastest, so check the times before you head out on the foreshore.
How can you find out the tide times?
A tide table or a tide clock tells you at a glance whether the sea is rising or falling, as long as you choose the right reference port.
Tide times change from one port to the next, so always pick the reference port closest to your beach. You can use the official tide tables published by SHOM or NOAA, or a tide clock. Its dial follows the 12 h 25 min cycle between two high tides and, once set to your port, shows you how long until the next high or low tide.
Our Marine tide clock is handmade in Hossegor, in the Landes, with a beech wood frame, a choice of dial language and an inscription of your choice. It is practical for planning a beach walk, a swim or a surf session, and it looks good on the wall too.
Keep exploring: why equinox tides are stronger, how to read tide times, the best time to go to the beach, low tide or high tide, family activities at low tide, riptides and how to swim safely and the history of the tide clock.
Keep an eye on the tide
Frequently asked questions
What is the difference between high tide and low tide?
At high tide the sea has reached its highest level and covers part of the beach. At low tide the water has retreated as far as it will go, leaving a wider area of sand or rocks uncovered.
Why are there two high tides a day?
Because the Earth turns in about 24 hours, each coast passes twice a day through the bulges of water created by the Moon's pull: once facing the Moon and once on the opposite side.
Does the Moon or the Sun have more influence on the tides?
The Moon, because it is much closer to the Earth. The Sun's effect is real but a little under half as strong.
What are spring tides and neap tides?
Spring tides are the biggest tides, at new and full moon, when the Sun, Earth and Moon are aligned. Neap tides are the smallest, at the first and last quarter, when the Moon and Sun pull at right angles.
How can I find out when high tide is?
Check the tide table for your nearest reference port (SHOM in France, NOAA in the United States), or use a tide clock set to that port, which shows the state of the tide at a glance. Each tide arrives about 50 minutes later than the day before.
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