What a tide is
A tide is the regular rise and fall of the sea. It is caused mainly by the gravitational pull of the moon, with a smaller contribution from the sun. These pulls slightly stretch the oceans, creating bulges of water that travel around the planet as the Earth rotates. Where those bulges reach a coast, the sea level rises; where they pass, it falls.
In Cornwall, as along most of the Atlantic coast of Europe, there are typically two high tides and two low tides a day. The cycle runs a little longer than 24 hours because the moon moves in its orbit, so high water arrives roughly an hour later each day than the day before. That drift explains why a beach that was perfect for a morning walk one week may be under water at the same hour a week later.
Tidal range
The difference in height between high water and low water is called the tidal range. It is not the same everywhere. As the tidal wave moves into shallow seas and narrow channels, the shape of the coast and seabed can magnify or reduce it.
The southwest of Britain generally has a large tidal range compared with many other coastlines, and the range in the Bristol Channel is among the largest in the world. Parts of the Cornish north coast therefore see substantial movements of water, while conditions can vary from one area to another. This guide does not give exact figures, since they differ by location and day. Published tide tables for a particular place are the reliable source.
A large range has practical effects:
- Wide beaches can appear at low water, and then be covered rapidly as the tide returns.
- Harbors may dry out, leaving boats resting on the mud or sand until the water comes back.
- Narrow coves can be completely submerged at high water.
- Estuaries and creeks fill and empty with strong currents.
Springs and neaps
The tidal range changes over about a two-week cycle, driven by the alignment of the sun, Earth and moon.
Spring tides occur around the time of the new moon and full moon, when the sun and moon pull in roughly the same line. They produce the highest highs and the lowest lows, so the tidal range is at its greatest. The name has nothing to do with the season; it comes from the idea of the water “springing up.”
Neap tides occur around the first and last quarter of the moon, when the sun and moon pull at right angles. The pulls partly cancel out, so high tides are lower, low tides are higher, and the range is smaller.
Because a spring tide stretches farther up and down the shore, it brings more of the beach into view at low water, and also sends the sea farther up at high water. Springs also tend to produce faster tidal currents. Neaps are gentler.
It follows that a cave or rock platform reachable on a neap low tide might be reachable only briefly, or not at all, on a different day. Small arrangements like this are covered in the article on sea caves and stacks.
Why the sea seems to come in so fast
On a gently sloping beach, even a modest rise in water level moves the waterline across a long stretch of sand. On a very flat beach, the tide may advance faster than people can walk. Hollows and channels in the sand can also fill before the surrounding area, trapping anyone who has wandered out to a bar or rock. This is one of the best-known hazards of tidal coastlines, and part of why guidance on rip currents and similar matters often begins with checking the tide.
The state of the tide also affects water movement in other ways. Tidal streams, the horizontal movement of water, are strongest at some points of the tide and around headlands, in narrows and across estuary mouths. Rocks and sandbars that are safe at one stage can be hazardous at another.
Other things that change the water level
The predicted tide is calculated from astronomical patterns, but actual sea level also depends on the weather:
- Low pressure can raise the sea above the prediction, while high pressure can lower it slightly.
- Strong onshore winds can pile water against the coast, increasing the height of the tide and the force of waves.
- Large swells make the waterline surge farther up the beach than the predicted level would suggest.
Storm surges, which combine these effects, can matter most when they coincide with a high spring tide.
Reading a tide table
A tide table for a given location lists the times and predicted heights of high and low water. A few tips apply generally:
- Check whether times are given in local time or in another time standard, and whether they have been adjusted for summer time.
- Look at the height as well as the time. A high tide of lesser height means a smaller range.
- Allow a safety margin: aim to be back well before the tide turns, not at the last moment.
- Remember that it is a prediction.
A fuller walkthrough is given in the guide to reading tide tables before a beach day.
Tides and the working coast
Tides have shaped daily life in Cornwall for centuries. Harbors were designed around them, as described in the article on harbor design, and sailors timed departures with the ebb and flood.
A short checklist
- Look up the tide times before you leave, and note whether the tide is rising or falling when you arrive.
- If in doubt, move to higher ground early.
- In an emergency, call the national emergency number and ask for the coastguard.
Tides are regular and predictable, but the coast they reach is not. Learning the rhythm makes a visit safer and more enjoyable.