From surfers to fishermen to lifeboat coxswains, people who rely on the sea need to be expert at understanding what it’s doing. But how well do you know your tides, waves and currents?

The sea is changing all the time. Whether it’s a storm whipping it into a frenzy or tides sweeping in and out, the sea is never the same from one day to the next.

Tides

Tides are caused by gravity. Gravity is a force of attraction between any two objects. It’s stronger the closer the objects are and the bigger the objects. So the enormous mass of the Earth and the fact that we stand directly on it literally keeps us grounded. However, gravity exists between all objects in the universe, creating a ‘tug-of-war’. The Moon and the Sun also have a gravitational pull on the Earth. We don’t notice their effect on us, but they do affect the sea.

As the Moon travels around the Earth, its gravity pulls the sea towards it. The Sun has a similar but smaller effect as, although it is far larger than the Moon, its much greater distance from the Earth weakens its influence overall. Their combined influence is what gives us our tides – normally two highs and two lows each day. However, the Moon orbits the Earth once in slightly more than 24 hours so the tide times slip later each day. Complicating the matter further is local geography, such as the shape of the coastline or the sea’s depth, which can create complex tidal systems, such as round the Isle of Wight.

As the tide comes up it is known as a flood tide and as it goes down again as an ebb. Slack water is the period of time at either high or low tide when the water is changing from flood to ebb and is briefly still. When the gravitational pulls of the Sun and Moon are aligned they combine to create tides with higher highs and lower lows, known as spring tides, or springs. (This is around new and full Moon.) And when they are at right angles the tidal range is smaller, creating neap tides or neaps. (Around quarter Moons.) The highest high tides of the year occur a day or two after the full or new Moon nearest to the Spring and Autumn equinoxes.

Tide heights also vary from year to year, with 1997 being the most recent year of exceptional high tides. And to experience Reading the sea From surfers to fishermen to lifeboat coxswains, people who rely on the sea need to be expert at understanding what it’s doing. But how well do you know your tides, waves and currents? COASTAL LIFE COASTAL LIFE some of the highest tides in the world, you need only look as far as the Bristol Channel, with a tidal range of up to 15m.

Although gravity is the dominant force on tides, it doesn’t act alone. The weather also has an impact, especially air pressure. Low pressure means that there is less resistance to the gravitational pull and gives us even higher high tides. In contrast, high pressure reduces the difference between high and low tides.

Despite their complexity, tide times are predictable. Checking a tide table will help us make informed decisions, whether it’s when to bring a craft into harbour or have a picnic on the beach. Tide tables can be found on public display or for sale at coastal locations and online. Some tables present the whole year’s data in GMT only, so it is important to check for this and adjust accordingly.

It is sometimes difficult to appreciate that millions of tonnes of water are on the move as the tide changes. The sheer weight of water can create strong tidal currents, moving at up to 7 or 8 knots. This is as fast as many boats can travel, leaving them entirely at the mercy of the tide. Away from the shore, it is easy to be swept up in a tidal stream, where few swimmers would have the strength to beat the incredible force of the sea. Areas with dangerous tidal currents close to shore are sometimes signposted with warnings and you should always check with any lifeguards.

Waves

Waves are formed by the wind blowing over the surface of the sea, just like the ripples formed when you blow on a cup of tea. As the winds increase and travel unrestricted across miles of open water, the waves grow. Far from shore, with no land to break them up, they travel hundreds or even thousands of miles and can build to enormous heights. Such formations of large, long, stable waves are known as swell. Wind over tide, when the tide is moving one way and the wind the other, can add to the effect as the conflicting forces push waves higher and higher.

The experience of being out in a large swell is one not quickly forgotten. The top may be well above your head allowing you just brief glimpses of the surrounding area as the boat climbs before sinking into the next trough. Smooth-topped swells are often referred to as rolling seas.

In high winds or closer to the shore, however, waves may break, creating a foamy top. This can reduce visibility drastically, making it much harder for lifeboat crews to spot a casualty. The RNLI’s all-weather lifeboats are designed to ride large swells at speed without damaging craft, crew or casualties. (See page 13 for news of the latest experimental work.) The latest models even have unique shock-absorbing seats and they are all inherently self righting in case of capsize.

As swell approaches the shore, the sea becomes shallower than the waves themselves, forcing them to rise up and collapse dramatically. There is a real beauty and majesty to waves crashing onto the shore. But this splendour masks quite incredible power. It is very easy to be knocked off your feet and then be pulled into the sea by the retreating water. This can be especially dangerous on a pebble or shingle beach, which will be scoured out from under your feet.

Rip currents

The constant movement of the sea throws up many strange and surprising phenomena, some of which can prove very costly if stumbled upon unknowingly. And for a beach user, none poses more risk to the unwary than the rip current. So knowing how to spot and avoid one can be a lifesaver.

When a wave crashes onto the shore it dumps large amounts of water high on the beach that will then flow back to the sea. But if the waves being pushed up behind it come too strongly and quickly, or if there are physical barriers such as sandbars or groynes, the excess water cannot escape. Instead it will flow parallel to the beach until it finds a faster route back out to sea, creating a rip current.

A powerful rip current will be far too strong to swim against, even for experienced swimmers. But, fortunately, they are quite easy to avoid or escape with a little knowledge. You can sometimes see a rip current from the shore, especially if you can get up high. Look along the beach. If there is a patch of clear water with breaking waves on either side, then this may be a rip. You may also see a muddy patch, where the current has stirred up the seabed. It may look calm and inviting but you should avoid it.

If you do find yourself in a rip, being taken away from the shore, follow a few simple instructions. Firstly, don’t panic. Although you are being pulled out you won’t get pulled under the water. Don’t try to swim straight back to shore as you will quickly get exhausted and then be at risk. Instead, swim across the rip, parallel to the shore, until you stop being pulled out to sea. Once you are out of the rip you can start to swim back to shore to safety.

Lifeguards will be looking out for rips and shepherding swimmers away from such areas – always take their advice.

(If you actively use the sea, see page 16 for news of the RNLI's membership for people like you. Read more about lifeguarding skills on pages 6 and 26.)