Reading a tide table before you swim

A tide table is a small grid of times and numbers, and it's one of the most useful things you can check before a sea swim. The RNLI warns that tide times and heights vary through the month and can easily catch you out if you haven't checked them.

What a tide table shows

Each day lists the times of high water and low water for a place, with a height for each. Around Britain there are two high tides and two low tides a day, according to the National Tidal and Sea Level Facility (NTSLF), and high tide to high tide takes roughly 12 hours 25 minutes. The times move from one day to the next. The RNLI puts it simply: a beach that was clear at 5pm yesterday might be completely covered by the sea at the same time today.

Two details catch people out:

• Low water isn't always halfway between the highs. NTSLF notes that at some places the tide stays low for maybe four or five hours, then floods in quickly.
• Check the time zone. ADMIRALTY's EasyTide gives its predictions in GMT and lets you add an hour for British Summer Time, but warns that it won't switch back for you when BST ends. Whatever table you use, check which time it's showing.

The heights are in metres above chart datum. ADMIRALTY describes chart datum as a level so low that the tide won't often fall below it, and in the UK it's normally around the level of the Lowest Astronomical Tide. A bigger number means more water, and a low-water height close to zero is a very low tide.

Springs and neaps

Tides are driven by the pull of the Moon and the Sun, the Met Office explains. When the Sun, Moon and Earth line up, around full and new moon, you get spring tides. When the Moon and Sun pull at right angles, around the half moons, you get neaps. Despite the name, springs have nothing to do with the season: each comes round twice a month.

Springs have the biggest range. The Outdoor Swimming Society (OSS) notes that high springs are higher and low springs lower than neaps, and the RNLI says that on springs the water comes further up the beach at high tide. NTSLF adds that springs arrive roughly 36 to 48 hours after the full or new moon, and that the springs nearest the March and September equinoxes are slightly bigger.

How big the range is depends hugely on where you are. NTSLF gives a spring range of 12.2 metres at Avonmouth, against 1.9 metres at Lowestoft. The RNLI says the UK and Ireland have some of the biggest tidal ranges in the world.

How the tide moves the water you swim in

A tide table gives you heights, but what you feel in the water is the flow. OSS explains that the most water moves during the middle two hours of a rising (flood) or falling (ebb) tide, so that's when currents are strongest, and they're stronger on springs than on neaps. Swim England adds that a current can change direction and speed at different states of the tide.

The slack water around high and low tide, when the flow eases, is often the calmer window. OSS says it lasts roughly an hour either side of high or low water in many places. But the actual slack can differ from the published times because of local geography and weather, and in some places low tide is the better time to swim.

How the tide changes your way in and out

As the tide comes and goes, the beach you're swimming from changes shape. The RNLI says the depth of water can change through the day by as much as 10 metres, and that if you've walked round a cove or an outcrop of rocks at low tide, the water can block your way back when the tide turns. Swim England warns that the tide can significantly change your entry and exit points over the course of a 30-minute swim, and asks whether the incoming tide will cover the beach you walked over to get in.

Low tide has its own hazard. The RNLI says plunging, or dumping, waves break powerfully in shallow water, and most commonly at low tide.

Before you get in, work out what the tide will be doing when you get out, not just when you start.

What a tide table can't tell you

Tide tables are predictions. EasyTide says its figures don't take account of the weather on the day: a low barometer tends to raise sea level and a high one tends to lower it, and storm surges can push the water higher or lower than predicted. NTSLF says strong winds and unusual air pressure are two of the main causes of altered tides, and recommends using local tide tables where you can.

Local really does mean local. NTSLF points out that places fairly close together on the same coast can have high tides hours apart. OSS warns that the nearest tide table might not apply exactly to where you're thinking of swimming, and that the tide doesn't always arrive at the predicted time. Ask experienced local swimmers, sailors, lifeguards or clubs how the water behaves at your spot.

Tides on Wild

Wild shows tide times on beach pages, alongside wave and marine forecasts, today's sea temperature and recent reports from swimmers. Use it to see the shape of the day, then plan your swim around it.

If you're wondering whether you could swim back against the flow, read swimming against the tide. If you swim where a river meets the sea, tidal rivers covers how the tide changes a river. Rips are a different thing from tides, and rip currents explains the difference. For everything else at the coast, start with the safe sea swimming guide.

Sources

1. RNLI, Tides
2. RNLI, Waves
3. National Oceanography Centre (National Tidal and Sea Level Facility), All about tides
4. ADMIRALTY (UK Hydrographic Office), EasyTide FAQs
5. Met Office, Tides
6. Outdoor Swimming Society, Understanding Tides
7. Outdoor Swimming Society, Understanding Tidal Rivers and Estuaries
8. Swim England, How to deal with currents while swimming outdoors

This guide is general information, not a substitute for local knowledge or training. Conditions change: check them on the day, and if in doubt, don't go in. Images on the blog are illustrations, not pictures of specific spots.