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Tide words, explained

Every term we use on the site, in plain English, and what it means for someone standing on West Kirby beach deciding whether to walk.

High water and low water HW, LW, high tide, low tide: The two turning points of each tide. At Hilbre there are usually two high waters and two low waters in a day, about six hours and a quarter apart, and each one arrives roughly 50 minutes later than it did the day before. That daily drift is why last week's times are no use this week.
Tide height for example "8.9 m": How high the water reaches above chart datum at the moment of high water. It is not the depth of water over the crossing. Around 7 m is a modest tide at Hilbre, 9 m is a big one, and anything over 9.5 m earns an extra half-hour of margin in our calculations.
Chart datum CD: The zero line that all tide heights are measured from. It is set close to the lowest tide that astronomy can produce at that place, so heights are almost always positive numbers. Sea charts use the same line, which is where the name comes from. Two places with the same tide height can have very different amounts of water over the sand, because their datums and their beaches differ.
Spring tide "big tide": Nothing to do with the season. Springs are the larger tides that arrive a day or two after a new moon and a full moon, when the sun and moon pull together. High water is higher, low water is lower, and the water moves faster in between. For Hilbre that means the sands cover earlier and the crossing window is shorter. We label a high water of 8.5 m or more as spring-like.
Neap tide "small tide": The gentler tides around the half moons, when the sun and moon pull at right angles. Less range, slower water, longer and more forgiving crossing windows. If you are planning a first visit or taking children, a neap tide is the kind day. We label a high water of 7.5 m or less as neap-like.
Tidal range: The difference in height between one low water and the following high water. The Dee estuary has one of the biggest ranges in Britain, often 8 to 10 m on springs. A big range means a lot of water has to move in a few hours, which is why the tide here can come in faster than you can walk.
The 3-hour rule: The safety rule the whole site is built on: do not set off until three hours after high water, and be back on the beach three hours before the next high water. When the highest tide of the pair is 9.5 m or more we take a further 30 minutes off each end. It is deliberately conservative. It does not account for weather, so add margin of your own when the wind is onshore.
Crossing window safe window, "sands open": The stretch of time between those two limits, when the sands are treated as open. Typically five to six hours around low water, of which two are spent walking. "Depart after" is the earliest you should leave West Kirby; "be back by" is the latest you should be off the sands, not the latest you should leave the island.
The gutter: The low channel in the sands nearest the mainland, between West Kirby beach and Little Eye. Because it is the lowest ground on the route it fills first when the tide turns, sometimes while the sands further out still look dry. That is how people get cut off on the way back: the water arrives behind them. Local walkers and the Friends of Hilbre call the channel between Middle Eye and Hilbre "the Hilbre Gut": it can hold water even at low tide and floods rapidly once the tide turns.
Hilbre Swash the Swash, the east channel: The fast tidal channel that runs along the east side of Hilbre and Middle Eye, between the islands and the East Hoyle Bank. It is why there is no route from Hoylake or Red Rocks, and why you must not head straight for Hilbre from West Kirby beach: the direct line crosses it. The Friends of Hilbre's leaflet puts it plainly: it "must not be crossed". The recognised route goes out to Little Eye first, then along the islands.
Slate Shed: The small building beside the composting toilets at the north end of Hilbre, kept open as a wet-weather and emergency shelter. If you are cut off on the island, this is where to wait for the tide. It has benches and a roof and nothing else, so it is a place to sit out a few hours, not a reason to cut your margins.
Surge storm surge, meteorological effect: Extra water on top of the predicted tide, caused by weather. A strong onshore wind from the south-west round to the north-west piles water into Liverpool Bay and the Dee, and low air pressure lets the sea stand higher, roughly a centimetre for every millibar below normal. Together they can bring high water in higher and earlier than the tables say. When our weather feed shows both, the site shows a surge-risk flag and you should widen your margins.
Intertidal sands tidal flats, foreshore: The ground between the low-water and high-water lines: everything you walk across to reach Hilbre. Firm rippled sand for most of the route, with softer patches and channels, and mud on the Hoylake side that is why nobody should try to cross from there.
West Hoyle Bank: The large sandbank off the north-west end of Hilbre where the grey seals haul out at low water. It is separated from the island by a channel and is not somewhere to walk to. Watch the seals from the island.
Tide predictions tide tables, Admiralty data: The times and heights on this site come from the UK Hydrographic Office's Admiralty predictions for the Hilbre Island station. They are calculated from astronomy and are usually accurate to a few minutes and a few tenths of a metre. They cannot see the weather, which is what surge is about. Predictions are not observations.
Rule of twelfths: A sailor's rule of thumb for how the tide moves between low and high water: in the six hours it rises by roughly 1, 2, 3, 3, 2 and 1 twelfths of the range, hour by hour. The middle two hours move half the water. On the route map we use a smooth version of the same curve to estimate the height at any minute.
GMT and BST: All times on HilbreTides are UK clock time, switching automatically between Greenwich Mean Time in winter and British Summer Time from late March to late October. Printed tide tables are often in GMT all year and need an hour adding in summer, which is a classic way to get caught out.