## Time & Tide

TIDAL TERMSTidal range: Difference in height between high and low tide Spring tide: Largest tidal range in a two-week period Neap tide: Smallest tidal range in a two-week period Flood: Incoming tide Ebb: Outgoing tide Slack: When the current arising from the tide stops before changing direction Height of tide: Depth of water relative to a datum, generally the lowest theoretical tide at a regional port, so that the height of the tide is always positive. |

**UNLIKE WEATHER**, tides are predictable years in advance. For any port, we can look up the time and height of high and low water from a set of tide-tables, or online on various websites.

In any region, tide-tables relate to a specific reference port. If our harbour or beach is along the coast a bit, but not at the next reference port, the times of tides need to be adjusted by adding or subtracting a fixed amount from the times of the reference port. Sometimes this is already done for us in more localised tide tables.

The other correction we may need to make is to add an hour for British Summer Time. I say “may”, because some tide-tables are already adjusted for BST. Look in the small print and it should tell you; you don’t want to make the adjustment twice, and arrive too late for the dive.

**TIDES AND ASTRONOMY**

Tides are caused by gradients in the pull of gravity from the Moon and the Sun. In its simplest form, the gravitational effect of the Moon pulls the sea about the Earth so that it is not perfectly spherical. We end up with a tidal bulge towards the Moon, where gravitational pull is stronger, and a corresponding bulge away from the Moon, where it’s weaker.

As the Earth rotates each day, the tidal bulges follow the Moon, causing high tides.

As the Moon is also orbiting the Earth every 28 days, tide times move on by an average of 1/28th of a day, or 51 minutes.

The same happens to a lesser extent with the Sun. It is much bigger, but it is also further away, so the gradient in the gravitational force is smaller, and a smaller tidal bulge about the Earth is caused.

When the tidal effects of the Moon and Sun line up we have the biggest tides, or spring tides. When they are 90° out, they cancel each other out a bit, and we have neap tides.

Springs correspond to a full Moon and new Moon, while neaps correspond to a half-Moon.

An equinox occurs when the tilt of the Earth’s axis lines up with the Sun, in spring and autumn. Because we’re lined up, the tidal effect of the Sun is slightly larger, and we get bigger spring tides and gentler neaps.

The orbits of the Moon about the Earth and the Earth about the Sun are not perfectly aligned. When they do happen to line up, we can get a solar or lunar eclipse, and the alignment also has a tiny effect on the size of tides.

Finally, the orbits of the Earth and Moon are not perfectly circular, but slightly elliptical. At closest approach, or perihelion, the tides are a little larger. Further away, at aphelion, the tides are smaller.

When we hear a prediction of “the highest tide in 20 years”, it is because all of these things come closest to matching up roughly that often.

The cycle is actually about 19 years, but 20 is a rounder number for the press.

**TIDES AND GEOGRAPHY**

If the surface of our planet was completely watery, and the sea a uniform depth, apart from one big pole that stuck out of the water on which we measured tides, then astronomy would be enough to predict tides. Tides everywhere would be small compared to what we’re used to, less than a metre.

But we have continents, deep oceans and continental shelf. Coastlines are irregular; there are headlands, inlets, islands, narrow channels and varying depths.

Where the sea shallows or narrows, such as in the English Channel, the movement of water with the tide funnels up to create bigger tides.

The astronomical origin of tides means that they are actually an oscillation. Constrain the oscillation of the tide within an ocean and we can get resonances and damping that result in bigger tides in some areas and smaller tides in others. Hence the big tides of the Channel Islands and small tides at Lerwick.

Geography can also delay the time of spring tides, in some locations by as much as a few days after the full or new Moon.

**TIDES AND WEATHER**

Weather by itself does not cause tides, but it can affect the height of tides through a storm surge or storm tide. Differences in atmospheric pressure influence the level of the sea as the weight of atmosphere pushes down more or less against the sea.

A big low-pressure system can result in a hump in the sea – not that high, but potentially over a big area. A 1-millibar drop in pressure equals 1cm height of water.

The hump moves with the low-pressure system and currents, and grows with the funnelling effect of coastal geography.

Coupled with a big low-pressure system are storm-force winds that also push water before them. The geography of the North Sea makes it particularly vulnerable to storm surges of up to 2m above normal tides, hence the Thames Barrier.

**TIDAL CURVES**

The height of the tide usually follows a roughly sinusoidal curve. As tides are just over 6 hours apart, we conveniently forget about the extra 10 minutes, and this leads to the rule of twelfths.

The approximation is that the tide moves 1/12 in the first hour, 2/12 in the second hour, 3/12 in each of the third and fourth hours, 2/12 in the fifth hour and 1/12 in the last hour.

With this approximation and a set of tide tables, we can calculate the depth of water for any hour of the tide.

For example, suppose low water occurs at 9am and is 1.4m, high tide occurs at 3:10pm and is 5m. This gives a tidal range of 3.6m.

Dividing by 12 we have 0.3m, and the height of the tide can then be tabulated:

9am1.4m

10am1.4 + 0.3 x 1 = 1.7m

11am1.7 + 0.3 x 2 = 2.3m

12am2.3 + 0.3 x 3 = 3.2m

1pm3.2 + 0.3 x 3 = 4.1m

2pm4.1 + 0.3 x 2 = 4.7m

3pm4.7 + 0.3 x 1 = 5m

Suppose we plan to dive a wreck charted as 16m to the seabed not far outside the harbour. We can add these depths to predict the maximum depth of the dive. A dive at low water would be 17.4m, at mid-day 19.2m and at high water 21m.

If there are depth-limited PADI Open Water or BSAC Ocean Divers in the group, we know at what times they would be permitted to make the dive. In practice, the rise of the tide further offshore will usually be a bit less.

Beware, however: there are places where the tides are not conveniently sinusoidal – places where resonances result in wobbles in the curve and even double tides, where the tide goes out a bit, then comes back a little before going all the way out.

Tide-tables produced specifically for navigation therefore often include tidal curves, a reference curve for the harbour that follows a complete cycle of the tide.

When scaled by the range of the tide on any day, we can use this to work out the height of the tide at any time on that day.

Many websites and computer programs that give tide-tables also include a tidal curve already adjusted for the day in question. Some of the most useful sites are shown in the panel.

ON THE WEB | |

TIDES ONLINEEasytide is a tidal prediction service from the UK Hydrographic Office. Predictions are for a week ahead, with a small charge for others. The map interface is a bit clunky, but if you know the port you want it’s easy enough to get the tide-table for a week ahead, including a full graph. www.easytide.ukho.gov.uk . | |

BBC The BBC presents UKHO data, the same as Easytide but with a slightly different interface, and the convenience of links to all the BBC weather forecast pages, such as the shipping and inshore forecasts. www.bbc.co.uk/weather/coast/tides | |

NATIONAL OCEANOGRAPHY CENTREThis website by the Proudman Oceanographic Laboratory and Natural Environment Research Council has a more limited range of ports, but makes up for this by giving tide-tables for the next 28 days with a seven-day graph. The times are a few minutes different to those given for the same port on Easytide. www.pol.ac.uk/ntslf/tides | |

RYAThe weather and tides page from the Royal Yachting Association contains several weather forecasts and eight-day tide-tables on a single page. All very convenient, but less so when you realise that you have to select the location for each item within the page separately. This could easily lead to mistakes. www.rya.org.uk | |

TIDE TIMES This website uses data from Easytide for seven-day tables and from the Proudman Oceanographic Laboratory for 28-day tables, both without a tidal graph. The map interface is built on Google Maps, so is very slick. www.tidetimes.org.uk | |

TIDE FORECASTThe display is a seven-day graph with high- and low-water times and heights presented as annotations on the graph. This could be useful if you want to go into detail, but the concept is too complex if all you want is a straightforward tide-table. www.tide-forecast.com |