How to read a tide table

What each entry tells you

Tables come in several layouts, but they all carry the same pieces of information: the station the predictions are for, the date, the time of each high and low water, and the height of the water at that moment. Four things are worth checking before trusting any number.

Station: the predictions apply to that place only. A harbour a short distance away can run on a noticeably different schedule.

Time zone: most tables use local time, but check whether daylight saving time is already applied or whether the hour has to be added by the reader.

Units: heights may be in metres or in feet. Mixing them up is an easy and serious mistake, since 1 metre is about 3.3 feet.

Year: tides do not repeat on the same calendar dates from one year to the next, so an old table is of no use.

How many tides to expect in a day

Most coasts see two high waters and two low waters in a little under 25 hours, a pattern called a semidiurnal tide. The cycle follows the moon more than the clock. The moon returns to the same position in the sky about 50 minutes later each day, so on average the tides run later by about that much from one day to the next. The daily shift is uneven, and it means that some days show only three entries, because the fourth has slipped past midnight.

Some coasts have only one high and one low a day, known as a diurnal tide. Many have a mixed tide, in which the two highs and the two lows of the day reach clearly different heights. On a mixed coast, one low may barely uncover the beach while the next exposes a wide stretch of it, so read every entry for the day and not only the first.

Height and tidal range

The height beside each time says how far the water surface is predicted to stand above the reference level. Suppose a table shows a high of 3.4 m (about 11.2 ft) followed by a low of 0.6 m (about 2.0 ft). The difference, 2.8 m (about 9.2 ft), is the tidal range for that tide: the vertical distance the water falls between the two.

Range changes through the month. It is largest around spring tides and smallest around neap tides, a cycle covered in the article on spring and neap tides. It also varies enormously around the world, from well under a metre (3 ft) on some coasts to more than 10 m (33 ft) in a few funnel-shaped bays.

What the heights are measured from

Every height in a tide table is measured from chart datum, a low-water reference level chosen so that the water seldom falls far below it. The same level is the zero for the depths printed on the nautical chart of that area. That is what makes the two work together: the charted depth at a spot plus the height of tide at that moment gives the approximate depth of water there.

Countries choose the reference differently. United States tables use mean lower low water, usually written MLLW, which is the average of the lower of each day's low waters over a long reference period. Canadian tables for tidal waters use a lower level, described as lowest normal tides or lower low water, large tide. The United Kingdom and many other countries use lowest astronomical tide, the lowest level that can be predicted under average weather conditions.

Because MLLW is an average, some low waters fall below it, and the table shows those as negative heights, often called minus tides. A negative height simply means the water is predicted to drop below the reference level, leaving less water than the chart shows.

Predicted is not the same as observed

Tide tables are calculated from the movements of the moon and sun and from long records of water level at the station. They describe what the tide alone is expected to do. They know nothing about next week's weather.

Wind and air pressure both move the sea. A strong wind blowing towards the coast piles water against it, and a wind blowing off the coast pushes water away. Low atmospheric pressure lets the sea surface rise and high pressure presses it down, by roughly one centimetre for each hectopascal (millibar) as a rule of thumb. In a severe storm these effects combine into a storm surge that can lift the water well above the predicted height. In estuaries, heavy river flow raises levels too.

Putting a tide table to use

CatchWindow location pages show tide predictions where a tide station applies, together with a tidal-range figure, which is a convenient way to see the shape of a day when planning a trip. For navigation, clearances and anything where safety depends on the answer, use the official tide tables and charts for the area and the current marine forecast.

Confirm the station, the date, the time zone and the units before reading a single number.

Remember that heights are measured above chart datum, that they are not depths, and that datums differ between countries.

Allow a margin for weather. Strong winds and unusual pressure make the real level differ from the table.