Did you know that a tide table can list two high tides on the same day, with different heights, and still be perfectly correct? A student may expect one high tide in the morning and one low tide at night, because everyday speech describes ‘the tide’ as if it has only one rise and one fall. Real coastal water levels can follow a richer rhythm. Reading that rhythm matters for science diagrams, fieldwork, coastal photography and sensible planning by the shore.
The immediate answer: a high tide is a local peak in water level during a tidal cycle, not necessarily the highest level in an entire day. At many coastal locations, a daily prediction table shows approximately two highs and two lows, and the two highs may have different predicted heights. Read each entry with its time, height, reference datum, location and date. Do not choose only the higher high and ignore the earlier one. In some locations, the tidal pattern has only one high and one low per tidal day, so the number of entries depends on the station and date.
- Read a tide table for a named station; nearby coastal locations can have different predictions.
- Identify whether each row is a high or a low, and read the local time and the height unit.
- Two high tides on one calendar date can have different heights. Neither is a mistake simply because the numbers differ.
- Check the height’s reference datum: a predicted height above a datum is not automatically the water depth at your feet.
- Keep track of midnight: the next low or high may fall on the following calendar date.
- Treat predicted levels as planning information, not as guarantees of safe shoreline access or exact observed water levels.
What actually makes the water rise and fall?
Tides are long ocean waves driven mainly by the Moon’s gravitational influence, with the Sun also contributing. Earth’s rotation, the relative positions of the Earth, Moon and Sun, the shape of ocean basins, and coastal geography make the pattern at a real harbour or beach more complicated than the familiar school picture of two tidal bulges. Water levels at one place can rise to a high, fall to a low, and then rise again. How often the sequence repeats depends on the location’s tidal pattern.
The US National Oceanic and Atmospheric Administration (NOAA) describes semidiurnal tides, with roughly two similar high and low tides; mixed semidiurnal tides, with two high and low tides of unequal heights; and diurnal tides, with one high and one low per tidal day. See NOAA’s Types and Causes of Tidal Cycles and How Many High Tides Are There Per Day?. The important lesson is not to memorise that every shore has precisely two high tides every civil date; it is to read the actual station’s table.
Another useful distinction is between a solar day, the ordinary 24-hour calendar cycle, and a lunar day, roughly 24 hours and 50 minutes. NOAA explains the difference in its lunar-day lesson. For an idealised semidiurnal rhythm, successive highs occur approximately 12 hours 25 minutes apart. Actual station times can vary because local water movements are more complex. Never copy the idealised interval into a real table without checking the issued prediction.
A completely fictional daily tide table, explained carefully
The following figures are invented for learning, not measurements, not the tide prediction for Singapore, and not safe coastal-access instructions. Imagine a hypothetical teaching station called Harbour A. The station reports predicted water level in metres above its stated chart datum. All dates and times in this example use one fictional local time zone. We will treat the numbers as entries from an imaginary table, not as real water depths on a beach.
- Monday, 06:15 — high tide, 2.3 m above chart datum.
- Monday, 12:30 — low tide, 0.7 m above chart datum.
- Monday, 18:45 — high tide, 2.8 m above chart datum.
- Tuesday, 00:55 — low tide, 0.5 m above chart datum.
There are two highs on Monday: 2.3 m at 06:15 and 2.8 m at 18:45. Both count as high tides because each is a local peak relative to the neighbouring low-water stages. The second is the higher of the two high tides in this example, but that does not make the first one ‘not a high tide’. Likewise, the lower predicted low of the sequence occurs just after midnight on Tuesday; a student who closes the table at 23:59 might miss the next stage entirely.
Now complete a hand-checked calculation. From the 06:15 high of 2.3 m to the 12:30 low of 0.7 m, the difference in listed heights is 2.3 − 0.7 = 1.6 m. From the 12:30 low of 0.7 m to the 18:45 high of 2.8 m, the rise is 2.8 − 0.7 = 2.1 m. From the evening high to the next low after midnight, the difference is 2.8 − 0.5 = 2.3 m. All three answers are changes in predicted water level relative to the same datum; none is a measurement of the water depth at a particular rock.
The arithmetic check can be reversed: 0.7 + 2.1 = 2.8 m, so the second rise agrees with the recorded evening high. The second high also exceeds the morning high by 2.8 − 2.3 = 0.5 m. Every subtraction in this example assumes the height numbers belong to one station and one reference datum. Subtracting heights printed relative to different datums without converting them would be an invalid comparison.
Why the time columns deserve as much care as the height columns
A tide table is a sequence of events, not a multiplication table with identical time intervals. The example’s first high is at 06:15 and the next high at 18:45, an interval of 12 hours 30 minutes. The interval between the 18:45 high and Tuesday’s 00:55 low is 6 hours 10 minutes: 5 hours 15 minutes from 18:45 to midnight, then 55 more minutes. Crossing midnight does not make the elapsed time negative or require pretending that Tuesday’s event happened on Monday.
A learner who simply subtracts clock times, 00:55 minus 18:45, may get a nonsensical negative number. One remedy is to represent the next day’s time as 24:55 for that one calculation: 24 hours 55 minutes minus 18 hours 45 minutes = 6 hours 10 minutes. But 24:55 is not a normal clock display; it is a temporary arithmetic representation of a following-day event. The proper table should retain Tuesday’s actual date.
Tidal time also depends on the prediction provider’s time standard and any time-zone or daylight-saving settings in the location concerned. A visitor copying a table into another country must check whether the provider lists local civil time, UTC or another convention. Do not assume the clock zone from where you happen to be reading the webpage.
Chart datum and real water depth are not the same thing
A predicted height of 2.8 m above chart datum does not mean every place along a harbour contains 2.8 m of water. The sea floor varies in elevation, and a chart may show depths relative to a reference plane at different points. Nor does the number by itself mean that a particular footpath is dry or covered. Local ground height, sea-level datum, waves, weather and the coastal layout all matter. The correct sentence for our fictional table is ‘Harbour A’s predicted water level is 2.8 m above its stated chart datum at 18:45,’ not ‘there will be 2.8 m of water everywhere’.
A tide prediction is also different from an observed tide-gauge reading. Wind and atmospheric pressure can raise or lower actual water levels relative to astronomical predictions, and waves can reach exposed rocks and shorelines. Strong wind, rain, currents and access restrictions can create additional risk. Never use a lesson’s fictional figures to select a safe crossing time, and never let children explore exposed shore areas without appropriate supervision and local guidance.
Diagnose what a student is actually misunderstanding
Ask four questions before handing out ten more graphs. One: why are 06:15 and 18:45 both highs? Two: which high is higher, and by how many metres? Three: why is the final low listed under Tuesday rather than Monday? Four: what does ‘above chart datum’ tell you—and what does it not tell you? A student who misses question one may confuse a local peak with a daily maximum. A student who misses question two may need decimal subtraction. Question three tests dates. Question four checks whether the number has been interpreted in its proper physical context.
Match the correction to the observed error. For a local-peak error, draw a wavy line with two summits of unequal height. For a calculation error, use a ruler to mark 2.3 m and 2.8 m on one vertical scale and show their 0.5 m difference. For a midnight error, put Monday and Tuesday on separate cards and move the final tide row to the correct card. For a datum error, label a drawing ‘reference level’, ‘water surface’ and ‘uneven seabed’.
Guided-to-independent practice using fresh numbers
Stage one: find highs and lows without any arithmetic
Provide four imaginary event entries labelled high, low, high, low, and ask the learner to put them in chronological order. They should say that ‘high’ and ‘low’ identify turning points relative to the adjacent water-level trend, not ranks within all numbers seen in the week. Give them a second day with only one high listed and ask what other information would be required before claiming the table is wrong.
Stage two: calculate one difference and reverse it
A fictional high is 2.6 m above datum and the following low is 0.9 m. The fall is 1.7 m. To check, add 0.9 m + 1.7 m and recover 2.6 m. Ask for units on each step. If a pupil answers 1.7 without units, prompt them to say whether that is metres, hours or an unrelated number.
Stage three: compare unequal high tides
A second invented station has morning high 1.9 m and evening high 2.4 m above the same datum. The evening high is 0.5 m higher. Explain that this numerical difference does not tell us why the tides differ. Causes require knowledge of the local tidal system, not just one pair of observations. Do not automatically label every two-high pattern ‘mixed’ based on one small difference without understanding the provider’s classification.
Stage four: interpret an overnight sequence
A fictional high is predicted at 22:40 on Friday and a low at 04:55 on Saturday. The elapsed time is 1 hour 20 minutes until midnight plus 4 hours 55 minutes after midnight, which equals 6 hours 15 minutes. Now ask whether a school trip scheduled for 02:00 can be judged safe from those two figures alone. The correct answer is no. Missing details include the actual site, weather, water-level reference, access rules and conditions on the day.
Stage five: read an authentic station’s table independently
With a teacher or suitable adult, obtain a current tide prediction from a trustworthy provider responsible for the chosen coast. Check station, date, time zone, unit and datum. Identify the predicted next high and low and explain whether the entries describe a semidiurnal, mixed or diurnal pattern over the relevant period. Separately look for local advisories or signs before any real-world shoreline activity. The goal of the exercise is interpretation and safety awareness, not a promise that every tide prediction can be followed as a timetable for access.
Common mistakes and why each changes the answer
- ‘Only the highest number of the day can be called high tide.’ Both daily highs may be local maxima.
- ‘The two high tides must be exactly equal.’ Real locations often have unequal highs.
- ‘All coasts have two highs every civil day.’ Diurnal and varying local patterns exist.
- ‘A predicted height is the seawater depth everywhere along the shore.’ It is referenced to a station and datum.
- ‘The following day’s 00:55 is earlier than today’s 18:45.’ The dates differ, so the chronology remains forward.
- ‘A tidy prediction proves the shore is safe.’ Weather, waves, ground conditions and access restrictions can matter.
- ‘The time gap must always be precisely 12 hours.’ Idealised lunar-period descriptions do not replace the location’s actual prediction.
What parents, teachers and learners can do next
A parent can turn the tide chart into a short reading-and-reasoning game at the kitchen table: point at the two highs, ask which is higher, and explain why ‘higher high’ still contains the word ‘high’. A teacher can have learners annotate an invented station table, then compare it against NOAA’s explanation of the three broad tidal patterns. A student can independently produce a labelled line sketch with time along the horizontal axis and water level relative to a named datum along the vertical axis.
Judge progress by observable actions: the learner identifies all turning points, keeps dates aligned across midnight, subtracts height differences with metres attached, and names at least two limits of a tide prediction. Repeat with a different station or time pattern after a gap; if they can still explain the method, it is more likely to be transferable learning. Do not imply this table-reading skill alone qualifies anyone to navigate vessels, walk tidal flats or make coastal safety decisions.
Frequently asked questions
Why can there be two high tides but one is lower than the other?
The word ‘high’ means a local high-water stage relative to its surrounding low-water stages. The astronomical forcing and regional ocean response can make successive highs unequal. NOAA calls a two-high, two-low pattern with noticeable differences mixed semidiurnal.
Does every coastal location have two high tides each day?
No. Broad tidal patterns differ. NOAA describes semidiurnal, mixed semidiurnal and diurnal cycles. Examine the prediction for the location and date rather than imposing one pattern on every coast.
Why do the high tide times shift from day to day?
The Moon’s motion makes the lunar day roughly 24 hours 50 minutes rather than 24 hours, an important influence on tidal timing. Local water dynamics complicate that simple picture, so calculate with the station’s forecast rather than automatically adding a fixed number of minutes.
Does ‘2.8 m high tide’ mean the water is 2.8 m deep at the beach?
No. The height is measured from the stated reference datum; depth at a particular beach or harbour point depends on ground or seabed elevation and other local factors. You cannot infer safe access from the number alone.
Should a family plan an island or shoreline walk solely from a tide table?
No. A tide table is one part of information gathering, not an access permit or safety guarantee. Read local authority guidance, current weather and marine advisories and any site-specific restrictions. Do not enter exposed or dangerous shore areas based on a classroom calculation.
Reliable sources, related learning and a useful closing habit
The science behind these reading steps is explained by NOAA’s guide to high tides, its tidal-cycle classifications, and its lunar-day lesson. Our previously published shore and tidal-range vocabulary guide explores complementary terms, while the Core Aim guide to compass readings and map bearings is another practical example of reading a number with its proper reference. The Education learning hub and immutable Clementi mathematics reference reinforce diagnosis, evidence and independent verification.
When a tide table shows two high tides, resist the urge to cross one out. Read the times and dates; identify both local peaks; compare their predicted heights in the same units and datum; then explain what the numbers cannot tell you. That is the larger aim of education: use information accurately, check it independently, and know where the limits of a calculation begin.