Tides & Water Levels

Start here: ten words every newcomer needs

Overheard on the job

Real-sounding lines from the floor, translated into plain English.

• Springs are making and there's an onshore blow, so the residual will push tonight's high over MHHW.

  TranslationTidal ranges are growing toward spring tides and the onshore wind is adding water above the prediction, so the high tide will exceed mean higher high water.

• Slack's an hour after high water here, don't mix up the stand with the turn of the current.

  TranslationAt this location the current stops about an hour after the water level peaks; the pause in level and the pause in flow are separate events.

• Bubbler flatlines on every low-low, the orifice is too high, do a staff shot and drop it a metre.

  TranslationThe gas-purge gauge reads a constant value at the lowest tides because its underwater opening is exposed; check it against the tide staff and lower it.

• Give me thirty clean days and we'll run a simultaneous comparison against the entrance gauge for MLLW.

  TranslationCollect a month of good water levels so the datum at the new site can be computed by comparison with a long-term station over the same period.

• Max ebb is 4 knots at the gate with wind against tide, so we anchor and go at slack.

  TranslationThe outgoing current peaks at 4 knots at the critical headland and opposes the wind, making rough seas, so the boat waits for the current to stop.

• King tide plus a 30 millibar low, that's sunny-day flooding turning into a real storm tide.

  TranslationAn exceptionally high tide combined with low air pressure, which raises the sea about 30 cm, will push routine tidal flooding toward storm-level water.

• Minus tide Saturday, dead low at 0610, bring the level and we'll tie in the bottom staff while it's dry.

  TranslationA low tide below chart datum will leave the lowest section of the tide staff out of the water, so the crew will be there at the bottom of the tide to level to it.

The tidal and land datums that give water levels a fixed zero, and the marks that preserve them.

• Bench markalso benchmark, tidal bench mark
  A permanent, stable mark of known height near a tide station that preserves the station's datums.
• Brass capalso disk, survey disk
  Surveyor slang for a bench mark disk, the stamped metal cap set in concrete or rock. Shop talk
• Chart datumalso CD, sounding datum
  The low-water zero that charted depths and tide table heights are measured from.
• LATalso lowest astronomical tide
  Lowest Astronomical Tide: the lowest level that tide predictions reach when the weather is held at average.
• MHHWalso mean higher high water
  Mean Higher High Water: the average height of the higher of each day's two high waters over the datum epoch.
• MHWalso mean high water
  Mean High Water: the mean height of every high tide across the datum epoch, often the line that defines the legal shoreline.
• MLLWalso mean lower low water
  Mean Lower Low Water: the average height of the lower of each day's two low waters, used as chart datum in the United States.
• MSLalso mean sea level, LMSL
  Mean Sea Level: the plain average of a station's hourly water levels across the tidal datum epoch.
• MTLalso mean tide level, half-tide level
  Mean Tide Level: the height midway between the mean high water and mean low water datums.
• NAVDalso North American Vertical Datum
  North American Vertical Datum: the geodetic height reference used for land elevations across the United States.
• Recoveryalso mark recovery, bench mark recovery
  Finding an existing bench mark in the field, checking its condition and reporting it as found, damaged or lost.
• Station datumalso gauge zero, staff zero
  An arbitrary fixed zero set below the lowest expected water at a station, used to record raw water levels.
• Tidal datumalso tide datum, water level datum
  A local zero for heights, computed by averaging one stage of the tide, such as high or low water, across many years.
• Tidal datum epochalso NTDE, National Tidal Datum Epoch
  The fixed 19-year period over which water levels are averaged to compute official tidal datums.

The astronomy, forces and harmonic constituents behind the rise and fall of the sea.

• Amphidromic pointalso amphidrome, amphidromic system
  A point in the sea where a tidal constituent has zero range and around which its high water rotates.
• Corange linealso co-range line
  A line joining places that share the same tidal range or the same constituent amplitude.
• Cotidal linealso co-tidal line
  A map line through every place where high water of a tide or constituent arrives at the same moment.
• Declinationalso lunar declination, solar declination
  The angle of the Moon or Sun north or south of the celestial equator, which makes the two daily tides unequal.
• Equilibrium tidealso equilibrium theory
  The theoretical tide an ocean covering the whole Earth would show if it responded instantly to the tidal forces.
• Harmonic constituentalso tidal constituent, constituent
  One of the regular cosine waves, each with a period fixed by astronomy, that together make up the tide.
• K1also lunisolar diurnal
  The lunisolar diurnal constituent, period about 23.93 hours, a main driver of once-daily tides.
• Kelvin wavealso coastal Kelvin wave, coastally trapped wave
  A long wave trapped against a coast by Earth's rotation, the form in which tides travel around ocean basins.
• Lunar dayalso tidal day
  The time for the Moon to return to the same meridian, about 24 hours 50 minutes, which sets the daily tide rhythm.
• M2also principal lunar semidiurnal
  The principal lunar semidiurnal constituent, period 12.42 hours, usually the largest part of the tide.
• Nodal cyclealso lunar nodal cycle, 18.6-year cycle
  The 18.6-year cycle in the tilt of the Moon's orbit that slowly raises and lowers tidal ranges.
• O1also principal lunar diurnal
  The principal lunar diurnal constituent, period about 25.82 hours, created by the Moon's declination.
• Overtidealso shallow-water constituent
  A higher-frequency constituent, such as M4, generated when a tide distorts in shallow water.
• S2also principal solar semidiurnal
  The principal solar semidiurnal constituent, period exactly 12 hours, which beats with M2 to make springs and neaps.
• Tide-generating forcealso tidal force, tractive force
  The small difference between the Moon's or Sun's pull at a point on Earth and its pull at Earth's centre.

Daily and fortnightly patterns: semidiurnal, diurnal and mixed tides, springs, neaps and their quirks.

• Aggeralso double high water, double low water
  A double high water or double low water, where the tide peaks or bottoms out twice in quick succession.
• Borealso tidal bore, eagre
  A wave or wall of water that runs up a river at the start of the flood tide in some funnel-shaped estuaries.
• Diurnal inequalityalso daily inequality, high water inequality
  How much the day's two high waters, or its two low waters, differ in height from each other.
• Diurnal tidealso daily tide
  A tide with a single high water and a single low water in each lunar day.
• Form factoralso form number, F ratio
  A ratio of diurnal to semidiurnal constituent amplitudes used to classify a tide as semidiurnal, mixed or diurnal.
• Higher high wateralso HHW
  The taller of a tidal day's two high waters, the one that matters most on mixed-tide coasts.
• Mixed tidealso mixed semidiurnal tide, mixed diurnal tide
  A tide with two highs and two lows a day of clearly unequal heights, combining semidiurnal and diurnal parts.
• Neap tidealso neaps
  A tide of smaller than average range, occurring near first and last quarter moon when Sun and Moon pull at right angles.
• Perigean spring tidealso perigean tide
  An especially large spring tide that occurs when new or full moon falls close to the Moon's nearest approach to Earth.
• Semidiurnal tidealso semi-diurnal tide, twice-daily tide
  A tide with two high waters and two low waters of similar height in each lunar day.
• Spring tidealso springs, spring tides
  A tide of larger than average range, occurring around new and full moon when Sun and Moon pull in line.
• Standalso stand of tide, high water stand
  The pause at the top or bottom of the tide, when the level hangs nearly still before turning.
• Tidal rangealso range, range of tide
  The vertical difference between a high water and the neighbouring low water.
• Tropic tidealso tropic tides
  A tide occurring when the Moon is near its greatest declination, giving the largest daily inequality.

The horizontal side of the tide: flood, ebb, slack and the words mariners use to time them.

• Current ellipsealso tidal ellipse
  The ellipse traced by the tip of a tidal current vector over one cycle, showing direction and speed at each stage.
• Ebbalso ebb current, ebb tide
  Tidal current running out toward the sea and down rivers and estuaries, mostly while the water is falling.
• Ebb-dominant
  Describes an estuary or inlet where ebb currents are stronger than flood currents, pushing sediment seaward.
• Fair tidealso fair stream, fair current
  A tidal current flowing in the same direction a vessel wants to go, adding to its speed over the ground. Shop talk
• Floodalso flood current, flood tide
  Tidal current running in from the sea and up rivers and estuaries, mostly while the water is rising.
• Foul tidealso foul stream, adverse tide
  A tidal current running against a vessel's intended direction, slowing its progress over the ground. Shop talk
• Hydraulic currentalso head-driven current
  Flow through a strait or canal driven by the water level difference between its two ends, not by local rise and fall.
• Max floodalso maximum flood, strength of flood
  The peak strength of the flood current in a cycle, and the time it occurs; max ebb is the seaward equivalent. Shop talk
• Reversing currentalso rectilinear current, reversing tidal current
  A tidal current that runs back and forth along one line, flood one way and ebb the other, with slack at each change.
• Rotary currentalso rotating current, rotary tidal current
  An offshore tidal current whose direction keeps swinging round the whole compass during each tidal cycle.
• Set and driftalso set, drift
  The direction a current flows toward (set) and its speed (drift).
• Slack wateralso slack, slack tide
  The short lull when a reversing current almost stops while it swaps between flood and ebb.
• Tidal currentalso tidal stream
  The horizontal flow of water that accompanies the rise and fall of the tide.
• Tidal diamondalso tidal stream diamond, diamond
  A lettered diamond symbol on British-style charts linked to a table of tidal stream direction and rate hour by hour.
• Tidal prismalso prism, tidal volume
  The volume of water that flows into a bay or estuary on the flood and out on the ebb.
• Tide-rodealso tiderode
  Describes an anchored vessel lying head to the tidal current rather than to the wind. Shop talk
• Wind against tidealso wind over tide, wind against current
  Wind blowing opposite to a tidal current, which builds short, steep and sometimes breaking seas. Shop talk

Tide gauges, staffs, levelling and current profilers, plus the field slang of water level observers.

• ADCPalso acoustic Doppler current profiler, ADP
  Acoustic Doppler current profiler: an instrument that measures current speed and direction at many depths at once.
• Binalso depth cell, cell
  One depth cell of an ADCP profile, the slice of water over which a single velocity is averaged.
• Bubbleralso gas-purge pressure gauge, bubbler gauge
  Field name for a gas-purge pressure gauge that measures water level by pushing gas through an underwater orifice. Shop talk
• Closing the loopalso loop closure, closing the run
  Finishing a level run back on the starting bench mark to measure how much error crept in. Shop talk
• Flatlinealso flat-lined data, stuck sensor
  A stretch of water level data stuck at one value, a sign of a frozen, failed or dry sensor. Shop talk
• Levelingalso levelling, running levels
  Precise measurement of height differences between bench marks and the gauge or staff using a level and rods.
• Microwave radar sensoralso radar gauge, microwave water level sensor
  A non-contact water level sensor mounted above the surface that measures the distance down to the water by radar.
• Pressure gaugealso pressure sensor, bottom pressure recorder
  A submerged sensor that measures the weight of water above it, converted to water level with density and air pressure.
• Six-minute dataalso 6-minute data, six-minute water levels
  Water level values recorded every six minutes, the standard sampling interval at many tide stations. Shop talk
• Staff shotalso staff reading, staff-gauge comparison
  A set of tide staff readings taken alongside the gauge to check that the gauge is still reading true. Shop talk
• Stilling wellalso well, stilling pipe
  An upright tube fed through a narrow bottom intake, filtering out waves so the gauge inside sees only the slow tide.
• Tide gaugealso water level gauge, tide gage
  An instrument that records water level continuously at a fixed point, the basic tool of tidal measurement.
• Tide housealso gauge house, tide hut
  The small shelter on a pier or wharf that houses a tide gauge's well, recorder and electronics. Shop talk
• Tide staffalso tide pole, tide board
  A graduated board fixed vertically in the water and read by eye to check a gauge against datum.

Harmonic analysis, tide tables and the methods that turn raw records into forecasts and datums.

• Astronomical tidealso predicted tide, gravitational tide
  The part of the water level caused only by the gravitational effects of the Moon and Sun.
• Harmonic analysisalso tidal analysis, harmonic reduction
  Breaking an observed water level or current record into its tidal constituents to find each one's amplitude and phase.
• Lunitidal intervalalso high water interval, low water interval
  How long a place waits after the Moon crosses its meridian before the next high or low water arrives.
• Phase lagalso kappa, Greenwich phase lag
  How far a constituent's local high water trails the corresponding astronomical forcing, given in degrees.
• Reference stationalso standard port, primary station
  A tide or current station with its own harmonic constants, for which full daily predictions are published.
• Residualalso non-tidal residual, surge residual
  The observed water level minus the predicted tide, showing the effect of weather, rivers and other non-tidal causes.
• Simultaneous comparisonalso datum transfer, comparison of simultaneous observations
  Computing datums at a short-term station by comparing its record with a long-term control station over the same days.
• Subordinate stationalso secondary port, secondary station
  A location whose tides are predicted by applying time and height corrections to a nearby reference station.
• Tidal predictionalso tide prediction, tide tables
  A forecast of tide heights or currents computed from harmonic constants and astronomy, with no weather included.

Storm surge, set-down, seiches and the long-term sea level trends recorded at tide gauges.

• Blowout tidealso blow-out tide, blowout
  An extreme low water caused by strong offshore winds pushing water out of a bay or sound. Shop talk
• High tide floodingalso nuisance flooding, tidal flooding
  Shallow coastal flooding that happens when ordinary high tides exceed a local flood threshold, often without a storm.
• Inverse barometer effectalso inverted barometer effect, IB effect
  The rise of sea level under low air pressure and fall under high pressure, about 1 cm per hectopascal.
• Relative sea level trendalso sea level trend, relative sea level rise
  The long-term rate of change in mean sea level at a gauge, including both ocean rise and land motion.
• Seichealso harbour oscillation, range action
  A standing oscillation of water sloshing back and forth in a lake, bay or harbour at its natural period.
• Set-downalso setdown, negative surge
  A drop in water level below the predicted tide caused by winds blowing water away from the coast.
• Storm surgealso surge, positive surge
  A rise of water above the predicted tide caused by a storm's winds and low pressure.
• Storm tidealso total water level, storm water level
  The full water level reached in a storm: astronomical tide and storm surge added together.
• Sunny-day floodingalso clear-sky flooding, nuisance flooding
  Popular name for high tide flooding that arrives in fine weather, with no rain or storm in sight. Shop talk
• Vertical land motionalso VLM
  Sinking or rising of the ground at a tide station, which changes the sea level trend the gauge records.

Waterfront and boating talk about tides, from king tides and minus tides to neaped boats.

• Dead lowalso dead low water, bottom of the tide
  The very bottom of low water, when the tide has stopped falling and not yet started to rise. Shop talk
• Dry outalso drying out, take the ground
  To let a boat settle deliberately on the bottom as the tide falls, against a wall, on legs or on twin keels. Shop talk
• King tidealso king tides, big tide
  Informal name for the highest tides of the year, usually perigean spring tides. Shop talk
• Low-lowalso lower low, big low
  Shorthand for the lower low water of the day in places with mixed tides. Shop talk
• Makingalso making tide
  Said of a tide that is rising, or of the fortnightly cycle building from neaps toward springs. Shop talk
• Minus tidealso negative tide, minus low
  A low tide predicted to fall below the chart datum zero, shown as a negative number in the tide table. Shop talk
• Neapedalso beneaped
  Stranded after grounding near spring high water and unable to refloat until the next spring tides return. Shop talk
• Rule of twelfthsalso twelfths rule, 1-2-3-3-2-1
  A rule of thumb for how much a tide rises or falls in each hour of a six-hour rise or fall. Shop talk
• Tidal gatealso gate
  A headland, strait or bar that can only be passed comfortably at certain states of tide, fixing the timing of a passage. Shop talk
• Tide bookalso tide table, tide calendar
  A pocket booklet of local tide predictions for the year, handed out by marinas, bait shops and chandlers. Shop talk
• Tide windowalso tidal window, tidal restriction
  The span of time when water is deep enough, or current slack enough, for a planned vessel move or job. Shop talk
• Top of the tidealso top of the flood
  High water, or the short period around it, as dock workers and boaters say it. Shop talk
• Turn of the tidealso the turn, tide turning
  The moment the tide changes from rising to falling or the current from flood to ebb, or the reverse. Shop talk

Frequently asked questions

More from Sea & Rail