DICTIONARY

A-Z of sea words 

A

 

 

Abyssal

The lowest depths of the ocean, generally below 3,700m (2,000 fathoms)

 

 

Anti-cyclone

Weather with high atmospheric pressure*, typically with clear blue skies and gentle winds if the isobars* are widely spaced. In the northern hemisphere winds blow clockwise around an anti-cyclone

 

 

Amphidromic Point

A no-tide point from which co-tidal* lines radiate (also known as a Tidal Node)

 

 

Aphelion

The furthest point of earth from the sun during its elliptical annual orbit

 

 

Apogean Neap Tide

A neap tide* that happens when the moon is at apogee*​

 

 

Apogee

The furthest point of the moon from earth on its elliptical monthly orbit

 

 

Archipelago

A group of islands

 

 

Astronomical Twilight

When the centre of the sun is 12-18 degrees below the horizon

 

 

Atmospheric Pressure

The weight of air at a specific point, commonly called 'air presure'. High pressure indicates cold air sinking (increasing the weight of air) and low pressure indicates warm air rising (decreasing the weight of air)

Atoll

A coral island consisting of a ring-shaped reef surrounding a lagoon

 

 

 

 

B 

 

Backing Wind

A counter-clockwise change in wind direction (opposite to veering*)

 

Bar

A ridge of shallow water extending across a river mouth or harbour entrance

 

 

Barometer

An instrument for measuring atmospheric pressure*

 

 

Bathymetry

The shape of the seabed

 

 

Bay

A wide indentation of the coast, bigger than a cove but smaller than a gulf

 

Beam Sea

Waves moving in a direction 90 degrees to the way you are heading

 

 

Beam Wind

Wind moving in a direction 90 degrees to the way you are heading

 

 

Beaufort Wind Scale

A measurement of wind pioneered by Admiral Beaufort in 1805, ranging from Force 0 (calm*) to Force 12 (hurricane*)

 

 

Billow

A great wave

 

 

Bore

A rare river wave that happens mostly around spring tides in estuaries with big tides connected to shallow and slow moving rivers

 

 

Breaker

A wave breaking in shallow water, either on the shore or on a reef*

 

 

Breeze

A wind of moderate strength

 

 

Buy Ballots Law

A technique whereby if you stand with your back to the wind in the northern hemisphere, the low pressure* will be to your left (opposites apply in the southern hemisphere)

 

 

C

 

 

Cable

A unit of distance equal to 1/10th of a nautical mile (185 metres)

 

 

Calm

Wind speeds of less than 2 knots

 

 

Cape

A piece of land projecting out to sea; also called a Bill, Foreland, Head

 

 

Cardinal Marker

A yellow and black buoy or post marking a danger either to the north, east, south or west

 

 

Cay

A low, flat island of sand

 

 

Celestial Body

A ‘heavenly’ object - notably the sun, moon, stars and planets

 

 

Celestial Sphere

An imaginary sphere projecting out from earth with an equator and poles in alignment with ours. Latitudinal co-ordinates are called Declination* while celestial longitude is measured as Right Ascension* or Sidereal Hour Angle*.

 

 

Chart

A map of the sea

 

 

Civil Twilight

When the centre of the sun is within 6 degrees of the horizon

 

 

Cold Front

A moving body of colder air replacing warmer air

 

 

Comber

A deep-water wave that breaks due to the force of the wind

 

 

Continental Shelf

Shallow waters close to continents

 

 

Continental Slope

A steep slope connecting the Continental Shelf* with Abyssal* Plain

 

 

Convergence

A line where two different currents or bodies of water meet

 

 

Coriolis Force

A force made by the rotation of earth that deflects particles to the right in the northern hemisphere and to the left in the southern hemisphere

 

 

Co-tidal Line

A line extending from an Amphidromic Point* (as known as tidal node) where all places experience high tide at the same time

 

Crescent Moon

When less than half the moon’s face is illuminated

 

Cross Sea

A swell* that moves across another swell at an angle, often creating rough conditions

 

 

Culmination

The moment a celestial body is highest in the sky each day. This happens when your meridian* lines up with the sun, moon, star or planet.

 

 

Cyclone

A weather system with low atmospheric pressure*

 

 

D 

 

Declination

The celestial equivalent of latitude, measured up to 90 degrees north or south of the celestial equator

 

 

Deviation

Compass error caused by magnetic disturbance nearby

 

 

Diurnal Tide

A tidal cycle with just one high tide a day, notably the Gulf of Mexico and Western Australia

 

 

Dock

The space between two piers

 

Doldrums

A line around the equator generally with low winds but frequent squalls and thunderstorms

Double High Tide

When two high tides happen in close succession, usually 1-​3 hours apart

Downwelling

The process by which surface ocean currents move downwards towards the seabed

 

 

Downwind

The direction the wind is blowing towards

 

 

E

 

 

Ebb Stream

The direction of tidal streams* when the tide is falling

 

 

Ebb Tide

The part of the tidal cycle from high tide to low tide, when the tide is falling

 

 

Ecliptic

The annual journey of the sun around the celestial sphere*

 

 

Eddy

A circular movement of water generally found downstream of obstructions

 

 

Eddyline

The edge of an eddy, indicated by turbulent water where the counter-current from the eddy collides with the main stream

 

 

Elliptical

Egg-shaped

 

 

Equinox

A twice-yearly event when north and south poles are the same distance from the sun (see Solstice*)

 

 

F 

 

Fathom

A measurement of depth; 1 fathom = 6 foot = 1.83cm

 

 

Ferrell Cell

A global weather system between 30-60 degrees latitude

 

Fetch

An area of the sea surface over which wind consistently blows, required to generate a swell*

 

First Quarter

A week after the New Moon*, when the right half of the moon is visible from the northern hemisphere 

Flood Tide

The part of the tidal cycle from low tide to high tide, when the tide is rising

 

Foam Line

The front of wave as it advances towards the shore, after it has broken

Fog

Suspended water droplets in the air that reduce visibility to less than 1 kilometre

 

 

Forel Scale

A scale determining the colour of seawater

 

 

Foreshore

The part of the shore between high and low tides 

 

 

Front

In meteorology, a surface separation between two air masses

 

 

G

 

 

Gale

Wind speeds between 32-37 knots (Force 8)

 

 

Gat

A deep channel between two shoals or banks

 

 

Gibbous Moon

When more than half the moon’s face is illuminated  

 

Greenwich Meridian

An imaginary line on earth from the north pole to south pole, passing through Greenwich in the UK. Longitude is measured east or west of this meridian

 

 

Groundswell

When the period* of a swell* is more than 10 seconds

Gust

A temporary increase in wind speed

 

 

 

Gyre

A closed circulatory system (like a large eddy*) typically made from ocean currents

 

 

 

H

 

 

Hadley Cell

A global weather system between 0-30 degrees latitude

 

H.A.T

‘Highest Astronomical Tide’; the highest tide a place experiences when all astronomical factors coincide, such as a perigean spring tide* at the equinox* when the moon has a low declination*.

 

 

Head Sea

When you are travelling in the opposite direction to the waves

 

High Seas

The open sea beyond any states jurisdiction

High Water

The formal way to describe a day's high tide

Horse Latitudes

Two bands of light winds around the the globe at 30 degrees latitude

 

Hurricane

The name for a tropical cyclone in the North Atlantic and North-West Pacific when wind speeds exceed 58 knots (Force 12)

 

 

Hydrography

A branch of science specialising in the measurement and description of the earth’s seas, oceans, rivers and lakes

 

 

I

 

 

Islet

A small island

Isobar

A line joining points of equal atmospheric pressure

 

 

J

 

 

Jetsam

Objects jettisoned overboard that do not float, as opposed to Flotsam that floats

 

 

K

 

 

Katabatic Wind

A downslope offshore* wind, commonly with powerful gusts

 

 

King Tide

A colloquial name for a perigean spring tide*

Knot

A measurement of speed at sea; 1 knot = 1 nautical mile per hour = 1.85kmh

 

 

 

L 

 

Lagoon

An enclosed body of salt or brackish water separated from the sea by a reef or low bank

 

 

Land Breeze

Another term for an onshore* wind, where wind blows from the sea onto the shore

 

 

Landfall

The first sighting of land when approaching from sea

 

 

Lateral Markers

Red and green buoys or posts that indicate you should pass to the left or right

 

 

L.A.T

‘Lowest Astronomical Tide’; the lowest tide a place experiences when all astronomical factors coincide, such as a perigean spring tide* at the equinox* when the moon has a low declination*.

 

 

Latitude

The angular distance of a place north or south of the equator, measured up to 90 degrees

 

 

Lee Shore

Land that is downwind* of your position at sea

 

 

Lipper

A slight roughness to the sea’s surface

 

 

Lithosphere

The solid, rocky part of earth

 

 

Longitude

The angular distance of a place east or west of the Greenwich Meridian, measured up to 180 degrees

 

 

Longshore Current

A current running along the shore in the surf zone*, leading to a rip current*

 

 

Low Water

The formal way to describe a day's low tide

 

 

Lull

A momentary decrease in the wind speed

Lunar Eclipse

A rare event when the earth blocks the sun's rays from ​illuminating a Full Moon*

 

 

M

 

 

Magnetic North

A constantly moving place that the north needle of compasses point towards 

 

 

Meridian

An imaginary line on earth from north pole to south pole, passing through your position

 

 

Meteorology

Science of the atmosphere

 

 

Meteotsunami

A tsunami with meteorological origins, made by a dramatic drop in atmospheric pressure* along a squall line*

 

 

MHWS

‘Mean High Water Springs’; the average height of high water at spring tides

 

 

MLWN

‘Mean Low Water Neaps'; the average height of low water at neap tides

 

 

Millibar (Mb)

A unit of atmospheric (air) pressure; the average at sea level is 1013 millibar

Mixed Tides

A type of semi-diurnal* cycle where one high tide (or low tide) is higher than the other

 

 

Mole

A massive structure of masonry or large stones extending out to sea

 

 

Monsoon

A seasonal wind of the Indian Ocean

 

 

Moonbow

Similar to a rainbow, but where the light comes from the moon instead of directly from the sun

 

 

N

 

 

Nadir

The point on the celestial sphere* directly beneath the observer (shown by a line drawn from your position through the centre of earth and extending onto the celestial sphere)

 

 

Nautical Twilight

When the centre of the sun is 6-12 degrees below the horizon

 

 

Neaps

Weak tides that happen 36 hours after the First Quarter* and Third Quarter* moon phases

 

New Moon

The beginning of a tidal month, when the moon is directly between the sun and earth

 

 

Noon

The moment your meridian* lines up with the sun, resulting in its culmination

 

 

O

 

 

Offshore Wind

A wind blowing from the land out to sea

 

 

Onshore Wind

A wind blowing from the sea onto the land

 

 

Outflow

The flow of water from a river or estuary out to sea

 

 

Overfalls

Steep breaking waves caused by fast currents colliding with an underwater obstruction and/or opposing wind and swell*

 

 

 

P

 

 

Passage

A narrow channel, commonly through reefs* or between islands

 

 

Peninsula

A piece of land that is almost an island, but is connected to the mainland by a narrow strip or causeway

 

 

Perigean Spring Tide

A powerful spring* tide that happens when the moon is at perigee* 

 

 

Perigee

The moment the moon is closest to earth on its elliptical* orbit

Perihelion

The moment the earth is closest to the sun on its elliptical* orbit

 

Period

The time interval between two consecutive wave peaks

Planet

A celestial body of the solar system reflecting sunlight and revolving around the sun

 

Polar Cell

A global weather system between 60-90 degrees latitude*

 

 

Polaris

The name of a star situated directly above the north pole

 

 

Pontoon

A floating structure that rises and falls with the tide

 

 

Port

The ‘left’ side of a boat when looking towards the front (bow)

 

 

Q

 

 

Quicksand

Sand saturated by water so that it cannot support any weight

 

 

R

 

 

Range Of Tide

The height difference between high and low tide

 

Rate

The speed of a current

Reef

A mass of shallow rock or coral, usually rising steeply from deep water

 

 

Refraction

The ‘bending’ of a wave caused by the changing depths of a seabed

 

 

Relief

The undulations of a coastline or seabed

 

 

Resonance

A bathtub-like phenomena in a bay or estuary where the outgoing high tide merges with the incoming high tide to form an extra high tide 

 

 

Right Ascension

The celestial equivalent of longitude, measured eastwards in hours and minutes (1 hour = 15 degrees / 4 minutes = 1 degree) from the position of the sun on the March equinox*

 

 

Rip Current

A current flowing out to sea in the surf zone*

 

 

Rogue Wave

A wave that is twice the size of the significant wave height (which is the average of the top third of waves at that time)

Roller

An informal name referring to a long-period* wave

 

 

Rotary Current

A place where tidal streams change direction every hour, rather than every 6 hours

 

 

S

 

 

Safe Water Mark

A red and white buoy or post with a single red ball, indicating navigable water all around

Salinity

The quantity of dissolved salts in seawater

 

 

Sandwave

A large wavelike seabed found in shallow waters, made of sand and potentially creating overfalls*

 

 

Saros

An 18-year cycle in which a series of eclipses closely repeat themselves

 

 

Satellite

A small celestial body* orbiting a planet

 

 

Seamanship

The art of handling a vessel

 

 

Seamount

An isolated feature rising to 1,000 metres or more from the sea floor

 

 

Seaward

Away from land

 

 

Seaway

A moderately rough sea

 

 

Seiche

See meteotsunami*

 

 

Semi-Diurnal Tide

A tidal cycle with two high tides a day

 

 

Set 

The direction towards which a current flows, or a group of waves

 

 

Sextant

An instrument for measuring the angle between two objects

 

 

Shelving

A gently sloping seabed

 

 

Shoal

A sudden shallowing of the seabed, usually far from land

 

 

Sidereal Hour Angle

The equivalent of celestial longitude, measured in degrees westwards from the position the sun at the March equinox

 

 

Slack Water

A set time before and after high tide when tidal streams* change direction

 

Solar Eclipse

A rare event when a New Moon* blocks the sun's rays from ​illuminating a place on earth

 

 

Solstice

A twice-yearly event when one pole is at maximum tilt towards the sun. At the June solstice the north pole is closest to the sun and on the December solstice the south pole is facing the sun

 

 

Sound

A broad term describing a large inlet

Speed Over Ground 

Your speed, taking into account the currents (if your speed through water* is 4 knots and the current is 2 knots in the same direction, your speed over ground is 6 knots)

Speed Through Water

Your speed, not taking into account the set* and rate* of currents  

 

Springs

Strong fortnightly tides that happen 36 hours after New Moon* and Full Moon*

 

 

Squall

A sudden and dramatic increase in the wind speed lasting a short time, often accompanied by showers and thunderstorms

 

 

Squall Line

An imaginary moving line along which squalls occur

 

 

Standing Wave

A wave that stays in one place, usually made by a currents flowing down an underwater ramp

Stand Of Tide

A condition at low or high tide when there is no vertical change in the tide

 

 

 

Starboard

The ‘right’ side of a boat when looking towards the front (bow)

 

 

Storm Surge

When a high astronomical tide coincides with low atmospheric pressure* and onshore* winds to create extra high tides that put a coastline at risk of flooding

 

 

Surf Zone

The area between the outermost breaker* and the limit of wave uprush*

 

 

Swash

A narrow channel or sound within a sandbank, or between a sandbank and the shore

Swell

A collection of waves with a similar period*

 

 

Synoptic Chart

A map showing fronts* and isobars

 

 

Syzygy

The momenttwo celestial bodies* are in direct alignment with the sun

 

 

T

 

 

Tail Wind

Wind that blows in the same direction you are travelling

 

 

Thallasophile

Somebody who loves the sea

 

Third Quarter

A week after the Full Moon*, when the left half of the moon is visible from the northern hemisphere

Tidal Diamond

A position on a chart* showing the set* (direction) and rate* (speed) of tidal streams* every hour before and after high water*

 

 

Tidal Streams

Currents flowing back and forth along a shore, generally changing direction every 6 hours at slack water*

 

Tide Gate

A passage* through which water runs with great speed because of the tide

 

Tide Wave

A wave that travels around an ocean or sea (anti-clockwise in the the northern hemisphere) bringing high water* at the peaks and low water* at the troughs

 

Tideway

A channel through which tidal streams* flow

 

Topography

The shape of the coastline

 

 

Tropical Cyclone

A cyclone of tropical origins, usually moving towards the poles and characterised by heavy rains and violent winds

 

 

True North

The north pole, as opposed to magnetic north*

 

 

Tsunami

A powerful wave created by a sudden displacement of water, usually earthquake or volcanic eruption

 

 

U

 

 

Undercurrent

A current below the surface, particularly one travelling in a different direction and speed to the surface current

 

 

Undertow

A seaward* flow of water caused by waves breaking on a steeply sloping beach

 

 

Universal Time (UT)

The time at Greenwich, London. For every 15 degrees of longitude west of this meridian, the time will be 1 hour later.

 

 

Uprush

The surge of water up a beach after the breaking of a wave

 

 

Upwelling

The process by which deep ocean currents move upwards towards the surface

 

 

Upwind

The direction the wind is coming from

 

 

V

 

 

Variation

The difference between magnetic north* and true north*

Veering Wind

A clockwise change in wind direction (opposite to backing*)

 

 

W

 

 

Waning Moon

The period from Full Moon* to New Moon* when the left side is illuminated (as seen from the northern hemisphere)

 

 

Warm Front

A moving body of warmer air replacing colder air

 

 

Waterline

The line marking the junction between land and sea

 

Waterspout

A rotating column on air and spray formed by a whirlwind over the sea or a body of water

Wavelength

The distance between the peaks of two waves

 

 

Waxing Moon

The period from New Moon* to Full Moon* when the right side is illuminated (as seen from the northern hemisphere)

 

 

Weather Shore

A shore that is to windward (upwind*) of your position

Whirlpool

Water in a rapid rotary motion, commonly found along eddylines*

 

 

Whitecap

The crest of a wave which becomes unstable in deep water, toppling over or breaking

 

 

Wind Rose

A star-shaped diagram showing the frequency of wind directions at a specific place

Windswell

When the period* of a swell* is less than 10 seconds

 

 

 

X

 

 

Y

 

 

Z

 

 

Zenith

The point on the celestial sphere* directly above an observer (shown by a line drawn from the centre of earth passing through your position and extending onto the celestial sphere)