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)