Royal Netherlands Meteorological Institute; Ministry of Transport, Public Works and Water Management

Marine Meteorological Glossary
21 November 2000
A Back to the top
Air mass A large volume of air with uniform properties of temperature and moisture.
Anabatic wind A wind that blows up the slope of a hill or mountain due to the sun heating the land.
Anticyclone See high.
Arctic sea smoke A type of fog which is formed when cold air moves over relatively warm water.
Average Hsig Average significant wave height recorded during a storm event.
Average TP1 Average spectral peak period recorded during a storm event.
Average Tsig Average significant wave period recorded during a storm event.
Average Wave Power Average wave power level recorded during a storm event.
B Back to the top
Backing A counterclockwise change in wind direction; opposite of veering
Backwash A term used to describe reflected waves from a steep-to shoreline.
Baroclinic Has vertical structure, in the form of variable density, temperature and winds.
Barotropic Single level, with no vertical structure.
Basin Either an oceanic region with coherent features (e.g. the north Indian Ocean Basin), or a set of bathymetric data for use in numerical models.
Bathymetry The shape of the sea bed.
Beaufort scale A scale of wind force, based on behaviour of the sea under varying degrees of wind speed.
Bore A broken swell wave travelling shorewards across the surf zone. See also tidal bore.
Breaker A wave that has broken into foam.
Breaker Index The ratio between the height of a wave and the water depth in which the wave breaks.
Breaker Zone That area of coastal waters where shoaling effects cause swell waves to break. This typically occurs in the shallow waters over an offshore bar.
Breaking Waves As waves increase in height through the shoaling process, the crest of the wave tends to speed up relative to the rest of the wave. Waves break when the speed of the crest exceeds the speed of the advance of the wave as a whole. Waves can break in three modes: spilling, surging and plunging.
C Back to the top
Calm Beaufort force 0. Wind speed less than 1 knot. Sea like a mirror.
Col The saddle-backed region occurring between two highs and two lows, arranged alternately.
Cold front A boundary separating cold and warm air masses at which the cold air is advancing.
Combined seas or combined waves See Seas.
Convection Describes the vertical air motion that occur when the atmosphere is unstable. Convection often gives rise to the formation of convective cloud (Cumulus, Towering Cumulus and Thunderclouds) and showery precipitation.
Convergence Consider an area on the earth's surface. On the sides which face the wind, air will flow into the area, while on the other sides air will flow out. If, however, the wind is not uniform, more air may flow in than flows out, and the amount of air in the area will tend to increase: this process is called convergence. The air cannot, however, go on accumulating and excess will have to flow out over the top, thus leading to a rising air current, and perhaps to clouds and rain. The contrary case is called divergence.
Coriolis force An apparent force exerted on moving objects due to the earth's rotation.
Cyclone A name given to the tropical revolving storms of the Bay of Bengal and Arabian Sea. Sometimes used as a general term for tropical revolving storms of all oceans, or in the form "tropical cyclone". Depressions of the temperate zones were formerly often referred to as cyclones, but depression or low is now used to distinguish them from the tropical storms. The term "cyclonic depression" is still sometimes used for a depression, as also is "extra-tropical cyclone". 
D Back to the top
Deepwater Water sufficiently deep that surface waves are little affected by the ocean bottom. Generally, water deeper than one-half the surface wave length is considered deepwater.
Depression See low.
Design Wave Height The wave height adopted for the purposes of designing coastal structures such as breakwaters and seawalls. It is chosen to ensure that the structures are not at undue risk of wave damage.
Dewpoint The temperature at which the air, cooled at constant pressure, becomes saturated with water vapour.
Diffraction The "spreading" of waves into the lee of obstacles such as breakwaters by the transfer of wave energy along wave crests. Diffracted waves are lower in height than incident waves.
Directional Waverider Buoy A floating device used to measure ocean wave height, period and direction. It is a registered trademark of the Dutch company Datawell.
Divergence Consider an area on the earth's surface. On the sides which face the wind, air will flow into the area, while on the other sides air will flow out. If, however, the wind is not uniform, more air may flow out than flows in, and the amount of air in the area will tend to decrease: this process is called divergence. In this case there is a deficit of air, which is balanced by a descent of the upper air layers above the area. This descent is called "subsidence". The subsiding air warms up, its relative humidity falls, and fine weather is the usual accompaniment of subsidence, though fog, stratus or stratocumulus clouds may occur under certain conditions. The contrary case is called convergence.
Doldrums The equatorial oceanic regions of calms and light variable winds, accompanied by heavy rains, thunderstorms and squalls. These belts are variable in position and extent, and as a whole move north and south with the annual changes of the sun's declination, although their movement is considerably less than that of the sun and lags behind it.
E Back to the top
Ebb tide The portion of the tide cycle between high water and the following low water. It is sometimes called "falling tide".
Eddy A small rotating area of water.
Electromagnetic Current Meter A device which measures current and water pressure variations. If deployed in shallow water current and pressure data can be converted to wave height, period and direction. Current meters manufactured by the American companies Marsh McBirney and InterOcean are used by MHL to collect wave data.
F Back to the top
Fetch The distance over which wind blows from a constant direction. Wave generation occurs along the fetch.
Flood tide The portion of the tide cycle between low water and the following high water. It is sometimes called "rising tide".
Fog Minute water droplets suspended in the air that reduce surface visibility to less than 1000m. Fog is cloud on the ground.
Freak waves Single waves that may arise from local amplification of intersecting waves, interactions between waves and currents, or development of solitary waves. Of considerable danger to mariners because of their unexpected nature (often coming from an unexpected direction) and potential for developing to great height.
Fresh breeze Beaufort force 5. Sustained wind speeds in the range of 17 to 21 knots. Moderate waves, taking a more pronounced long form; many white horses are formed (chance of some spray).
Front The boundary between two different air masses.
Frontogenesis The development or marked intensification of a front.
Frontolysis The disappearance or marked weakening of a front. Subsidence (see divergence) is the most important factor in causing frontolysis.
G Back to the top
Gale Beaufort force 8. A sustained wind speed of 34 to 40 knots. Moderate high waves of greater length; edges of crests begin to break into spindrift; the foam is blown in well-marked streaks along the direction of the wind.
Gentle breeze Beaufort force 3. Sustained wind speeds in the range of 7 to 10 knots. Large wavelets, crests begin to break; foam of glassy appearance, perhaps scattered white horses.
Gravity wave A perturbation along a vertical discontinuity, or gradient, which has gravity as its restoring force; for example ocean waves, storm surges. In the atmosphere, gravity waves are generated whenever an imbalance between the mass and wind fields develops.
Gust A sudden, brief increase in wind speed, generally lasting less than 20 seconds.
H Back to the top
H1 Average height of the waves that comprise the top 1%.
H10 Average height of the waves which comprise the top 10%.
High A region of high pressure. Air flows outward and (on the northern hemisphere) clockwise around high pressure areas. A high is usually associated with fair weather. Also called "anticyclone".
Hindcast The prediction of wave characteristics using meteorological information as opposed to the measurements of these features.
Hmax Maximum wave height in a recorded burst of raw data.
Hmean Mean wave height.
Horse Latitudes The Horse Latitudes is the name of the belt of calms, light winds and fine, clear weather between the trade wind belts and the prevailing westerly winds of higher latitudes. The belts move north and south after the sun in a similar way to the doldrums. The story of the name, in which was explained that it was an old practice to throw overboard horses, which were being transported to America or the West Indies when the ship's passages were unduly prolonged, was recalled by the Belgian "weather professor" Poppe. The sea area, where there is little or no wind, was named after Sir James Ross, a British naval-officer from last century. In 1870, a German geographer, read about the "Ross-latitudes" and translated "Ross" into the German word "Pferd", which is an understandable error beacause Ross in German is slang for Horse (or Pferd). By translating the "Pferde Breidten" back to English, the "Horse Latitudes" were introduced. 
Hrms Root mean square wave height.
Hsig Significant wave height = average height of the waves which comprise the highest 33% of waves in a given sample period (typically 20 to 30 minutes).
Hurricane Beaufort force 12. Sustained wind speed greater than 63 knots. The air is filled with foam and spray; sea completely white with driving spray; visibility very seriously affected.
I Back to the top
Iceberg A chunk of fresh water ice "calved" from a glacier. Icebergs are harder than pack ice.
Intertropical Convergence Zone The zone separating the wind circulations of the northern and southern hemispheres. Over the Atlantic and Pacific Oceans, where it is closely related to the doldrums, it is the zone between the north-east and the south-east trade winds. It was formerly known as the "intertropical front".
Inversion More fully known as a "temperature inversion". This occurs when warm air resides over colder air. Inversions occur in stable conditions.
Isobar Line on a weather map joining points of equal pressure.
Isotherms Lines drawn through positions with the same temperatures.
K Back to the top
Knot A unit of speed in the nautical system equal to one nautical mile per hour (= 1.852km/hr; = 0.51444 m/s).
L Back to the top
Land breeze A small-scale wind circulation set off by differences in water and land temperatures along the coast. The land breeze develops at night and always blows from the land. Its counterpart is the sea breeze.
Leeward Downwind, situated away from the wind: opposite of windward.
Light air Beaufort force 1. Sustained wind speed between 1 and 3 knots. Ripples with the appearance of scales are formed, but without foam crests.
Light breeze Beaufort force 2. Sustained wind speeds in the range of 4 to 6 knots. Small wavelets, still short but more pronaounced; crests have a glassy appearance and do not break.
Line squall A sudden increase in wind speed, generally lasting from a few minutes up to half an hour. Line squalls are caused by instability of cold air moving over warmer water.
Logger Device for recording digitised data.
Long Wave Waves with periods greater than 30 seconds.
Low A region of low pressure. Air flows inwards and (on the northern hemisphere) counterclockwise around a low. A low centre is usually accompanied by precipitation and strong winds. Also called "depression".
M Back to the top
Micro-climate A detailed description of the climate of a small region.
Millibar The old unit used to measure barometric pressure. Replaced by hectoPascal.
Moderate breeze Beaufort force 4. Sustained wind speeds in the range of 11 to 16 knots. Small waves, becoming longer, fairly frequent white horses.
Moderate winds Wind speeds in the range of 13 to 19 knots.
Monsoon The term originally referred solely to the winds of the Arabian Sea which blow for about six months from the north-east (in winter) and for six months from the south-west (in summer), but now is also used for other marked seasonal winds. The rainy season associated with the south-west monsoon is termed "the monsoon" without reference to the wind.
Mountain wave A lee wave resulting from air flowing over a mountain range.
Murphy's Law Anything that can go wrong, will go wrong. There are two useful corrolaries: 1. The change of something going wrong is directly proportional to the damage that will ensue; 2. Murphy was an optimist.
N Back to the top
Nautical mile The unit of distance in the nautical system. There are 60 nautical miles in one degree of latitude. 1 nautical mile = 1.15 statute miles.
Near gale Beaufort force 7. Sustained wind speeds in the range of 28 to 33 knots. Sea heaps up and white foam from breaking waves begins to blow in streaks along the direction of the wind.
Nearshore Zone Coastal waters between the offshore bar and the 60m depth contour. Swell waves in the nearshore zone are unbroken, but their behaviour is influenced by the presence of the seabed.
O Back to the top
Occlusion The end result of a cold front overtaking a warm front. It marks the dying stages in the life of a low.
Offshore Bar Also known as a longshore bar. Submerged sandbar formed offshore by the processes of beach erosion and accretion. Typically, swell waves break on the offshore bar.
Offshore Zone Coastal waters to the seaward of the nearshore zone. Swell waves in the offshore zone are unbroken and their behaviour is not influenced by the presence of the seabed.
P Back to the top
Pack ice Sea water which freezes and forms into large floes. Also known as sea ice, it is different from icebergs.
Peak Hmax Highest maximum wave height recorded during a storm event.
Peak Hsig Highest significant wave height recorded during a storm event.
Peak Wave Power Maximum wave power level recorded during a storm event.
Plunging Waves The wave crest breaks suddenly and with tremendous force by curling over a near vertical wave face.
Points of sail The headings of a sailboat in relation to the wind, (upwind, close reach, reach, broad reach, downwind.).
Polar front The line of discontinuity, which is developed in suitable conditions between air originating from polar regions and air from low latitudes, and on which the majority of the depressions of temperate latitudes develop. It can sometimes be traced as a continuous wavy line thousands of miles in length, but it is interrupted when polar air breaks through to feed the trade winds, and is often replaced by a very complex series of fronts, or by continuous gradients of temperature.
Pressure The force exerted by the weight of the atmosphere, also known as atmospheric or barometric pressure.
Pressure gradient The difference in pressure between two points divided by the distance between them. On a weather map, the closeness of the isobars is a measure of the pressure gradient.
Pressure gradient wind The wind that results from the pressure gradient. The greater the pressure gradient between two points, the greater the wind. The pressure gradient wind blows parallel to the isobars with (on the northern hemisphere) lower pressure on the left.
R Back to the top
Reflection of waves When waves encounter land that rises steeply from the ocean bottom, they will reflect or bounce off the land rather than break or refract as they would in shallow water. Reflected waves, also known as backwash, will interact with oncoming seas to create confused crossing sea conditions.
Refraction The tendency of wave crests to become parallel to bottom contours as waves move into shallower waters. This effect is caused by the shoaling process which slows down waves in shallower waters.
Return Period Expected average interval between the occurrences of events at a particular threshold.
Ridge (ice) A ridge of ice is caused by ice piling upat a coastline, due to the action of onshore winds.
Ridge (pressure) An elongated area of high pressure.
S Back to the top
Salinity A measure of the quantity of dissolved salts in sea water.
Sea breeze A small-scale wind circulation set off by differences in water and land temperatures along the coast. The sea breeze develops during the daytime and always blows from the sea. Its counterpart is the land breeze.
Seas Combined wind-waves and swell. Mariners often use the term to describe wind waves only. Forecasters will normally use the term "combined sea" or "combined waves" to avoid this ambiguity.
Seastate A description of the properties of the wind-generated waves and swell on the surface of the sea.
Sea Waves Waves in coastal waters resulting from the interaction of different wave trains and locally generated waves. Typically, sea waves are of short wave length and of disordered appearance.
Shallow water Water depths less than or equal to one half of the wavelength of a wave. Therefore, water may be "shallow" for some waves, but not for others.
Shelf Wave Long period waves of low height that travel along the continental shelf and may modify coastal water levels off New South Wales by up to 0.2m. Shelf waves are generated e.g. by the pressure gradients associated with atmospheric disturbances in Bass Strait.
Shoaling The influence of the seabed on wave behaviour. Such effects only become significant in water depths of 60m or less. Manifested as a reduction in wave speed, a shortening in wave length and an increase in wave height.
Significant wave height/period Average height/period of the highest one-third of the waves present.
Solitary wave Single waves, or an isolated train of waves that can be of considerable height and different to the prevailing ocean wave conditions and have been associated with some spectacular maritime incidents. These may take the form of solitons, tsunamis, tidal bores, and tidal rip or hydraulic jumps.
Soliton A single wave, or short wave train that has developed from either a special combination of wind swells or interactions of tides with local bathymetrie. On the surface these consist of a well-defined set of wave crests. Solitons also may develop internal density gradients in the ocean, and are manifested at the surface by a boiling effect.
Spring tide The tide when ranges between high and low water are greatest. It occurs near full and new moons.
Squall A brief, violent windstorm, usually, but not necessarily associated with rain or snow.
Squall line A line of thunderstorms, or other heavy weather, often running parallel to, and ahead of, a cold front.
Stable A non-convective state in the atmosphere, opposite to unstable.
Standing men Steep waves generated when ocean waves propagate against a strong current.
Steep-to An expression used when water depth increases rapidly away from shore or away from shallow underwater features such as a reef.
Storm Beaufort force 10. Sustained wind speeds in the range of 48 to 55 knots. Very high waves with long overhanging crests; the resulting foam, in great patches, is blown in dense white streaks along the direction of the wind; on the whole, the surface of the sea takes on a white appearance; the tumbling of the sea becomes heavy and shock-like; visibility affected.
Strong Breeze Beaufort force 6. Sustained wind speeds in the range of 22 to 27 knots. Large waves begin to form; the white foam crests are more extensive everywhere (probably some spray).
Strong Gale Beaufort force 9. A sustained wind speed of 41 to 47 knots. High waves; dense streaks of along the direction of the wind. crests of waves begin to topple, tumble and roll over; spray may affect visibility.
Storm Event Period of high wave activity. For the New South Wales (Australia) coastline is normally defined as the time when a Hsig greater than 3 metres is recorded at an offshore wave recording station.
Storm surge A change in water level caused by atmospheric pressure and wind. A storm surge can cause floodings, especially when it occurs at the same time as a high tide.
Spilling Waves The wave crest breaks gradually as the wave travels to the shore. Characterised by the appearance of white water at the crest.
Surf Beat Periodic rise and fall in coastal water levels caused by two or more wave trains arriving at the shoreline.
Surf Zone Coastal waters between the breaker zone and the swash zone characterised by broken swell waves moving shorewards in the form of bores.
Surging Waves The wave does not "break" but maintains its basic shape as it moves towards the shore, where it surges up the beach. Very little white water is evident before surging waves reach the shore.
Swash Zone That area of the shoreline characterised by regularly spaced wave crests
Swell Waves Long waves formed by winds blowing over a distant area of ocean (fetch), that travel rapidly over the surface, having a uniform and orderly appearance characterised by regularly spaced wave crests.
Synoptic scale A scale of distance used by meteorologists to describe large weather disturbances. Weather systems which span thousands to millions of square miles and which exist for several days are on the "synoptic scale".
T Back to the top
Tc Crest period = average time between successive crests.
Thunderstorm A local storm, usually produced by a cumulonimbus cloud (thunderhead), and accompanied by thunder and lightning.
Tidal bore Generated in estuaries and rivers with high tidal range and a sharp narrowing and shoaling at the entrance. Manifested as a single wave, which may be breaking, that travels along the river or estuary and is followed by a rise in water level. See also bore.
Tidal race See "Tidal rip".
Tidal range The difference between the heights of high water and low water.
Tidal rip A heavy boil on the sea surface often accompanied by breaking waves. Rips are produced by strong tidal currents over irregular sea bottoms.
Topography The shape of the land.
TP1 Period of the peak of the energy spectrum.
TP2 Period corresponding to the second biggest peak of the energy spectrum.
Trade winds Trade winds are the winds that blow from the tropical high pressure belts (situated at about 30°) towards the equatorial region of low pressure. In the northern hemisphere that is generally from the NE, while in the southern hemispere the trade winds blow from the SE. The trade winds are present over all the oceans except the Indian Ocean, north of the equator. There the monsoons reign the wind pattern. The trade wind areas are between 1000-2000 miles wide. The weather in the trade wind areas varies a lot between different locations. On the east side of the oceans more days with haze occur, while on the western side of the ocean more days with rain are reported. Typical trade wind weather is described as nice, dry, relatively cool for its latitude, stable with NE'ly winds (SE'ly in the southern hemisphere) force 4-5, small "nice weather" cumili. This type of weather is only found in the hearts of the trade wind areas. Towards the equator the heating of the seawater causes the air to become unstable and form large convective clouds like cumulus congestus.
Tropical depression A closed-circulation low pressure area of tropical origin with maximum sustained winds of less than 34 knots.
Tropical storm A storm of tropical origin with maximum sustained winds in the range of 34-63 knots.
Trough An elongated area of low pressure, often associated with a wind shift and showery weather.
Trowal A through of warm air aloft. The stage in an occlusion process where the warm air lifts completely off the surface. The significant weather stays with the trowal as it pulls away from the parent low pressure centre.
Tsig Significant period = average period of the waves used to define Hsig.
Tsunami Long period ocean wave generated by an earthquake or volcanic explosion; travels through deep ocean with height of a few cm and a speed of 200-300 m/s; on reaching shallow coastal regions the wave slows and grows to huge heights of potentially 60-70 m. These are often incorrectly referred to as tidal waves.
Typhoon A name given to the tropical revolving storms of the China Sea and the North Pacific Ocean.
Tz Zero crossing period = mean period.
U Back to the top
Unstable A turbulent state in the atmosphere, often caused by cold air moving over warm air. Unstable conditions are also induced by cold air moving over warm water and by strong heating of the ground by the sun. Convection develops when the atmosphere is unstable.
V Back to the top
Veering A clockwise change in wind direction. Opposite of backing.
Violent storm Beaufort force 11. Sustained wind speeds in the range of 56 to 63 knots. Exceptionally high waves (long and medium-sized ships might be for a time lost to view behind the waves); the sea is completely covered with long white patches of foam lying along the direction of the wind; everywhere the edges of the wave crests are blown into foam; visibility affected.
W Back to the top
Warm front A boundary separating cold and warm air masses at which the warm air is advancing.
Waterspout A small whirling storm over water which can either be spawned from the base of a thunderstorm, or formed in a cold outbreak of Arctic air. They are similar, but generally no as severe as tornadoes.
Wave Direction The direction from which ocean waves approach a location. Generally, the principal wave direction is represented by the direction which corresponds to the peak period of the energy spectrum (TP1).
Wave height or amplitude The vertical distance between a wave crest and a trough.
Wave length The horizontal distance between two successive wave crests.
Wave period The time taken for two wave crests to pass a fixed point.
Wave Power The rate at which wave energy is transmitted in the direction of wave propagation. Normally expressed in kilowatts per metre of wave crest length.
Waverider Buoy A floating device used to measure water level variations caused by ocean waves. It is a registered trademark of the Dutch company Datawell.
Wave Runup The vertical distance above mean water level reached by the uprush of water from waves across a beach or up a structure.
Wave Setup The increase in water level within the surf zone above mean still water level caused by the breaking action of waves.
Wave steepness The ratio of wave height to wave length.
Wave Train A series of waves originating from the same fetch with similar wave characteristics.
White Squall A white squall is the culprit of many sea stories and blamed for quite a few tragedies. It is described as a sudden increase in wind velocity in tropical and sub-tropical waters, and lacks the usual dark, ominous squall clouds. The white squall may be myth, or it may be a microburst. If they form during daylight you might see the approach as a line of broken water or whitecaps rushing at your vessel, but usually they appear out of nowhere.

"The Pride Of Baltimore, a fine 137 foot schooner, was reportedly struck by a white squall. The 121-ton vessel sank about 240 miles north of Puerto Rico, casting the surviving crew members adrift for five days. The Toro, a Norwegian freighter picked them up at 2:30 a.m. May 19th, 1986."

Here is an eyewitness account of the sinking: "A tremendous whistling sound suddenly roared through the rigging and a wall of wind hit us in the back. The Pride heeled over in a matter of seconds. The 70-knot wind pushed a 20 foot high wall of water into the starboard side. She sank in minutes."

A graphic shows what a microburst is. While the graphic shows what a microburst can do to an airplane that's taking off or landing and much of the research into microbursts was prompted by the danger to aviation, microbursts have caused other kinds of damage on the ground and I know of at least one case in which a microburst overturned a boat, killing 11 people. This accident occurred on July 7, 1984 when a sternwheeler tour boat, with 18 people on board, capsized on the Tennessee River south of Huntsville, Ala. A weather station about five miles away measured a sudden 70 mph wind gusts and winds flattened trees and overturned sheds near the where the boat capsized. These sudden winds were probably faster than 70 mph when they hit the boat. This accident is one that the late Ted Fujita examined in detail. It's described in his book, The Downburst: Microburst and Macroburst, published by the University of Chicago in 1985.

(Jack Williams, Weather editor, 3-20-00)
Wind The horizontal movement of air relative to the earth's surface.
Wind Setup The increase in mean sea level caused by the "piling up" of water on the coastline by wind.
Windward Upwind, or the direction from which the wind is blowing: the opposite of leeward.
Wind Waves The waves initially formed by the action of wind blowing over the sea surface. Wind waves are characterised by a range of heights, periods and wave lengths. As they leave the area of generation (fetch), wind waves develop a more ordered and uniform appearance and are referred to as swell or swell waves.
Wirlwind General term for a small-scale rotating column of air. They can be caused by uneven frictional effects.