Navis.gr - Terms Used in Marine Meteorology

A Short Glossary of Terms Used in Marine Meteorology

Air Mass	A mass of air which is largely homogeneous in a horizontal direction. Its physical properties are determined by the nature of the surface over which it forms, and may be subsequently modified when the air mass moves over a different type of surface. Air masses are often separated from each other by frontal surfaces, which are discontinuities.
Anemometer	An instrument for determining the velocity or speed of the wind.
Anticyclone	A region characterized in the barometric pressure distribution by a system of closed isobars, with the highest pressure on the inside. It is also known as a 'high'.
Anti-Trades	In trade-wind regions at a height of 6000 feet or more above the surface the wind direction is sometimes reversed, giving, for example, a S.W. wind on the Peak of Teneriffe. These winds are believed to be the return currents carrying the air of the trade-winds back to higher latitudes, hence they are termed 'anti-trades', or 'counter-trades'. but they are not regularly developed.
Aurora	Bright streamers of light, ascending from the horizon towards the zenith, or luminous arcs, which are manifestations of electrical energy in the upper atmosphere. The aurora is seen in both hemispheres, in high and sometimes in medium latitudes. In the northern hemisphere it is known as Aurora Borealis, in the southern as Aurora Australis.
Backing	A change in the direction of the wind, in an anti-clockwise direction.
Blizzard	A high wind accompanied by great cold and drifting or falling snow.
Brave West Winds	See Roaring Forties.
Col	The saddle-backed region occurring between two anticyclones and two depressions, arranged alternatively.
Cold Front	The boundary line between the advancing cold air at the rear of a depression and the warm sector. Line squalls may occur at the passage of this front, which was formerly called the squall line.
Cold Sector	That part of a depression occupied by cold air on earth's surface.
Convection	In convection, heat is carried from one place to another by the bodily transfer of the matter containing it. In particular, this is the method by which heat raises the temperature of a fluid mass. That part in close contact with the heating agent expands and rises, moving away from it, while colder fluid moves in to take its place. This action in the atmosphere gives rise to convectional currents which may produce cumulus or cumulonimbus cloud.
Corona (ae)	A series of coloured rings round the sun caused by diffraction of the light by water-drops, chiefly of alto-clouds. The innermost ring is usually a brownish red and this is often the only one visible. Within it there is a clear space, which, with this inner ring, forms the 'aureole'. When the corona is fully developed there is a violet ring outside the brownish red ring, followed by blue, green, yellow and red rings outwards. Additional series of coloured rings in the same order, are occasionally seen on the outside of the first set, forming a double or triple corona. (See Halo).
Corposants	Luminous brush discharges of electricity, sometimes observed at the mastheads and on projecting parts of ships during electrical storms. Also known as St. Elmo's Fire. Due to atmospheric electricity.
Cyclone	A name given to the tropical cyclones of the Bay of Bengal and the Arabian Sea. Sometimes used as a general term for tropical cyclones 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'.
Dangerous Quadrant	The forward quadrant of the dangerous semi-circle of a cyclone, which before recurvature is nearer the pole (in both hemispheres).
Depression	A region characterised in the barometric distribution by a system of closed isobars, having lowest pressure on the inside.
Dew	Water drops deposited by condensation of water vapour from the air, mainly on horizontal surfaces cooled by nocturnal radiation.
Dew Point	The temperature to which air can be cooled without causing condensation.
Diffraction	The diversion of a ray of light from the straight path by a material obstacle. Thus light may be diffracted by water drops, producing coronae (q.v.).
Diurnal Variation	This term is used to indicate the changes, in the course of an average day, in the magnitude of a meteorological element. The most striking example of this is the diurnal variation of barometric pressure in the tropics, the chief component of which has a 12-hourly period. The maxima of this variation are about 10 a. m. and 10 p.m., the minima about 4 a.m. and 4 p.m. local time.
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.
Eddy	A name given to the deviation from steady motion which occurs in any viscous fluid which flows past an obstacle, or in which neighbouring streams flow past or over each other. Air and water eddies are formed over and around a ship as she moves along.
Eye of Storm	The calm, central area of a tropical cyclone. The most noticeable feature of this area is the sudden drop in wind from hurricane force to light unsteady breezes or even to a complete calm, with more or less cloudless sky and absence of rain. Over the ocean the sea in the eye of the storm is usually very high and turbulent.
Front	The line of separation at the earth's surface between cold and warm air masses.
Frontogenesis	The development or marked intensification of a front.
Frontolysis	The disappearance or marked weakening of a front. Subsidence is the most important factor in causing frontolysis.
Further Outlook	A statement in brief and general terms appended to a detailed forecast and giving the condition likely to be experienced in the 24 hours or more following the period covered by the actual forecast.
Gust	A comparatively rapid fluctuation in the strength of the wind, characteristic of winds near the surface of the earth. Gusts are mainly due to the turbulence or eddy motion arising from the friction offered by the ground to the flow of the current of air. (See Squall).
Hail	Hard pellets of ice, of various shapes and sizes, and more or less transparent, which fall from cumulonimbus clouds and are often associated with thunderstorms.
Halo	Halo phenomena constitute a large group of phenomena produced by the refraction or reflection of the light of the sun or moon by the ice crystals composing cirrus or cirrostratus cloud. The commonest is the halo of about 22° radius round the sun or moon; other fairly common forms are mock suns (or moons), sun pillars, the halo of 46° radius, etc. (See Corona).
Horse Latitudes	The belts of calms, light winds and fine, clear-weather between the trade-wind belts and the prevailing westerly winds of higher latitudes.
Hurricane	A name given to the tropical cyclone of the West Indian region. Also applied to force 12 in the Beaufort scale, whatever its cause.
Intertropical Convergence Zone	The zone of separation between the wind circulations proper to the northern and southern hemispheres. Over the Atlantic and Pacific Oceans, where it is closely related to the Doldrums, it is the boundary between the north-east and south-east trade-winds.
Inversion	An abbreviation for 'inversion of temperature gradient'. The temperature of the air, generally decreases with increasing height but occasionally the reverse is the case; when the temperature thus increases with height there is said to be an inversion. When an inversion exists at lower levels fog, often occurs.
Isallobars	Isallobars are lines drawn upon a chart through places at which equal changes of pressure have occurred in some period of time. Lines of equal change, or isallobars, are drown to enclose regions of rising or of falling pressure.
Isobars	Lines drawn through positions having the same barometric pressure, when reduced to sea level.
Isotherms	Lines drawn through positions having the same temperatures.
Katabatic Wind	A wind that flows down slopes, usually at night. The air at the top of the slope is cooled a greater amount by radiation than the air lower down, becomes heavier, and flows down the slope under the influence of gravity. The opposite of katabatic is anabatic, applied to a wind blowing up a slope, if it is caused by the convection of heated air.
Land and Sea Breezes	These are caused by the unequal heating and cooling of land and water under the influence of solar radiation by day and radiation to the sky at night, which produce a gradient of pressure near the coast. During the day-time the land is warmer than the sea and a breeze, the sea-breeze, blows onshore; at night and in the early morning the land is cooler than the sea and the land-breeze blows offshore. The land-breeze is usually less developed than the sea-breeze.
Line Squall	A more or less violent squall, accompanying the passage of the cold front of a depression, distinguished by a sudden or rapid rise of the wind strength; a change of wind direction, a rapid rise of the barometer and a fall of temperature. There is usually heavy rain or hail, sometimes a thunderstorm or snow. The accompanying low black cloud forms a line or arch.
Local Winds	Winds that are prevalent in particular areas at particular times and have special features. Usually they have special names. The Bora, the Pampero, the Mistral, the Levanter and the Sumatra are examples.
Looming	The vertical extension of an object due to abnormal refraction, making it appear unusually tall. The word 'loom' is also used in such expressions as 'the loom of a light' or 'the loom of the land' when the light or the land cannot be seen directly, but its presence is inferred from reflections seen in the sky, or from other optical effects.
Luminescence	A luminous appearance of the sea, mainly caused by biological processes in microscopic organisms.
Mirage	The appearance of one or more images of a terrestrial object in the sky; also all forms of distortion of objects due to abnormal refraction.
Monsoons	Seasonal winds, those in the Indian Ocean, China Sea, and off the West Coast of Africa being the principal examples.
Occlusion, occluded depression	When the whole of the warm sector of a depression has been pushed up from the earth's surface by the advance of the cold front behind it, this is known as an occlusion, and the depression in which it occurs is called an occluded depression.
Orographic Rain	Rain caused by the interference of rising land in the path of moisture-laden air. A horizontal air current striking a mountain slope is deflected upwards and the consequent dynamical cooling associated with the expansion of the air produces cloud and rain, if the air contains sufficient aqueous vapour. Banner clouds, such as the 'Table Cloth' over Table Mountain, 'Tursui' over Mount Fuji, and the cloud over the Rock of Gibraltar during the Levanter, are examples of orographic cloud.
Polar Front	The line of discontinuity, which is developed in suitable conditions between air originating in 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.
Precipitation	Any aqueous deposit in liquid or solid form, derived from the atmosphere. The precipitation at a given station during a given period includes not only the rainfall but also dew and the water equivalent of any solid deposits (snow, hail, or hoar frost) received in the rain-guage.
Psychrometer	An alternative name for the dry-and wet-bulb hygrometer. In the aspirated psychrometer, a definite rate of ventilation is secured by drawing the air over the bulbs by the agency of a fan.
Radiation	The passing of heat from one body to another by electromagnetic waves. Radiated heat should not be confused with convected or conducted heat. The heat of the sun is radiated through 'empty' space, where convection and conduction would be impossible.
Rainbow	An arch of coloured light ill the sky, made by raindrops breaking up the white light of the sun into its component colours. The colours of the rainbow are, from the outer to the inner edge, red, orange, yellow, green, blue, indigo and violet. The radius of the bow is 42°. One or more narrow, faint supernumerary bows may be seen inside the main bow, touching it. A secondary bow, less bright than the primary bow, and with colours reversed, is often seen about 9° outside it.
Recurvature of Storm	This expression refers to the recurvature of the track of a tropical cyclone, which is a typical feature of the great majority of these phenomena. It is also known as the 'recurve'. In the northern hemisphere a tropical cyclone, after preceding in a more or less westerly direction, recurves and normally takes a north-easterly direction; in the southern hemisphere the final direction is normally south-easterly.
Ridge	An extension of an anticyclone or high-pressure area shown on a pressure chart, corresponding with a ridge running out from the side of a mountain.
Roaring Forties	A nautical expression for the region of westerly winds in south temperate latitudes, which reach their greatest development south of 40°S. A general term for the prevailing westerly winds in the temperate latitudes of both hemispheres is Brave West Winds.
St. Elmo's Fire	See Corposants.
Saturation	A given volume of ordinary air which is exposed to a plane surface of water or ice has for a given temperature a definite saturation pressure of water-vapour; this saturation pressure increases rapidly with increasing temperature. A fall of temperature would lead to condensation of some of this water-vapour, while a rise of temperature would make the air unsaturated and therefore able to take up more water-vapour.
Scud	A word used by sailors to describe ragged fragments of cloud drifting rapidly in a strong wind, often underneath rain clouds. The meteorological term is Stratus fractus.
Sea Breeze	See Land and Sea Breezes.
Secondary Depression or 'Secondary'	The isobars around a depression are frequently not quite symmetrical; they sometimes show bulges or distortions which are accompanied by marked deflections in the general circulation of the wind in the depression; such distortions are called secondaries; they may appear merely as sinuosities in the isobars, but al other times they enclose separate centres of low pressure and show separate wind circulations from that of the parent depression.
Shower	In describing present or past weather, the following distinction is made between the use of the terms 'showers' and 'occasional precipitation'. In general, showers are of short duration, and the fair periods between them are usually characterised by definite clearance of the sky. The clouds which give the showers are, therefore, isolated. The precipitation does not usually last more than 15 minutes, though it may occasionally last for half an hour or more. Occasional precipitation, on the other hand, usually lasts for a longer time than the showers, and the sky in the periods between the precipitation is usually cloudy or overcast.
Sleet	Precipitation of snow and rain together, or of melting snow and rain.
Snow	Precipitation of ice crystals of feathery or needlelike structure. The crystals may fall singly, or a large number of them may be matted together in the form of large flakes.
Squall	A strong wind that rises suddenly, lasts for some minutes, and dies away comparatively rapidly. It is frequently, but not necessarily, associated with a temporary change of direction. (See Gust).
Squall Line	See Cold Front.
Stratosphere	The region of the atmosphere immediately above the troposphere (q.v.). In the lower stratosphere temperature may continue to decrease with increase of height (but more slowly than in the troposphere) or may remain practically constant, or may increase with height. The transition front troposphere, to stratosphere, judged by change of temperature with height, is not always abrupt. At greater heights are other regions with special characteristics, e.g. (a) the ozonosphere, where the concentration of ozone gas is greatest, centred at a height of about 20 miles; (b) the ionosphere, the highly-electrically conducting region of ionised gases, extending upwards from the height of 50 or 60 miles. This region plays an important part in radio propagation. The main subdivisions of this region in order of increasing height are usually referred to as the D. E. (or Kennelly-Heaviside), F (or Appleton) regions or layers.
Subsidence	Descent of air over a wide area, associated with a developing ridge or anticyclone. The subsidising air warms up, its relative humidity falls, and fine weather is the usual accompaniment of subsidence, though fog may occur under certain conditions.
Swell	This is a wave motion in the ocean caused by a disturbance which may be at some distance away; the swell may persist after the originating cause of the wave motion has ceased or passed away.
Synoptic	An adjective derived from the noun 'synopsis', a brief or condensed statement presenting a combined or general view of something. Thus a synoptic chart shows the weather conditions over a large area at a given instant of time.
Tendency of the Barometer	The amount of change in barometric pressure in the 3 hours preceding the time of observation. The characteristic of the tendency is the type of change during the same period, e.g. 'rising', 'falling at first then rising', 'steady', etc.
Thunder	The noise made by an electric discharge (lightning) from charged raindrops in a cloud to another cloud (or other part of the same cloud) or to the earth, or to the air surrounding the charged cloud. Sound travels 1 mile in about 5 seconds, while the lightning flash is seen almost as soon as it occurs, hence the interval of time between the two will give the distance from the observer.
Tornado	A violent whirl, generally cyclonic in sense, averaging about 100 miles in diameter and with an intense vertical current at the centre. Associated winds may attain speeds of about 200 knots. Heavy rain, and generally thunder and lightning, occur with the tornado. The term 'tornado' has also been used for thunderstorm squalls in West Africa.
Trade-Winds	The name given to the winds which blow from tile sub-tropical high-pressure belts towards the equatorial region of low pressure from the N.E. in the northern hemisphere and from the S.E. in the southern hemisphere. The name originated in the nautical phrase 'to blow trade' meaning to blow in a regular course or constantly in the same direction.
Troposphere	The lower region of the atmosphere throughout which temperature in general decreases as height increases, and within which occur practically all clouds and the various other phenomena normally styled 'weather'. The upper boundary of the region is known as the tropopause. The height of the tropopause varies with the latitude from all average of about 5.5 miles in polar regions to about 11 miles at the equator, but the height also varies from summer to winder and with the general meteorological situation. (See Stratosphere).
Trough	The trough line of a circular depression is the line, through the centre, perpendicular to the line of advance of the centre. During the passage of a depression over any given place the pressure at first falls and later rises; the trough line passes over the place during the period of transition from the falling to the rising barometer. The word trough is also used in a more general sense for any 'valley' of low pressure, and is thus the opposite of a 'ridge' of high pressure.
Typhoon	A name given to the tropical cyclones of the China Sea and the west part of the North Pacific Ocean.
Veering	A change in the direction of the wind, in a clockwise direction.
Visibility	A term used in describing the transparency of the atmosphere, and defined by the maximum distance at which a suitable object can be seen.
Warm Sector, Warm Front	Most depressions in their earlier stages have an area of warm air on the side nearest the equator known as the warm sector. The warm front is the boundary between the front of the warm sector, as the depression advances and the colder air in front of it.
Waterspout	An air whirl, normally with a funnel-shaped cloud projecting downwards from a cumulonimbus cloud, accompanied by an agitation of the sea surface beneath it, and the formation of a cloud of spray. The complete waterspout is formed when the funnel-shaped cloud has descended far enough to join up with the cloud of spray, The spout then assumes the appearance of a column of water.