French-English lexicon of the main terms used in the weather reports

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High Warning or SWS Breezes Fog Normal weather report Centres of action Cyclogenesis Tropical cyclone Low Thermal low Shallow low Complex low Tropical low Ridge line Landwards maritime flow State of the sea Flow Disturbed flow Wind force Warm front Cold front Secondary cold front Alternating cold front Pressure gradient Squalls Swell Reliability index Instability Isobar Flat low Wind sea Easterly wave Occlusion Hurricane Disturbance Gust Regime Warm sector GMDSS Storm Tropical storm Actual weather Subsequent trend Trough line Rear of a depression Water-pillar Mean wind Cyclonic winds Visibility ZCIT

 

 

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  Anticyclone - high
High-pressure zone compared with the surroundings represented by a closed isobar. A high is said to form, to strengthen or weaken and then to dissipate or die out. It is often stable over time and not very mobile. In the northern hemisphere, the winds flow clockwise around the high-pressure systems.
  Warning or SWS - warning
A text drafted and issued as soon as current or forecasted weather conditions present a danger for navigation. In temperate regions, these warnings cover : - Strong wind warnings or SWS (Special weather statement); - Strong swell warnings; - Cancellation notices concerning a dangerous phenomenon forecast. SWS include coastal SWS, open-sea SWS, high-seas SWS depending on whether they are associated with coastal, open seas or high seas reports. Coastal SWS are issued as soon as the observed or forecasted winds reach or exceed Beaufort force 7. Coastal SWS : - near gale warnings (force 7), - gale warnings (force 8) strong gale warnings (force 9), - storm warnings (force 10), - severe storm warnings (force 11), - hurricane warnings (force 12). Open-seas SWS and high-seas SWS are issued as soon as the observed or forecasted winds reach or exceed force 8. They are generally not issued more than 24 hours before the start of the gale. A gale is said to be "occurring" if it exists at the time of the issuance of the report, "imminent" if it is to arise within 3 hours following the issuance of the report or "forecasted" as of a time indicated in the text. SWS are issued when there is little or no doubt concerning the weather situation. In general, the threat of a gale or storm will have already been mentioned in the "general weather situation" and "subsequent trends" of preceding reports. A cancellation notice (coast, open seas, high seas) is issued to cancel all or part of a SWS when the dangerous phenomenon announced will not occur or if it is to finish earlier than forecasted.
  Breezes - Breeze phenomena - Breeze regime
Situation characterised by alternating sea breezes (daytime) and land breezes (night-time). Breezes are essentially significant during sunny weather (not necessarily anticyclonic) and unstable air
  Fog
Water drops suspended in the atmosphere reducing visibility. In marine weather reports, the term "fog" is used when visibility is reduced to less than 0.5 mile (1 km). See Visibility also.
  Regular report
Text drafted and issued at specific hours. In France, regular marine weather reports generally comprise a reminder of strong wind warnings, a description of the general situation, a forecast whose range varies according to the type of report and possibly semaphore observations. We distinguish between the following reports : - "Beach" report : >From June 1 to September 30, talking summary reports provide weather conditions, wind, air and sea temperature, and sea state at different points of the coastal departments. - "Funboard report : This report describes in detail the measured wind and its evolution in the coming hours at all the different "spots" in the department. - "Shoreline" report : departemental report intended for users on the seashore and covering a coastal strip up to 2 miles offshore, beaches and any inland lakes in the department. It is drafted three times per day or more if required by the conditions, and is issued by answering machine and Minitel. - "coastal" report : report covering the coastal strip running the length of the French coastline (including Corsica) and extending up to 20 miles offshore. The "coastal" reports are drafted twice daily and are broadcast by VHF. - "Open seas" report : its range extends from the coast to 200 to 300 miles offshore. It is broken down into forecast zones each with a name. Drafted twice daily, it is broadcast over SSB radio, by Radio France and the Navtex system. - "high seas" report : The high seas reports cover all of the oceans. Drafted once or twice daily, they are broadcast over the Inmarsat-C satellite network and by Radio France Internationale. The "shoreline", "coastal" and "open seas" reports issued by telephone and Minitel comprise a 7-day forecast with a reliability index varying from 1 to 5, with 5 being the highest level of reliability. The "coastal", "open seas" and "high seas" reports comprise a detailed forecast limited to a 24-hour range and a concise subsequent trend focusing on dangerous phenomena (gale threats).
  Centres of action
vast highs or lows that control a part of the atmospheric circulation.
  Cyclogenesis
Deepening or formation of a fairly significant low.
  Tropical cyclone - hurricane
Under tropical latitudes, vorticity phenomenon leading to mean winds equal or exceeding 64 knots (force 12). The Anglo-Saxon expression "tropical cyclone" refers to any vorticity phenomenon regardless of its intensity.
  Low
Zone in which pressure is low with respect to its surroundings and represented by a closed isobar. The pressure gets lower as the centre is reached. A low is said to deepen or, on the contrary, to fill. In the northern hemisphere, the winds circulate counter clockwise around a low.
  Thermal low
Zone in which pressure is low with respect to its surroundings and represented by a closed isobar. The pressure gets lower as the centre is reached. A low is said to deepen or, on the contrary, to fill. In the northern hemisphere, the winds circulate counter clockwise around a low.
  Shallow low
Low whose pressure in the centre remains high in absolute terms (higher than 1010 hPa for example) but lower than in the surroundings.
  Complex low
Depressionary zone that is usually quite extensive with several centres.
  Tropical low
Under tropical latitudes, vorticity phenomenon leading to winds lower than 34 knots.
  Ridge
High pressure axis extending a high. Like a high, a ridge forms a barrier creating an obstacle to the passage of disturbances; however, this barrier is not always impassable. It can also be mobile. A ridge is said to form, to develop in a zone or along an axis or to dissipate.
  Landwards maritime flow
Refers to the arrival on the coast of a maritime air mass that is humid. It results in a drop in temperature during summer and sometimes in a decrease in visibility.
  Sea state
State of the surface resulting from the superimposing of swell and waves caused by the wind (wind sea). To describe the sea state, we don't use the maximum wave height (too high) or the mean height (too low) but the mean height of a third of the highest waves. This characteristic height is called H1/3.
  Flow
Air flow at a large scale. The flow direction is given by the isobar orientation. On the ground, due to the relief and thermal effects, the local wind can be quite different from flow.
  Disturbed flow
Atmospheric flow transporting fronts that temporarily "disturb" the wind direction. Hence, in a disturbed westerly flow, the wind will come from the south-west at the leading edge of the front and from the north-west at the rear.
  Wind force
Mean wind force is generally expressed in degrees of the Beaufort scale. Gust speed is always expressed in knots. In weather reports, the mean wind force is also qualified by an adjective ranging from light to severe. There are no standardized definitions for corresponding wind forces. We can nevertheless put forward the following correspondence : - Light wind = 0 to 6 knots (Beaufort force 0 to 2 ) - Moderate wind = 7 to 21 knots (force 3 to 5) - Fairly strong wind = 22 to 27 knots (force 6) - Strong wind = 28 to 40 knots (force 7 and 8) - Very strong wind = 41 to 47 knots (force 9) - Severe wind = above 47 knots (force 10 or more). On meteorological maps, a wind arrow indicates the wind direction. It has barbs on it : a small for five knots and a big one for ten knots. A triangle replaces the barbs to indicate fifty knots.
  Warm front
Separation surface between two air masses of different temperatures, with the warm air pushing back the cold air. On weather maps, the intersection of this surface with the ground is represented by a line of half-circles (red). The arrival of a warm front is generally indicated by an increase in cloud cover, arriving rain then by a rotation of winds to the south (in the northern hemisphere).
  Cold front
Separation surface between two air masses of different temperatures, with the warm air pushing back the cold air. On weather maps, the intersection of this surface with the ground is represented by a line of triangles (blue). The arrival of a cold front is generally signalled by a strengthening of the wind and a temporary strengthening of precipitation. Often, at the rear of a cold front, the wind is irregular in force and direction, clearings appear, the clouds become cumuliform, visibility improves and precipitation takes the shape of showers. This is when marine squalls occur (rear zone sky). The cold front associated with a disturbance progressing eastwards at the latitudes of the British Isles or of northern France triggers, 8 or 9 times out of 10, the Mistral and Tramontana when it hits the south-eastern France. Since the air mass dries out while crossing France, a cold front is rarely accompanied by precipitation in the Mediterranean.
  Secondary cold front
In the cold air of the disturbance, a secondary cold front materializes the arrival of an even colder air mass and results in a strengthening of the squalls.
  Waving cold front
Cold front with waves. The presence of this type of front is always indicated in the reports since there is often a risk of a deepening of a small low at the "peak" of the wave which could lead to a worsening of the weather.
  Pressure gradient
Spacial variation in pressure. The higher the horizontal pressure gradient (big difference in pressure between two close points with the same altitude) the stronger the wind. On weather maps, a strong pressure gradient is shown by tightly-packed isobars.
  Squalls
A severe gust of wind lasting at most several minutes that starts and ends suddenly. This phenomenon is generally linked to the presence of large cumulus or cumulonimbus clouds, characterised by a sudden but passing worsening of the weather : severe precipitation, sometimes stormy, strong gusts of wind exceeding the mean wind by at least 15 knots. It's under the cloud's wind that the strongest gusts can occur.
  Swells
In a given area, waves not caused by the local wind but formed elsewhere and propagated Swells always have a more regular, less chaotic appearance than the wind sea.
  Reliability index
Index giving information concerning the reliability index that the forecaster attributes to his forecast. The reliability index ranges from 1 to 5, with 5 being the highest level. The marine weather reports issued by telephone, fax, Internet and Minitel comprise two reliability indexes, one for the forecast of days D + 4 and D + 5, and the other for the forecast of days D + 6 and D + 7.
  Unstable (unstable atmosphere, unstable air or unstable flow)
The atmosphere is unstable when the low altitude layers are abnormally warm compared with the medium and upper layers of the atmosphere. An unstable atmosphere is the site of vertical movements that cause gusts, improve visibility and favour clouds with a vertical development (cumulus, cumulonimbus), generators of precipitation in the shape of showers. Conversely, stable air "sticks" to the sea : the wind is steady, visibility is mediocre or poor and the clouds form a layer
  Isobar
Line connecting points on the earth's surface with the same pressure to sea level at a given moment.
  Flat low
Vast zone where the pressure varies little from one point to the next. There is either no wind or light winds in variable directions. Flat lows are situations conducive to the development of stormy cumulonimbus, and the formation of mist and fog.
  Wind sea
In a given area, the waves caused by the prevailing wind at that spot and at that moment. The wind sea always appears more chaotic than swells.
  Easterly wave or tropical wave
Trough line propagating from east to west in a trade wind flow and giving rise to a line of squalls.
  Occlusion (or occluded front)
Front resulting from the meeting of a warm front and a cold front. The warm sector was pushed up into altitude but continues to generate precipitation.
  Hurricane
Phenomenon giving rise to force 12 winds.
  Disturbance
In general, all meteorological phenomena causing a worsening of the weather. Under our latitudes, it is used to designate the combination of warm front, warm sector, cold front and possibly occlusion. We also use it to designate the cloudy zone associated with this combination, or even the cloudy zone associated with an isolated cold front. Each disturbance is associated with a low; but a part of the cold front continues outside of the low.
  Gust
Sudden and passing strengthening of the wind. A gust can reach a speed 50% higher than the mean wind speed. The speed of a gust is expressed in knots, the Beaufort scale being reserved for mean wind. French weather reports use the term "gust" when the instantaneous speed exceeds mean wind speed by 10 to 15 knots; "strong gust" when the difference reaches 15 to 25 knots and "severe gusts" when it exceeds 25 knots.
  Regime
Synonym of "conditions" (breeze regime, high regime, westerly regime, etc.).
  Warm sector - warm air
The relatively warm air mass between the warm front and the cold front is called the warm sector of the disturbance.
  GMDSS
Global Maritime Distress and Safety System. International regulation governing the procedures and equipment, on land or aboard ship, required for the safety of ships at sea. The broadcasting of weather reports (Inmarsat-C and Navtex) is regulated by the GMDSS.
  Storm
Under temperate latitudes phenomenon causing winds of Beaufort force 10.
  Tropical storm
Under tropical latitudes phenomenon causing winds of Beaufort force 8 to 11 .
  Actual weather
Describes the predominant weather conditions in a given zone : rain, showers, squalls, hail, fog, snow, thunderstorm, etc. sometimes with a notion of duration : at times, temporary, occasional, dissipating or dying out, etc.
  Subsequent trends
Last section of some scheduled weather reports. It schematically describes the evolution in the forecasted situation beyond the next 24 hours while focusing on dangerous phenomenon.
  Trough
Axis of low pressures extending a low. The isobars have a V shape. Its passage is marked by a sudden rotation of the wind (grain).
  Rear of a depression
Rear section of a cloud system. See cold front.
  Water-spout
Intense whirlwind between the base of a cumulonimbus and the sea. Fairly frequent phenomenon in the Mediterranean, more rare in the Atlantic or the Channel. Tornadoes are the more severe land equivalent of water spouts.
  Mean wind
According to convention, in meteorology, the mean wind is wind speed averaged over 10 minutes and measured at a height of 10 metres. French weather reports always refer to mean wind. Gusts can exceed mean wind by 50%. Independently of the hemisphere, the wind is said to be "veering" when its direction varies clockwise and to be "backing" when its direction varies counter-clockwise.
  Cyclonic wind
Counter clockwise variations in wind direction (northern hemisphere) around and near the centre of a low. We also use the expression "depressionary winds".
  Visibility
In French Marine weather reports, visibility is expressed in nautical miles, either "fair to good" (greater or equal to 5 nautical miles), "mediocre to moderate" (between 2 and 5 nautical miles) or "bad to poor" (between 0.5 and 2 nautical miles). Below 0.5 nautical miles, the term "mist to fog" is used.
  ZCIT - ITCZ
Zone of intertropical convergence. Zone in which the trade winds from the northern and southern hemispheres meet. Squalls, storm related or not, are frequent. Sailors gave it the name doldrums.