| Air Mass | A mass of airwhich is largely homogeneous in a horizontal direction. Its physical propertiesare determined by the nature of the surface over which it forms, and may besubsequently modified when the air mass moves over a different type of surface.Air masses are often separated from each other by frontal surfaces, which arediscontinuities. |
| Anemometer | An instrument fordetermining the velocity or speed of the wind. |
| Anticyclone | A regioncharacterized in the barometricpressure distribution by a system of closed isobars, with the highest pressureon the inside. It is also known as a 'high'. |
| Anti-Trades | In trade-windregions at a height of 6000 feet or more above the surface the wind direction issometimes 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 thetrade-winds back to higher latitudes, hence they are termed 'anti-trades', or'counter-trades'. but they are not regularly developed. |
| Aurora | Bright streamersof light, ascending from the horizon towards the zenith, or luminous arcs, whichare manifestations of electrical energy in the upper atmosphere. The aurora isseen in both hemispheres, in high and sometimes in medium latitudes. In thenorthern hemisphere it is known as Aurora Borealis, in the southern as AuroraAustralis. |
| Backing | A change in thedirection of the wind, in an anti-clockwise direction. |
| Blizzard | A high windaccompanied by great cold and drifting or falling snow. |
| BraveWest Winds | See RoaringForties. |
| Col | The saddle-backedregion occurring between two anticyclones and two depressions, arrangedalternatively. |
| ColdFront | The boundary linebetween 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 calledthe squall line. |
| ColdSector | That part of adepression 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 mattercontaining it. In particular, this is the method by which heat raises thetemperature of a fluid mass. That part in close contact with the heating agentexpands and rises, moving away from it, while colder fluid moves in to take itsplace. This action in the atmosphere gives rise to convectional currents whichmay produce cumulus or cumulonimbus cloud. |
| Corona (ae) | A series ofcoloured 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 isoften the only one visible. Within it there is a clear space, which, with thisinner ring, forms the 'aureole'. When the corona is fully developed there is aviolet ring outside the brownish red ring, followed by blue, green, yellow andred rings outwards. Additional series of coloured rings in the same order, areoccasionally seen on the outside of the first set, forming a double or triplecorona. (See Halo). |
| Corposants | Luminous brushdischarges of electricity, sometimes observed at the mastheads and on projectingparts of ships during electrical storms. Also known as St. Elmo's Fire. Due toatmospheric electricity. |
| Cyclone | A name given tothe tropical cyclones of the Bay of Bengal and the Arabian Sea. Sometimes usedas a general term for tropical cyclones of all oceans, or in the form 'TropicalCyclone'. Depressions of the Temperate Zones were formerly often referred to ascyclones but 'depression' or 'low' is now used to distinguish them from thetropical storms. The term 'cyclonic depression' is still sometimes used for adepression, as also is 'extra-tropical cyclone'. |
| DangerousQuadrant | The forwardquadrant of the dangerous semi-circle of a cyclone, which before recurvature isnearer the pole (in both hemispheres). |
| Depression | A regioncharacterised in the barometric distribution by a system of closed isobars,having lowest pressure on the inside. |
| Dew | Water dropsdeposited by condensation of water vapour from the air, mainly on horizontalsurfaces cooled by nocturnal radiation. |
| Dew Point | The temperatureto which air can be cooled without causing condensation. |
| Diffraction | The diversion ofa ray of light from the straight path by a material obstacle. Thus light may bediffracted by water drops, producing coronae (q.v.). |
| DiurnalVariation | This term is usedto indicate the changes, in the course of an average day, in the magnitude of ameteorological element. The most striking example of this is the diurnalvariation of barometric pressure in the tropics, the chief component of whichhas a 12-hourly period. The maxima of this variation are about 10 a. m. and 10p.m., the minima about 4 a.m. and 4 p.m. local time. |
| Doldrums | The equatorialoceanic regions of calms and light variable winds, accompanied by heavy rains,thunderstorms, and squalls. These belts are variable in position and extent, andas a whole move north, and south with the annual changes of the sun'sdeclination. |
| Eddy | A name given tothe deviation from steady motion which occurs in any viscous fluid which flowspast 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. |
| Eyeof Storm | The calm, centralarea of a tropical cyclone. The most noticeable feature of this area is thesudden drop in wind from hurricane force to light unsteady breezes or even to acomplete calm, with more or less cloudless sky and absence of rain. Over theocean the sea in the eye of the storm is usually very high and turbulent. |
| Front | The line ofseparation at the earth's surface between cold and warm air masses. |
| Frontogenesis | The developmentor marked intensification of a front. |
| Frontolysis | The disappearanceor marked weakening of a front. Subsidence is the most important factorin causing frontolysis. |
| FurtherOutlook | A statement inbrief and general terms appended to a detailed forecast and giving the conditionlikely to be experienced in the 24 hours or more following the period covered bythe actual forecast. |
| Gust | A comparativelyrapid fluctuation in the strength of the wind, characteristic of winds near thesurface of the earth. Gusts are mainly due to the turbulence or eddy motionarising from the friction offered by the ground to the flow of the current ofair. (See Squall). |
| Hail | Hard pellets ofice, of various shapes and sizes, and more or less transparent, which fall fromcumulonimbus clouds and are often associated with thunderstorms. |
| Halo | Halo phenomenaconstitute a large group of phenomena produced by the refraction or reflectionof the light of the sun or moon by the ice crystals composing cirrus orcirrostratus cloud. The commonest is the halo of about 22° radius round the sunor moon; other fairly common forms are mock suns (or moons), sun pillars, thehalo of 46° radius, etc. (See Corona). |
| HorseLatitudes | The belts ofcalms, light winds and fine, clear-weather between the trade-wind belts and theprevailing westerly winds of higher latitudes. |
| Hurricane | A name given tothe tropical cyclone of the West Indian region. Also applied to force 12 in the Beaufortscale, whatever its cause. |
| IntertropicalConvergence Zone | The zone ofseparation between the wind circulations proper to the northern and southernhemispheres. Over the Atlantic and Pacific Oceans, where it is closely relatedto the Doldrums, it is the boundary between the north-east and south-easttrade-winds. |
| Inversion | An abbreviationfor 'inversion of temperature gradient'. The temperature of the air, generallydecreases with increasing height but occasionally the reverse is the case; whenthe temperature thus increases with height there is said to be an inversion.When an inversion exists at lower levels fog,often occurs. |
| Isallobars | Isallobars arelines drawn upon a chart through places at which equal changes of pressure haveoccurred in some period of time. Lines of equal change, or isallobars, are drownto enclose regions of rising or of falling pressure. |
| Isobars | Lines drawnthrough positions having the same barometric pressure, when reduced to sealevel. |
| Isotherms | Lines drawnthrough positions having the same temperatures. |
| KatabaticWind | A wind that flowsdown slopes, usually at night. The air at the top of the slope is cooled agreater amount by radiation than the air lower down, becomes heavier, and flowsdown 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 ofheated air. |
| Land andSea Breezes | These are causedby the unequal heating and cooling of land and water under the influence ofsolar radiation by day and radiation to the sky at night, which produce agradient of pressure near the coast. During the day-time the land is warmer thanthe sea and a breeze, the sea-breeze, blows onshore; at night and in the earlymorning the land is cooler than the sea and the land-breeze blows offshore. Theland-breeze is usually less developed than the sea-breeze. |
| LineSquall | A more or lessviolent 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 winddirection, a rapid rise of the barometerand a fall of temperature. There is usually heavy rain or hail, sometimes athunderstorm or snow. The accompanying low black cloud forms a line or arch. |
| LocalWinds | Winds that areprevalent 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 verticalextension of an object due to abnormal refraction, making it appear unusuallytall. 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 otheroptical effects. |
| Luminescence | A luminousappearance of the sea, mainly caused by biological processes in microscopicorganisms. |
| Mirage | The appearance ofone or more images of a terrestrial object in the sky; also all forms ofdistortion of objects due to abnormal refraction. |
| Monsoons | Seasonal winds,those in the Indian Ocean, China Sea, and off the West Coast of Africa being theprincipal examples. |
| Occlusion,occluded depression | When the whole ofthe warm sector of a depression has been pushed up from the earth's surface bythe advance of the cold front behind it, this is known as an occlusion, and thedepression in which it occurs is called an occluded depression. |
| OrographicRain | Rain caused bythe interference of rising land in the path of moisture-laden air. A horizontalair current striking a mountain slope is deflected upwards and the consequentdynamical cooling associated with the expansion of the air produces cloud andrain, if the air contains sufficient aqueous vapour. Banner clouds, such as the'Table Cloth' over Table Mountain, 'Tursui' over Mount Fuji, and the cloud overthe Rock of Gibraltar during the Levanter, are examples of orographic cloud. |
| PolarFront | The line ofdiscontinuity, which is developed in suitable conditions between air originatingin polar regions and air from low latitudes, and on which the majority of thedepressions of temperate latitudes develop. It can sometimes be traced as acontinuous wavy line thousands of miles in length, but it is interrupted whenpolar air breaks through to feed the trade-winds, and is often replaced by avery complex series of fronts, or by continuous gradients of temperature. |
| Precipitation | Any aqueousdeposit in liquid or solid form, derived from the atmosphere. The precipitationat a given station during a given period includes not only the rainfall but alsodew and the water equivalent of any solid deposits (snow, hail, or hoar frost)received in the rain-guage. |
| Psychrometer | An alternativename for the dry-and wet-bulb hygrometer. In the aspirated psychrometer, adefinite rate of ventilation is secured by drawing the air over the bulbs by theagency of a fan. |
| Radiation | The passing ofheat from one body to another by electromagnetic waves. Radiated heat should notbe confused with convected or conducted heat. The heat of the sun is radiatedthrough 'empty' space, where convection and conduction would be impossible. |
| Rainbow | An arch ofcoloured light ill the sky, made by raindrops breaking up the white light of thesun into its component colours. The colours of the rainbow are, from the outerto the inner edge, red, orange, yellow, green, blue, indigo and violet. Theradius of the bow is 42°. One or more narrow, faint supernumerary bows may beseen inside the main bow, touching it. A secondary bow, less bright than theprimary bow, and with colours reversed, is often seen about 9° outside it. |
| Recurvatureof Storm | This expressionrefers to the recurvature of the track of a tropical cyclone, which is a typicalfeature 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 lesswesterly direction, recurves and normally takes a north-easterly direction; inthe southern hemisphere the final direction is normally south-easterly. |
| Ridge | An extension ofan anticyclone or high-pressure area shown on a pressure chart, correspondingwith a ridge running out from the side of a mountain. |
| RoaringForties | A nauticalexpression for the region of westerly winds in south temperate latitudes, whichreach their greatest development south of 40°S. A general term for theprevailing westerly winds in the temperate latitudes of both hemispheres is BraveWest Winds. |
| St.Elmo's Fire | See Corposants. |
| Saturation | A given volume ofordinary air which is exposed to a plane surface of water or ice has for a giventemperature a definite saturation pressure of water-vapour; this saturationpressure increases rapidly with increasing temperature. A fall of temperaturewould lead to condensation of some of this water-vapour, while a rise oftemperature would make the air unsaturated and therefore able to take up morewater-vapour. |
| Scud | A word used bysailors to describe ragged fragments of cloud drifting rapidly in a strong wind,often underneath rain clouds. The meteorological term is Stratus fractus. |
| SeaBreeze | See Land andSea Breezes. |
| SecondaryDepression or 'Secondary' | The isobarsaround a depression are frequently not quite symmetrical; they sometimes showbulges or distortions which are accompanied by marked deflections in the generalcirculation of the wind in the depression; such distortions are calledsecondaries; they may appear merely as sinuosities in the isobars, but al othertimes they enclose separate centres of low pressure and show separate windcirculations from that of the parent depression. |
| Shower | In describingpresent or past weather, the following distinction is made between the use ofthe terms 'showers' and 'occasional precipitation'. In general, showers are ofshort duration, and the fair periods between them are usually characterised bydefinite clearance of the sky. The clouds which give the showers are, therefore,isolated. The precipitation does not usually last more than 15 minutes, thoughit may occasionally last for half an hour or more. Occasional precipitation, onthe other hand, usually lasts for a longer time than the showers, and the sky inthe periods between the precipitation is usually cloudy or overcast. |
| Sleet | Precipitation ofsnow and rain together, or of melting snow and rain. |
| Snow | Precipitation ofice 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 windthat rises suddenly, lasts for some minutes, and dies away comparativelyrapidly. It is frequently, but not necessarily, associated with a temporarychange of direction. (See Gust). |
| SquallLine | See ColdFront. |
| Stratosphere | The region of theatmosphere immediately above the troposphere (q.v.). In the lower stratospheretemperature may continue to decrease with increase of height (but more slowlythan in the troposphere) or may remain practically constant, or may increasewith height. The transition front troposphere, to stratosphere, judged by changeof 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 aheight of about 20 miles; (b) the ionosphere, the highly-electricallyconducting region of ionised gases, extending upwards from the height of 50 or60 miles. This region plays an important part in radio propagation. The mainsubdivisions of this region in order of increasing height are usually referredto as the D. E. (or Kennelly-Heaviside), F (or Appleton) regions or layers. |
| Subsidence | Descent of airover a wide area, associated with a developing ridge or anticyclone. Thesubsidising air warms up, its relative humidity falls, and fine weather is theusual accompaniment of subsidence, though fog may occur under certainconditions. |
| Swell | This is a wavemotion 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 ceasedor passed away. |
| Synoptic | An adjectivederived from the noun 'synopsis', a brief or condensed statement presenting acombined or general view of something. Thus a synoptic chart shows the weatherconditions over a large area at a given instant of time. |
| Tendencyof the Barometer | The amount ofchange 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 byan electric discharge (lightning) from charged raindrops in a cloud to anothercloud (or other part of the same cloud) or to the earth, or to the airsurrounding the charged cloud. Sound travels 1 mile in about 5 seconds, whilethe lightning flash is seen almost as soon as it occurs, hence the interval oftime 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 anintense vertical current at the centre. Associated winds may attain speeds ofabout 200 knots. Heavy rain, and generally thunder and lightning, occur with thetornado. The term 'tornado' has also been used for thunderstorm squalls in WestAfrica. |
| Trade-Winds | The name given tothe winds which blow from tile sub-tropical high-pressure belts towards theequatorial region of low pressure from the N.E. in the northern hemisphere andfrom the S.E. in the southern hemisphere. The name originated in the nauticalphrase 'to blow trade' meaning to blow in a regular course or constantly in thesame direction. |
| Troposphere | The lower regionof the atmosphere throughout which temperature in general decreases as heightincreases, and within which occur practically all clouds and the various otherphenomena normally styled 'weather'. The upper boundary of the region is knownas the tropopause. The height of the tropopause varies with the latitude fromall average of about 5.5 miles in polar regions to about 11 miles at theequator, but the height also varies from summer to winder and with the generalmeteorological situation. (See Stratosphere). |
| Trough | The trough lineof a circular depression is the line, through the centre, perpendicular to theline of advance of the centre. During the passage of a depression over any givenplace the pressure at first falls and later rises; the trough line passes overthe 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 lowpressure, and is thus the opposite of a 'ridge' of high pressure. |
| Typhoon | A name given tothe tropical cyclones of the China Sea and the west part of the North PacificOcean. |
| Veering | A change in thedirection of the wind, in a clockwise direction. |
| Visibility | A term used indescribing the transparency of the atmosphere, and defined by the maximumdistance at which a suitable object can be seen. |
| WarmSector, Warm Front | Most depressionsin their earlier stages have an area of warm air on the side nearest the equatorknown as the warm sector. The warm front is the boundary between the front ofthe 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 cumulonimbuscloud, accompanied by an agitation of the sea surface beneath it, and theformation of a cloud of spray. The complete waterspout is formed when thefunnel-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. |
