CHAPTER 35WEATHERELEMENTSGENERALDESCRIPTIONOFTHEATMOSPHERE3500.IntroductionThe standard atmosphere is a conventional verticalstructure oftheatmospherecharacterized bya standard sea-Weather is the state ofthe earth's atmosphere with re-levelpressureof1013.25millibarsofmercury(29.92inches)and a sea-level air temperature of 15C (59F).Thespectto temperature,humidity,precipitation,visibilitycloudiness,and otherfactors.Climate refers to the averagetemperature decreases with height (i.e., standard lapselong-termmeteorological conditions ofaplace orregion.rate)being a uniform 2°C (3.6°F)per thousand feetto11All weathermaybetraced totheeffectofthesunonthekilometers(36.089feet)andthereafterremains constantat56.5°C (69.7° F)earth.Mostchanges in weather involve large-scale horizon-Research has indicated that the jet stream is importanttal motion of air.Air in motion is called wind.Thismotionisproducedbydifferencesof atmosphericpressure,whichinrelationto the sequenceof weather.The jetstream refersto relatively strong (<60 knots) quasi-horizontal winds,are attributablebothtodifferencesof temperatureandthenatureofthemotionitselfusually concentrated within a restricted layer of the atmo-Weather is of vital importance to the mariner.Thesphere.There are two commonlyknown jet streams.Thewind and stateof the sea affect dead reckoning.Reducedsub-tropicaljet stream (STJ)occurs in theregion of 30°Nvisibility limitspiloting.Thestateoftheatmosphereaffectsduring the northern hemispherewinter, decreasing in sum-electronic navigationandradiocommunication.Iftheskiesmer.The core of highest winds in the STJ is found at aboutare overcast,celestial observations arenotavailable,and12kmaltitude(40.000feet)anintheregionof70°W,40°Eundercertainconditionsrefractionanddiparedisturbedand150°E,althoughconsiderablevariabilityiscommonWhen wind was the primarymotive power, knowledge ofThe polar frontal jet stream (PFJ) is found in middle totheareas offavorable winds was of great importance.Mod-upper-middle latitudes and is discontinuous and variableern vessels are still affected considerablybywind and sea.Maximum jetstreamwindshavebeenmeasuredbyweatherballoons at 291knots.3501.TheAtmosphere3502.GeneralCirculationOfThe AtmosphereThe atmosphere is a relatively thin shell of air, watervapor,and suspended particulates surrounding the earthThe heat required to warm the air is supplied originallyAir is amixturegases and,like any gas,is elastic and highlyby the sun.As radiant energy from the sun arrives at thecompressible.Although extremelylight,it has a definiteearth, about29percent isreflected back into spacebytheweightwhichcanbemeasured.Acubicfootofairatstan-earth and its atmosphere, 19percent is absorbed by the at-dard sea-level temperature and pressure weighs 1.22mosphere,andtheremaining 52percent is absorbed bytheounces, or about '/g17th the weight of an equal volume ofsurface of the earth.Much of the earth's absorbed heat is ra-water.Because ofthis weight,the atmosphere exerts a pres-diated back into space.Earth'sradiation is in comparativelysure upon the surface of the earth of about 15 pounds perlong waves relativetotheshort-waveradiationfromthesunsquare inch.because it emanates from a cooler body.Long-wave radia-As altitude increases,airpressuredecreases dueto thetion, readily absorbed by the water vapor in the air, isdecreased weight ofair above.With less pressure, the den-primarilyresponsible for thewarmth of the atmospheresity decreases. More than three-fourths of the air isnear the earth's surface.Thus, the atmosphere acts muchlike the glass on the roof of a greenhouse.It allows part ofconcentratedwithinalaveraveragingabout7statutemilesthick, called the troposphere.This is the region of mostthe incoming solarradiationto reachthesurface ofthe earth"weather."asthetermiscommonlyunderstoodbut is heated by the terrestrial radiation passing outwardThe top of the troposphere is marked by a thin transi-Over theentire earth and for long periods of time, the totaltion zone called the tropopause, immediately above whichoutgoing energy must be equivalent to the incoming energyis the stratosphere.Beyond this lie several other layers(minus anyconverted to anotherform andretained),orthehaving distinctive characteristics.The average height of thetemperature ofthe earth and its atmospherewould steadilytropopauserangesfrom about5miles or lessat high lati-increaseordecrease.Inlocalareas,oroverrelativelyshorttudestoabout10milesatlowlatitudesperiods of time, such a balance is not required, and in fact483
483 CHAPTER 35 WEATHER ELEMENTS GENERAL DESCRIPTION OF THE ATMOSPHERE 3500. Introduction Weather is the state of the earth’s atmosphere with respect to temperature, humidity, precipitation, visibility, cloudiness, and other factors. Climate refers to the average long-term meteorological conditions of a place or region. All weather may be traced to the effect of the sun on the earth. Most changes in weather involve large-scale horizontal motion of air. Air in motion is called wind. This motion is produced by differences of atmospheric pressure, which are attributable both to differences of temperature and the nature of the motion itself. Weather is of vital importance to the mariner. The wind and state of the sea affect dead reckoning. Reduced visibility limits piloting. The state of the atmosphere affects electronic navigation and radio communication. If the skies are overcast, celestial observations are not available; and under certain conditions refraction and dip are disturbed. When wind was the primary motive power, knowledge of the areas of favorable winds was of great importance. Modern vessels are still affected considerably by wind and sea. 3501. The Atmosphere The atmosphere is a relatively thin shell of air, water vapor, and suspended particulates surrounding the earth. Air is a mixture gases and, like any gas, is elastic and highly compressible. Although extremely light, it has a definite weight which can be measured. A cubic foot of air at standard sea-level temperature and pressure weighs 1.22 ounces, or about 1/817th the weight of an equal volume of water. Because of this weight, the atmosphere exerts a pressure upon the surface of the earth of about 15 pounds per square inch. As altitude increases, air pressure decreases due to the decreased weight of air above. With less pressure, the density decreases. More than three-fourths of the air is concentrated within a layer averaging about 7 statute miles thick, called the troposphere. This is the region of most “weather,” as the term is commonly understood. The top of the troposphere is marked by a thin transition zone called the tropopause, immediately above which is the stratosphere. Beyond this lie several other layers having distinctive characteristics. The average height of the tropopause ranges from about 5 miles or less at high latitudes to about 10 miles at low latitudes. The standard atmosphere is a conventional vertical structure of the atmosphere characterized by a standard sealevel pressure of 1013.25 millibars of mercury (29.92 inches) and a sea-level air temperature of 15° C (59° F). The temperature decreases with height (i.e., standard lapse rate) being a uniform 2° C (3.6° F) per thousand feet to 11 kilometers (36,089 feet) and thereafter remains constant at –56.5° C (69.7° F). Research has indicated that the jet stream is important in relation to the sequence of weather. The jet stream refers to relatively strong (≤60 knots) quasi-horizontal winds, usually concentrated within a restricted layer of the atmosphere. There are two commonly known jet streams. The sub-tropical jet stream (STJ) occurs in the region of 30°N during the northern hemisphere winter, decreasing in summer. The core of highest winds in the STJ is found at about 12km altitude (40,000 feet) an in the region of 70°W, 40°E, and 150°E, although considerable variability is common. The polar frontal jet stream (PFJ) is found in middle to upper-middle latitudes and is discontinuous and variable. Maximum jet stream winds have been measured by weather balloons at 291 knots. 3502. General Circulation Of The Atmosphere The heat required to warm the air is supplied originally by the sun. As radiant energy from the sun arrives at the earth, about 29 percent is reflected back into space by the earth and its atmosphere, 19 percent is absorbed by the atmosphere, and the remaining 52 percent is absorbed by the surface of the earth. Much of the earth’s absorbed heat is radiated back into space. Earth’s radiation is in comparatively long waves relative to the short-wave radiation from the sun because it emanates from a cooler body. Long-wave radiation, readily absorbed by the water vapor in the air, is primarily responsible for the warmth of the atmosphere near the earth’s surface. Thus, the atmosphere acts much like the glass on the roof of a greenhouse. It allows part of the incoming solar radiation to reach the surface of the earth but is heated by the terrestrial radiation passing outward. Over the entire earth and for long periods of time, the total outgoing energy must be equivalent to the incoming energy (minus any converted to another form and retained), or the temperature of the earth and its atmosphere would steadily increase or decrease. In local areas, or over relatively short periods of time, such a balance is not required, and in fact
484WEATHERELEMENTSdoes not exist, resulting in changes such as those occurringthe heatbalanceofthese areas.Thesefactors,coupled withfrom one year to another, in different seasons and in differ-others, result in constantly changing large-scale movementsentparts of theday.ofair.Forexample,therotation oftheearthexertsan appar-ent force, known as Coriolis force, which diverts the airThe more nearly perpendicular the rays of the sunfroma directpathbetweenhighand lowpressureareas.Thestrikethe surface ofthe earth, the more heat energy per unitdiversion of the air is toward the right in the Northernarea is receivedatthatplace.PhysicalmeasurementsshowHemisphere and toward the left in the Southern Hemi-that in the tropics, more heat per unit area is received thansphere. At some distance above the surface of the earth, theis radiated away,and that in polar regions, theopposite iswind tends to blow along lines connecting points of equaltrue.Unlessthereweresomeprocesstotransferheatfrompressure called isobars. The wind is called a geostrophicthetropicsto polarregions,thetropics would bemuchwind if the isobars are straight (great circles)and a gradi-warmer than they are, and the polar regions would be muchent wind if they are curved. Near the surface of the earth,colder.Atmospheric motions bring about the requiredfriction tends to divert thewind fromthe isobarstowardthetransfer ofheat. The oceans also participate in the process,center oflow pressure. At sea, where friction is less than onbut to a lesser degree.land,the windfollows theisobars more closely.If the earth had a uniform surface and did not rotate onA simplified diagram of thegeneral circulation patternits axis,withthesunfollowingitsnormal pathacrosstheisshowninFigure3502b.Figure3502candFigure3502dsky (solar heating increasing with decreasing latitude), agiveageneralized pictureof the world's pressure distribu-simplecirculation would result,as shown inFigure3502ation and wind systems as actuallyobserved.However, the surface of the earth is far from uniform, beingcovered with an irregular distribution of land and water.Achange in pressurewith horizontal distance is calledAdditionally,the earth rotates about its axis so that the por-a pressure gradient. It is maximum along a normal (per-tion heated by the sun continually changes.In addition, thependicular)to theisobars.Aforce results whichis calledaxisofrotation is tilted sothatastheearthmoves along itspressure gradient force and is always directed from highorbitaboutthe sun,seasonal changes occur in theexposuretolow pressure.Speed ofthe wind is approximately propor-of specificareas to the sun's rays,resulting invariations intional tothis pressuregradient.NORTHPOLEPOLARREGIONArea of Least HeatingEQUATORIALREGION1Area of Greatest HeatingPOLARREGIONAreaof Least HeatingSOUTHPOLEFigure3502a.Ideal atmosphericcirculationfora uniformand nonrotatingearth
484 WEATHER ELEMENTS does not exist, resulting in changes such as those occurring from one year to another, in different seasons and in different parts of the day. The more nearly perpendicular the rays of the sun strike the surface of the earth, the more heat energy per unit area is received at that place. Physical measurements show that in the tropics, more heat per unit area is received than is radiated away, and that in polar regions, the opposite is true. Unless there were some process to transfer heat from the tropics to polar regions, the tropics would be much warmer than they are, and the polar regions would be much colder. Atmospheric motions bring about the required transfer of heat. The oceans also participate in the process, but to a lesser degree. If the earth had a uniform surface and did not rotate on its axis, with the sun following its normal path across the sky (solar heating increasing with decreasing latitude), a simple circulation would result, as shown in Figure 3502a. However, the surface of the earth is far from uniform, being covered with an irregular distribution of land and water. Additionally, the earth rotates about its axis so that the portion heated by the sun continually changes. In addition, the axis of rotation is tilted so that as the earth moves along its orbit about the sun, seasonal changes occur in the exposure of specific areas to the sun’s rays, resulting in variations in the heat balance of these areas. These factors, coupled with others, result in constantly changing large-scale movements of air. For example, the rotation of the earth exerts an apparent force, known as Coriolis force, which diverts the air from a direct path between high and low pressure areas. The diversion of the air is toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere. At some distance above the surface of the earth, the wind tends to blow along lines connecting points of equal pressure called isobars. The wind is called a geostrophic wind if the isobars are straight (great circles) and a gradient wind if they are curved. Near the surface of the earth, friction tends to divert the wind from the isobars toward the center of low pressure. At sea, where friction is less than on land, the wind follows the isobars more closely. A simplified diagram of the general circulation pattern is shown in Figure 3502b. Figure 3502c and Figure 3502d give a generalized picture of the world’s pressure distribution and wind systems as actually observed. A change in pressure with horizontal distance is called a pressure gradient. It is maximum along a normal (perpendicular) to the isobars. A force results which is called pressure gradient force and is always directed from high to low pressure. Speed of the wind is approximately proportional to this pressure gradient. Figure 3502a. Ideal atmospheric circulation for a uniform and nonrotating earth
485WEATHERELEMENTSROTATIONOFTHE WORLDNORTHPOL60,000FEETNORTHEASTERLIES60*PREVAILING WESTERLIES30*HORSE LATITUDESHIGHPRESSURENORTHEAST TRADES0.OWPRESSUREDOLDRUMSDUTHEASTTRADESJIGHPRESSUR30*AODSELATITUDESPREVAILINGWESTERLIES60°SOUTHEASTERLIES25.000SOUTH POLFigure3502b.Simplifieddiagramof thegeneral circulationof the atmosphereLWCEHIOR++10012g140160KEYPREVAILING WINDSLENGTHOIadoatalizeddepreeofCONSTANCY OEWIND DIRECTIONWDTHofamewindicateyeFORCEOFWIND20Knet15KnotneFigure3502c.Generalizedpatternof actual surfacewinds in JanuaryandFebruary
WEATHER ELEMENTS 485 Figure 3502b. Simplified diagram of the general circulation of the atmosphere. Figure 3502c. Generalized pattern of actual surface winds in January and February
486WEATHERELEMENTSXLOWOW14THIGHFigure3502d.Generalized pattern of actual surfacewinds in JulyandAugust.(SeekeywithFigure3502c.)MAJORWINDPATTERNS3503.TheDoldrumshigh pressure toward the equatorial belts of low pressure.Because of the rotation of the earth, the moving air is de-A belt of low pressure at the earth's surface near theflected toward the west.Therefore, the trade winds in theequatorknown as the doldrums occupies a positionapprox-NorthernHemispherearefromthenortheastandarecalledthe northeast trades, while those in the Southern Hemi-imatelymidwaybetweenhighpressurebeltsataboutlatitude300to35oneachside.Exceptfor significantintradiurnalsphereare from the southeast and are called the southeastchanges,theatmosphericpressurealongtheequatorial lowistrades. The trade-wind directions are best defined overalmost uniform. With minimal pressure gradient, windeasternoceanareasspeeds are light and directions are variable, Hot, sultry dayThe trade winds are generally considered among theare common.The sky is often overcast, and showers andmost constant of winds,blowing for days or even weeksthundershowers are relatively frequent, in these atmospheri-with little change of direction or speed.However,at timescallyunstableareas,briefperiods of strong wind occurthey weaken or shift direction, and there are regions whereThe doldrums occupy a thin belt near the equator, thethe general pattern is disrupted. A notable example is foundeastern part in both the Atlantic and Pacific being widerin the island groups of the South Pacific, where the tradesthan the western part. However, both the position and ex-are practically nonexistent during January and Februarytent of the belt vary with longitude and season. During allTheir bestdevelopment is attained in the South Atlantic andseasonsintheNorthernHemisphere,thebeltiscenteredinintheSouthIndianOcean.Ingeneral,theyarestrongerdur-the eastern Atlantic and Pacific however, there are wideing thewinter than duringthe summer season.excursions of the doldrum regions at longitudes with con-In July and August, when the belt of equatorial lowsiderablelandmass.On theaverage,theposition is at5°N,pressuremovesto a position some distancenorth of thefrequently called the meteorological equator.equator,the southeasttrades blowacross the equator,intotheNorthern Hemisphere,wheretheearth's rotationdiverts3504.The Trade Windsthemtowardtheright,causingthemtobesoutherlyandsouthwesterly winds.The“southwestmonsoons"oftheAf-Thetradewinds at thesurfaceblowfromthebelts ofrican and Central Americancoastsoriginatepartlyin these
486 WEATHER ELEMENTS MAJOR WIND PATTERNS 3503. The Doldrums A belt of low pressure at the earth’s surface near the equator known as the doldrums occupies a position approximately midway between high pressure belts at about latitude 30° to 35° on each side. Except for significant intradiurnal changes, the atmospheric pressure along the equatorial low is almost uniform. With minimal pressure gradient, wind speeds are light and directions are variable. Hot, sultry days are common. The sky is often overcast, and showers and thundershowers are relatively frequent; in these atmospherically unstable areas, brief periods of strong wind occur. The doldrums occupy a thin belt near the equator, the eastern part in both the Atlantic and Pacific being wider than the western part. However, both the position and extent of the belt vary with longitude and season. During all seasons in the Northern Hemisphere, the belt is centered in the eastern Atlantic and Pacific; however, there are wide excursions of the doldrum regions at longitudes with considerable landmass. On the average, the position is at 5°N, frequently called the meteorological equator. 3504. The Trade Winds The trade winds at the surface blow from the belts of high pressure toward the equatorial belts of low pressure. Because of the rotation of the earth, the moving air is deflected toward the west. Therefore, the trade winds in the Northern Hemisphere are from the northeast and are called the northeast trades, while those in the Southern Hemisphere are from the southeast and are called the southeast trades. The trade-wind directions are best defined over eastern ocean areas. The trade winds are generally considered among the most constant of winds, blowing for days or even weeks with little change of direction or speed. However, at times they weaken or shift direction, and there are regions where the general pattern is disrupted. A notable example is found in the island groups of the South Pacific, where the trades are practically nonexistent during January and February. Their best development is attained in the South Atlantic and in the South Indian Ocean. In general, they are stronger during the winter than during the summer season. In July and August, when the belt of equatorial low pressure moves to a position some distance north of the equator, the southeast trades blow across the equator, into the Northern Hemisphere, where the earth’s rotation diverts them toward the right, causing them to be southerly and southwesterly winds. The “southwest monsoons” of the African and Central American coasts originate partly in these Figure 3502d. Generalized pattern of actual surface winds in July and August. (See key with Figure 3502c.)
WEATHER ELEMENTS487diverted southeasttrades.3507.Polar WindsCyclones from the middle latitudes rarely enterthe re-gions of the trade winds, although tropical cyclonesPartlybecause ofthe lowtemperatures nearthegeo-graphical poles of the earth,the surfacepressuretends tooriginate withintheseareasremain higher than in surrounding regions,since cold air ismore dense than warm air.Consequently,the winds blow3505.TheHorseLatitudesoutwardfromthepoles.andaredeflectedwestwardbytherotation of the earth, to become northeasterlies in the Arc-Alongthepoleward sideofeachtrade-windbelt,and cor-tic, and southeasterlies in the Antarctic. Where the polarresponding approximately with the belt of high pressure ineasterliesmeettheprevailing westerlies,near 50°N andeach hemisphere, is another region with weak pressure gradi-50os on the average,a discontinuity in temperature andents and correspondinglylight, variable winds.These arewind exists.This discontinuity is called the polar front.called the horse latitudes, apparently so named because be-Here the warmer low-latitude air ascends over the coldercalmed sailing ships threw horses overboard in this regionpolar aircreating a zone of cloudiness and precipitationwhen water supplies ran short The weather is generallygoodIn the Arctic, the general circulation is greatly modi-althoughlowcloudsarecommon.Comparedtothedoldrums.fied by surrounding landmasses.Winds over the Arcticperiods of stagnation in the horse latitudes are less persistentOceanaresomewhatvariable,and strongsurfacewinds areThedifferenceisdueprimarilytotherisingcurrents of warmrarely encountered.airin theequatorial low,which carrylarge amounts ofmoisIn the Antarctic, on the other hand, a high central land-ture.Thismoisturecondensesastheaircoolsathigherlevelswhile in the horse latitudes the air is apparently descendingmass is surrounded by water,a condition which augments,rather than diminishes, the general circulation. The highand becoming less humid as it is warmed at lower heights.pressure, although weaker than in the horse latitudes, is3506.The Prevailing Westerliesstronger than in the Arctic, and of great persistence espe-ciallyineasternAntarctica.Thecoldairfromtheplateauareasmoves outwardand downward toward the sea and isOn thepoleward sideof thehighpressurebelt in eachdeflected towardthewestbytheearth'srotation.Thewindshemisphere,the atmospheric pressure again diminishes.remain strong throughout the year,frequently attaininghur-The currents of air set in motion along these gradients to-ricaneforcenearthebaseofthemountains.Thesearesomeward thepoles arediverted bythe earth's rotation towardofthe strongest surfacewinds encountered anywhere in thethe east, becoming southwesterly winds in the Northernworldwiththepossibleexceptionofthoseinwell-develHemisphere and northwesterly in the Southern Hemi-oped tropical cyclones.sphere. These two wind systems are known as theprevailing westerlies of the temperate zones.3508.ModificationsOfTheGeneral CirculationIn theNorthern Hemispherethis relatively simplepat-ternisdistortedconsiderablybysecondarywindThegeneral circulation oftheatmosphere isgreatlycirculations,dueprimarilyto the presence of large landmodifiedbyvarious conditions.masses.In theNorth Atlantic,between latitudes 40°and50°windsblowfrom somedirection between south andThe high pressure in the horse latitudes is not uniform-ly distributed around the belts,but tends tobe accentuatednorthwestduring74percentof thetime,beingsomewhatmorepersistent inwinterthan insummer.Theyarestrongerat several points,as shown in Figure 3502c and Figurein winter, too, averaging about 25 knots (Beaufort 6) as3502d.These semi-permanent highs remain at about thecompared with 14 knots (Beaufort 4) in the summer.same places withgreatpersistence.In the Southern Hemisphere thewesterlies blowSemi-permanent lows also occur in various places, thethroughout the year with a steadiness approaching that ofmostprominent onesbeing west of Iceland, and over thethe trade winds. The speed, though variable,is generally be-Aleutians (winter only) in the Northern Hemisphere,and intween17and27knots(Beaufort5and6).Latitudes40°Stothe Ross Sea and Weddell Sea in the Antarctic areas. The re-50°s (or 55°s)where these boisterous winds occur,aregions occupied bythese semi-permanent lows are sometimescalled the roaring forties. These winds are strongest atcalled the graveyards of the lows, since many lows move di-about latitude50°srectly into these areas and lose their identity as they mergeThegreater speed and persistence of thewesterlies inwithandreinforcethesemi-permanentlows.Thelowpres-sure in theseareas is maintained largely by the migratory lowsthe Southern Hemisphere aredue to thedifference in the at-which stall there,withtopography also important,especiallymospheric pressure pattern, and its variations, from thein Antarctica.NorthernHemisphere.In thecomparativelylandless South-Anothermodifying influence is land,which undergoesernHemisphere,the averageyearlyatmospheric pressurediminishes much morerapidly on thepoleward side ofthegreater temperature changes than does the sea. During thehighpressurebelt.andhasfewerirregularitiesduetoconti-summer.a continent iswarmerthanits adjacent oceansTherefore, low pressures tend to prevail over theland.Ifa cli-nental interference,than intheNorthern Hemisphere
WEATHER ELEMENTS 487 diverted southeast trades. Cyclones from the middle latitudes rarely enter the regions of the trade winds, although tropical cyclones originate within these areas. 3505. The Horse Latitudes Along the poleward side of each trade-wind belt, and corresponding approximately with the belt of high pressure in each hemisphere, is another region with weak pressure gradients and correspondingly light, variable winds. These are called the horse latitudes, apparently so named because becalmed sailing ships threw horses overboard in this region when water supplies ran short. The weather is generally good although low clouds are common. Compared to the doldrums, periods of stagnation in the horse latitudes are less persistent. The difference is due primarily to the rising currents of warm air in the equatorial low, which carry large amounts of moisture. This moisture condenses as the air cools at higher levels, while in the horse latitudes the air is apparently descending and becoming less humid as it is warmed at lower heights. 3506. The Prevailing Westerlies On the poleward side of the high pressure belt in each hemisphere, the atmospheric pressure again diminishes. The currents of air set in motion along these gradients toward the poles are diverted by the earth’s rotation toward the east, becoming southwesterly winds in the Northern Hemisphere and northwesterly in the Southern Hemisphere. These two wind systems are known as the prevailing westerlies of the temperate zones. In the Northern Hemisphere this relatively simple pattern is distorted considerably by secondary wind circulations, due primarily to the presence of large landmasses. In the North Atlantic, between latitudes 40° and 50°, winds blow from some direction between south and northwest during 74 percent of the time, being somewhat more persistent in winter than in summer. They are stronger in winter, too, averaging about 25 knots (Beaufort 6) as compared with 14 knots (Beaufort 4) in the summer. In the Southern Hemisphere the westerlies blow throughout the year with a steadiness approaching that of the trade winds. The speed, though variable, is generally between 17 and 27 knots (Beaufort 5 and 6). Latitudes 40°S to 50°S (or 55°S) where these boisterous winds occur, are called the roaring forties. These winds are strongest at about latitude 50°S. The greater speed and persistence of the westerlies in the Southern Hemisphere are due to the difference in the atmospheric pressure pattern, and its variations, from the Northern Hemisphere. In the comparatively landless Southern Hemisphere, the average yearly atmospheric pressure diminishes much more rapidly on the poleward side of the high pressure belt, and has fewer irregularities due to continental interference, than in the Northern Hemisphere. 3507. Polar Winds Partly because of the low temperatures near the geographical poles of the earth, the surface pressure tends to remain higher than in surrounding regions, since cold air is more dense than warm air. Consequently, the winds blow outward from the poles, and are deflected westward by the rotation of the earth, to become northeasterlies in the Arctic, and southeasterlies in the Antarctic. Where the polar easterlies meet the prevailing westerlies, near 50°N and 50°S on the average, a discontinuity in temperature and wind exists. This discontinuity is called the polar front. Here the warmer low-latitude air ascends over the colder polar air creating a zone of cloudiness and precipitation. In the Arctic, the general circulation is greatly modified by surrounding landmasses. Winds over the Arctic Ocean are somewhat variable, and strong surface winds are rarely encountered. In the Antarctic, on the other hand, a high central landmass is surrounded by water, a condition which augments, rather than diminishes, the general circulation. The high pressure, although weaker than in the horse latitudes, is stronger than in the Arctic, and of great persistence especially in eastern Antarctica. The cold air from the plateau areas moves outward and downward toward the sea and is deflected toward the west by the earth’s rotation. The winds remain strong throughout the year, frequently attaining hurricane force near the base of the mountains. These are some of the strongest surface winds encountered anywhere in the world, with the possible exception of those in well-developed tropical cyclones. 3508. Modifications Of The General Circulation The general circulation of the atmosphere is greatly modified by various conditions. The high pressure in the horse latitudes is not uniformly distributed around the belts, but tends to be accentuated at several points, as shown in Figure 3502c and Figure 3502d. These semi-permanent highs remain at about the same places with great persistence. Semi-permanent lows also occur in various places, the most prominent ones being west of Iceland, and over the Aleutians (winter only) in the Northern Hemisphere, and in the Ross Sea and Weddell Sea in the Antarctic areas. The regions occupied by these semi-permanent lows are sometimes called the graveyards of the lows, since many lows move directly into these areas and lose their identity as they merge with and reinforce the semi-permanent lows. The low pressure in these areas is maintained largely by the migratory lows which stall there, with topography also important, especially in Antarctica. Another modifying influence is land, which undergoes greater temperature changes than does the sea. During the summer, a continent is warmer than its adjacent oceans. Therefore, low pressures tend to prevail over the land. If a cli-
488WEATHER ELEMENTS20°W20NTON10N63'EWANE70100'E70'E100°EFigure3508b.ThewintermonsoonFigure3508a.Thesummermonsoon.matological belt of high pressure encounters a continent,itscyclonicorigin and various local winds.Somecommon lo-calwindsarelistedbylocalnamebelowpattern is distorted or interrupted,whereas a belt of lowpres-sure is intensified over the samearea Inwinter,the oppositeeffect takes place,belts ofhigh pressurebeing intensified overAbroholosA squall frequent from May throughAugust between Cabode SaoTomelandandthoseof lowpressurebeingweakened.and CaboFrioon the coast of Brazil.Themoststrikingexampleofawindsystemproducedbythe alternate heating and cooling of a landmass is themon-Bali windA strong east wind at the eastern endsoon (seasonal wind)of the China Sea and Indian Ocean.AofJava.portion of thiseffect is shown in Figure3508a and FigureBaratA heavy northwest squall in Manado Bay3508b.Inthesummer,lowpressureprevails overthewarmon the north coast of the island ofCelebes,continentof Asia,and relativelyhigherpressureprevailsoverprevalent from December to February.the adjacent sea.Between these two systems thewind blowsBarberA strongwind carryingdamp snoworinanearlysteadydirection.Thelowerportionof thepatternsleet and spray that freezes upon contactis in the Southern Hemisphere,extending to about 10° southwith objects, especially the beard and hair.latitude.HeretherotationoftheearthcausesadeflectiontoBayamoAviolentwindblowingfromthelandthe left, resulting in southeasterly winds. As they cross theonthesouthcoastofCuba,especiallyequator, the deflection is in the opposite direction, causingnear theBight of Bayamo.them to curve toward the right, becoming southwesterlywinds.Inthewinter,thepositions ofhighand lowpressurear-Bentu de SoliAn east wind on the coast of Sardiniaeasare interchanged,and thedirection offlow is reversedBoraAcold, northerlywind blowing fromIntheChina Sea,thesummermonsoonblowsfromthethe Hungarian basin into the Adriaticsouthwest, usuallyfrom May to September.The strongSea. See also FALL WIND.winds are accompanied by heavy squalls and thunder-A thunderstorm or violent squall,Borascostorms, the rainfall being much heavierthan during theespecially in the Mediterranean.wintermonsoon.As the seasonadvances,squalls andrainbecomeless frequent.In someplacesthewind becomes aBrisa, Briza1. A northeast wind which blows onthe coast of South America or an eastlight breeze which is unsteady in direction, or stops alto-wind whichblows on PuertoRicogether,while in other places it continues almostduring the trade wind season.2. Theundiminished, with changes in direction or calms being in-northeast monsoon in the Philippines.frequent.Thewintermonsoonblows from thenortheast,BrisoteThe northeast trade wind when it isusuallyfromOctobertoApril.Itblowswitha steadinessblowing stronger than usual on Cuba.similartothat ofthetradewinds, often attainingthe speedof a moderate gale (28-33 knots). Skies are generally clearBrubuA name for a squall in the East Indies.during this season,and there is relatively littlerain.Bull's Eye SquallA squall forming in fair weather,The general circulation is further modified by winds of
488 WEATHER ELEMENTS matological belt of high pressure encounters a continent, its pattern is distorted or interrupted, whereas a belt of low pressure is intensified over the same area. In winter, the opposite effect takes place, belts of high pressure being intensified over land and those of low pressure being weakened. The most striking example of a wind system produced by the alternate heating and cooling of a landmass is the monsoon (seasonal wind) of the China Sea and Indian Ocean. A portion of this effect is shown in Figure 3508a and Figure 3508b. In the summer, low pressure prevails over the warm continent of Asia, and relatively higher pressure prevails over the adjacent sea. Between these two systems the wind blows in a nearly steady direction. The lower portion of the pattern is in the Southern Hemisphere, extending to about 10° south latitude. Here the rotation of the earth causes a deflection to the left, resulting in southeasterly winds. As they cross the equator, the deflection is in the opposite direction, causing them to curve toward the right, becoming southwesterly winds. In the winter, the positions of high and low pressure areas are interchanged, and the direction of flow is reversed. In the China Sea, the summer monsoon blows from the southwest, usually from May to September. The strong winds are accompanied by heavy squalls and thunderstorms, the rainfall being much heavier than during the winter monsoon. As the season advances, squalls and rain become less frequent. In some places the wind becomes a light breeze which is unsteady in direction, or stops altogether, while in other places it continues almost undiminished, with changes in direction or calms being infrequent. The winter monsoon blows from the northeast, usually from October to April. It blows with a steadiness similar to that of the trade winds, often attaining the speed of a moderate gale (28–33 knots). Skies are generally clear during this season, and there is relatively little rain. The general circulation is further modified by winds of cyclonic origin and various local winds. Some common local winds are listed by local name below. Figure 3508a. The summer monsoon. Figure 3508b. The winter monsoon. Abroholos A squall frequent from May through August between Cabo de Sao Tome and Cabo Frio on the coast of Brazil. Bali wind A strong east wind at the eastern end of Java. Barat A heavy northwest squall in Manado Bay on the north coast of the island of Celebes, prevalent from December to February. Barber A strong wind carrying damp snow or sleet and spray that freezes upon contact with objects, especially the beard and hair. Bayamo A violent wind blowing from the land on the south coast of Cuba, especially near the Bight of Bayamo. Bentu de Soli An east wind on the coast of Sardinia. Bora A cold, northerly wind blowing from the Hungarian basin into the Adriatic Sea. See also FALL WIND. Borasco A thunderstorm or violent squall, especially in the Mediterranean. Brisa, Briza 1. A northeast wind which blows on the coast of South America or an east wind which blows on Puerto Rico during the trade wind season. 2. The northeast monsoon in the Philippines. Brisote The northeast trade wind when it is blowing stronger than usual on Cuba. Brubu A name for a squall in the East Indies. Bull’s Eye Squall A squall forming in fair weather
489WEATHERELEMENTSBull's Eye Squallcharacteristic ofthe ocean off the coastKona StormA storm over the Hawaian Islands,(continued)of South Africa. It is named for thecharacterized by strong southerly orpeculiar appearance of the smallsouthwesterly winds and heavy rains.isolated cloud marking the top oftheLesteA hot,dry,easterly wind of theinvisible vortex of the stormMadeira and Canary Islands.The strong southeast wind whichCape DoctorLevanterA strong easterly wind of the Mediterrane-blows on the South African coast. Alsoan, especially in the Strait ofGibraltar,called the DOCTORatended by cloudy,foggy,and sometimsCaver, KaverA gentle breeze in the Hebrides.rainy weather especially in winter.A violent squall with thunder andChubascoLevanteraA persistent east wind of the Adriatic,lightning, encountered during the rainyusually accompanied by cloudy weather.season along the west coast of CentralLevantoA hot southeasterly wind which blowsAmericaover the Canary Islands.ChuradaA severe rain squallin the Mariana IslandsLevecheA warm wind in Spain,either a foehnduring the northeast monsoon. They occuror a hot southerly wind in advance of afrom November to April or May,lowpressure area movingfrom theespecially from January through March.Sahara Desert.Called a SIROCCO inCierzoSee MISTRAL.other parts of the Mediterranean area.MaestroContrastesWinds a short distance apart blowing fromA northwesterly wind with fineoppositequadrantsfquent inthespringweather which blows,especiallyinand fall in the western Mediterranean.summer, in the Adriatic. It is mostfrequent on the western shore. ThisCordonazoThe"Lash of St.Francis."Namewind is also found on the coasts ofapplied locally to southerly hurricaneCorsica and Sardiniawinds along the west coast of Mexico.Matanuska WindIt is associated with tropical cyclonesA strong, gusty, northeast wind whichin the southeastern North Pacificoccasionally occurs during the winterOcean.These storms mayoccurfromin the vicinity of Palmer, Alaska.May to November, but ordinarily affectMistralA cold, dry wind blowing from thethe coastal areas most severely near ornorth overthe northwest coast oftheaftertheFeastofSt.Francis,October4Mediterranean Sea, particularly overAnight land breezeprevailingfromCoromellthe Gulf of Lions. Also calledNovember to May at La Paz, near theCIERZO.See also FALL WINDsouthern extremity of the Gulf ofNashi, N'aschiA northeast wind which occurs inCalifornia.winter on the Iranian coast of the1. A cooling sea breeze in the Tropics.Persian Gulf, especially near theDoctor2. See HARMATTAN. 3. The strongentranceto thegulf,and also on theSE wind which blows on the southMakran coast. It is probably associatedAfrican coast.Usually called CAPEwithanoutflowfromthecentral AsiaticDOCTORanticyclone which extends over the highland of Iran.It is similar in character butA strong southerly or southeasterlyElephantaless severe than the BORA.wind which blows on the MalabarNortecoast of India during the months ofA strong cold northeasterly wind whichSeptember and October andmarks theblows in Mexico and on the shores ofendof thesouthwest monsoon.the Gulf of Mexico. It results from anoutbreak of cold air from the north.It isEtesianA refreshing northerly summer windtheMexicanextensionofanortherof the Mediterranean, especially overPapagayoA violet northeasterly fall wind on thethe Aegean Sea.Pacific coast of Nicaragua andA strong northeast wind ofthe centralGregaleGuatemala.It consists of the cold airMediterranean.mass of a nortewhich has overriddenHarmattanThe dry, dusty trade wind blowing offthemountains of Central America. Seethe Sahara Desert across the Gulfofalso TEHUANTEPECER.Guinea and the Cape Verde Islands.Santa AnaA strong, hot, dry wind blowing out intoSometimes called the DOCTOR, becauseSan Pedro Channel from the southermof its supposed healthful properties.California desert through Santa Ana Pass.A strong southeast wind in the vicinityKnik WindShamalA summer northwesterly wind blowingof Palmer,Alaska, most frequent in theover Iraq and the Persian Gulf, oftenwinter.strong during the day, but decreasingat night
WEATHER ELEMENTS 489 Bull’s Eye Squall (continued) characteristic of the ocean off the coast of South Africa. It is named for the peculiar appearance of the small isolated cloud marking the top of the invisible vortex of the storm. Cape Doctor The strong southeast wind which blows on the South African coast. Also called the DOCTOR. Caver, Kaver A gentle breeze in the Hebrides. Chubasco A violent squall with thunder and lightning, encountered during the rainy season along the west coast of Central America. Churada A severe rain squall in the Mariana Islands during the northeast monsoon. They occur from November to April or May, especially from January through March. Cierzo See MISTRAL. Contrastes Winds a short distance apart blowing from opposite quadrants, frequent in the spring and fall in the western Mediterranean. Cordonazo The “Lash of St. Francis.” Name applied locally to southerly hurricane winds along the west coast of Mexico. It is associated with tropical cyclones in the southeastern North Pacific Ocean. These storms may occur from May to November, but ordinarily affect the coastal areas most severely near or after the Feast of St. Francis, October 4. Coromell A night land breeze prevailing from November to May at La Paz, near the southern extremity of the Gulf of California. Doctor 1. A cooling sea breeze in the Tropics. 2. See HARMATTAN. 3. The strong SE wind which blows on the south African coast. Usually called CAPE DOCTOR. Elephanta A strong southerly or southeasterly wind which blows on the Malabar coast of India during the months of September and October and marks the end of the southwest monsoon. Etesian A refreshing northerly summer wind of the Mediterranean, especially over the Aegean Sea. Gregale A strong northeast wind of the central Mediterranean. Harmattan The dry, dusty trade wind blowing off the Sahara Desert across the Gulf of Guinea and the Cape Verde Islands. Sometimes called the DOCTOR, because of its supposed healthful properties. Knik Wind A strong southeast wind in the vicinity of Palmer, Alaska, most frequent in the winter. Kona Storm A storm over the Hawaiian Islands, characterized by strong southerly or southwesterly winds and heavy rains. Leste A hot, dry, easterly wind of the Madeira and Canary Islands. Levanter A strong easterly wind of the Mediterranean, especially in the Strait of Gibraltar, attended by cloudy, foggy, and sometimes rainy weather especially in winter. Levantera A persistent east wind of the Adriatic, usually accompanied by cloudy weather. Levanto A hot southeasterly wind which blows over the Canary Islands. Leveche A warm wind in Spain, either a foehn or a hot southerly wind in advance of a low pressure area moving from the Sahara Desert. Called a SIROCCO in other parts of the Mediterranean area. Maestro A northwesterly wind with fine weather which blows, especially in summer, in the Adriatic. It is most frequent on the western shore. This wind is also found on the coasts of Corsica and Sardinia. Matanuska Wind A strong, gusty, northeast wind which occasionally occurs during the winter in the vicinity of Palmer, Alaska. Mistral A cold, dry wind blowing from the north over the northwest coast of the Mediterranean Sea, particularly over the Gulf of Lions. Also called CIERZO. See also FALL WIND. Nashi, N’aschi A northeast wind which occurs in winter on the Iranian coast of the Persian Gulf, especially near the entrance to the gulf, and also on the Makran coast. It is probably associated with an outflow from the central Asiatic anticyclone which extends over the high land of Iran. It is similar in character but less severe than the BORA. Norte A strong cold northeasterly wind which blows in Mexico and on the shores of the Gulf of Mexico. It results from an outbreak of cold air from the north. It is the Mexican extension of a norther. Papagayo A violet northeasterly fall wind on the Pacific coast of Nicaragua and Guatemala. It consists of the cold air mass of a norte which has overridden the mountains of Central America. See also TEHUANTEPECER. Santa Ana A strong, hot, dry wind blowing out into San Pedro Channel from the southern California desert through Santa Ana Pass. Shamal A summer northwesterly wind blowing over Iraq and the Persian Gulf, often strong during the day, but decreasing at night
490WEATHER ELEMENTSSharkiA southeasterly wind which sometimesofthe Taku River, after which it is named,blows in the Persian Gulf.it sometimes attains hurricaneforceSiroccoA violent squally wind from north orA warm wind of the MediterraneanTehuantepecernorth-northeast in the Gulf ofarea, either a foehn or a hot southerlywind in advance ofa lowpressure areaTehuantepec (south of southern Mexico)moving from the Sahara or Arabianin winter. It originates in the Gulf ofdeserts. Called LEVECHE in Spain.Mexico as a norther which crosses theisthmus and blows through the gapSquamishA strong and often violent wind occurringbetween the Mexican and Guatamalanin many of the fjords of British Columbia.mountains.It maybefelt up to 100Squamishes occur in those fjords orientedmiles out to sea. See also PAPAGAYOin a northeast-southwest or east-westdirection where cold polar air can beTramontanaA northeasterly or northerly winter windfunneled westward. They are notable inoff the west coast of Italy.It is a freshJervis, Toba, and Bute inlets and in Deanwind ofthe fine weather mistral type.Channel and Portland Canal. SquamishesAcoldfall windblowingfromtheVardarnorthwest down the Vardar valley inlosetheirstrengthwhenfreeoftheconfining fjords and are not noticeable15Greece to the Gulf of Salonica.Itto 20 miles offshoreoccurs when atmospheric pressureover eastern Europe is higher than overSuestadoA stom with southeastgales,causedbythe Aegean Sea, as is often the case inintense cyclonic activity off the coasts ofwinter.Also called VARDARACArgentina and Uruguay, which affects theA foehn wind in the Schouten Islandssouthem part ofthe coast of Brazil in theWarmBrawnorth of NewGuinea.winter.White SquallA sudden, strong gust of wind comingSumatraA squall with violent thunder,up without warning, noted bylightning,and rain, which blows atwhitecaps or white, broken water,night in the Malacca Straits, especiallyusually seen in whirlwind form in clearduring the southwest monsoon. It isweather in the tropics.intensified by strong mountain breezes.WilliwawA sudden blast of wind descending fromTaku WindA strong, gusty, east-northeast wind,amountainous coastto the sea,in theoccurring in the vicinity of Juneau, Alaska,Strait of Magellan or the Aleutian Islandsbetween October and March.At the mouthAIRMASSES3509.TypesOfAir MassesorAntarctic (A),the northor south polar regions of iceandsnow.This classification isageneral indication ofrelativeBecause of large differences in physical characteristicstemperature, as well as latitude of origin.of the earth's surface, particularly the oceanic and continen-Air masses are further classified as maritime (m) ortal contrasts, the air overlying these surfaces acquirescontinental (c), depending upon whether they form overdifferingvalues oftemperatureandmoisture.Theprocesseswater or land.This classification is an indication of therel-ofradiation andconvectionin thelowerportions ofthetro-ativemoisturecontentoftheairmass.Tropical airmightbeposphere act in differing characteristic manners for adesignated maritimetropical (mT)or continental tropicalnumber of well-definedregions ofthe earth.The air overly-(cT).Similarly,polar air may beeither maritimepolar (mP)ingtheseregionsacquirescharacteristics commontotheor continental polar (cP).Arctic/Antarcticair, dueto theparticular area,but contrastingto those ofother areas.Eachpredominance of landmasses and ice fields in the high lati-distinctive part of the atmosphere, within which commontudes, is rarely maritime Arctic (mA)Equatorial air ischaracteristicsprevail overareasonably largearea,is calledfoundexclusivelyovertheoceansurfaceandisdesignatedanairmassneither (cE) nor (mE), but simply (E).Air masses are named according to their source re-Athirdclassification sometimesappliedtotropical andpolar air masses indicates whether the air mass is warm (w)gions,Fourregions aregenerallyrecognized:(1)equatorial(E), the doldrums area between the north and south trades;or cold (k)relative to the underlying surface. Thus, the sym-(2) tropical (T), the trade wind and lower temperate regions;bol mTw indicates maritime tropical air which is warmer(3) polar (P), the higher temperate latitudes; and (4) Arcticthantheunderlyingsurface.andcPkindicatescontinental
490 WEATHER ELEMENTS AIR MASSES 3509. Types Of Air Masses Because of large differences in physical characteristics of the earth’s surface, particularly the oceanic and continental contrasts, the air overlying these surfaces acquires differing values of temperature and moisture. The processes of radiation and convection in the lower portions of the troposphere act in differing characteristic manners for a number of well-defined regions of the earth. The air overlying these regions acquires characteristics common to the particular area, but contrasting to those of other areas. Each distinctive part of the atmosphere, within which common characteristics prevail over a reasonably large area, is called an air mass. Air masses are named according to their source regions. Four regions are generally recognized: (1) equatorial (E), the doldrums area between the north and south trades; (2) tropical (T), the trade wind and lower temperate regions; (3) polar (P), the higher temperate latitudes; and (4) Arctic or Antarctic (A), the north or south polar regions of ice and snow. This classification is a general indication of relative temperature, as well as latitude of origin. Air masses are further classified as maritime (m) or continental (c), depending upon whether they form over water or land. This classification is an indication of the relative moisture content of the air mass. Tropical air might be designated maritime tropical (mT) or continental tropical (cT). Similarly, polar air may be either maritime polar (mP) or continental polar (cP). Arctic/Antarctic air, due to the predominance of landmasses and ice fields in the high latitudes, is rarely maritime Arctic (mA). Equatorial air is found exclusively over the ocean surface and is designated neither (cE) nor (mE), but simply (E). A third classification sometimes applied to tropical and polar air masses indicates whether the air mass is warm (w) or cold (k) relative to the underlying surface. Thus, the symbol mTw indicates maritime tropical air which is warmer than the underlying surface, and cPk indicates continental Sharki A southeasterly wind which sometimes blows in the Persian Gulf. Sirocco A warm wind of the Mediterranean area, either a foehn or a hot southerly wind in advance of a low pressure area moving from the Sahara or Arabian deserts. Called LEVECHE in Spain. Squamish A strong and often violent wind occurring in many of the fjords of British Columbia. Squamishes occur in those fjords oriented in a northeast-southwest or east-west direction where cold polar air can be funneled westward. They are notable in Jervis, Toba, and Bute inlets and in Dean Channel and Portland Canal. Squamishes lose their strength when free of the confining fjords and are not noticeable 15 to 20 miles offshore. Suestado A storm with southeast gales, caused by intense cyclonic activity off the coasts of Argentina and Uruguay, which affects the southern part of the coast of Brazil in the winter. Sumatra A squall with violent thunder, lightning, and rain, which blows at night in the Malacca Straits, especially during the southwest monsoon. It is intensified by strong mountain breezes. Taku Wind A strong, gusty, east-northeast wind, occurring in the vicinity of Juneau, Alaska, between October and March. At the mouth of the Taku River, after which it is named, it sometimes attains hurricane force. Tehuantepecer A violent squally wind from north or north-northeast in the Gulf of Tehuantepec (south of southern Mexico) in winter. It originates in the Gulf of Mexico as a norther which crosses the isthmus and blows through the gap between the Mexican and Guatamalan mountains. It may be felt up to 100 miles out to sea. See also PAPAGAYO. Tramontana A northeasterly or northerly winter wind off the west coast of Italy. It is a fresh wind of the fine weather mistral type. Vardar A cold fall wind blowing from the northwest down the Vardar valley in Greece to the Gulf of Salonica. It occurs when atmospheric pressure over eastern Europe is higher than over the Aegean Sea, as is often the case in winter. Also called VARDARAC. Warm Braw A foehn wind in the Schouten Islands north of New Guinea. White Squall A sudden, strong gust of wind coming up without warning, noted by whitecaps or white, broken water; usually seen in whirlwind form in clear weather in the tropics. Williwaw A sudden blast of wind descending from a mountainous coast to the sea, in the Strait of Magellan or the Aleutian Islands
491WEATHER ELEMENTSpolar air which is colder than the underlying surface. The wcirculation,which in thetemperate latitudes is usually in anand k classifications are primarily indications of stabilityeasterlyand slightlypoleward direction(i.e., change of temperature with increasing height).If theAlong the leading edge of the wave, warmer air is re-air is cold relative tothe surface,thelowerportion ofthe airplacing colder air.This is called the warm front. Themass will be heated, resulting in instability (temperaturetrailing edge is the cold front, where colder air is under-markedly decreases with increasing height)as the warmerrunning and displacing warmerair.air tends to rise by convection. Conversely, if the air isThe warm air,being less dense,tendsto ride up greatlywarm relativetothesurface, the lower portion of the airover the colder air it is replacing.Partly becauseof the re-massiscooled.tendingtoremainclosetothesurface.Thisplacement of cold, dense air with warm, light air, theis a stable condition (temperature increases with increasingpressure decreases. Since the slope is gentle, the upper partheight).of a warmfrontal surfacemaybemanyhundredsof milesTwoothertypesofairmassesaresometimesrecognizedahead ofthe surface portion.The decreasingpressure, indi-These are monsoon (M),a transitional form between cP andcated by a"falling barometer,"is often an indication of theE;and superior (S),a special typeformed in the free atmo-approach of sucha wave.In a slow-moving,well-devel-spherebythe sinking and consequentwarming ofairaloftopedwave,thebarometermaybegintofall several daysbeforethewavearrives.Thus.theamountandnatureofthe3510.Frontschange of atmospheric pressure between observations.called pressure tendency,is of assistance in predicting theAs air masses move within thegeneral circulation, theyapproachofsuchasystem.travel from theirsourceregions tootherareas dominated byThe advancing cold air,being more dense,tends to rideairhavingdifferentcharacteristics.Thisleadstoazoneofunder the warmer air at the cold front, lifting it to greaterseparation between the two air masses, called a frontalheights.The slopehere is such that the upper-air portion ofthe cold front is behind the surface position relativeto itszone orfront,across whichtemperature,humidity,andwind speed and direction change rapidly.Fronts are repre-motion. After a cold front has passed, the pressure increas-sented on weather mapsby lines, acold front is shown withes, giving a rising barometer.pointed barbs,a warm frontwithroundedbarbs,and an oc-In thefirst stages, these effects arenotmarked, but asthe wave continues to grow, they become more pro-cluded front with both, alternating. A stationary front isshown withpointedand rounded barbs alternatingand onnounced,asshown inFigure3510b.Astheamplitudeoftheoppositesidesofthelinewiththepointedbarbsawayfromwave increases,pressure near the center usuallydecreases,the colder air.The front may take on a wave-like charac-and the low is said to"deepen."As it deepens, its forwardter,becominga"frontal wave."speed generally decreases.Before theformation offrontal waves, the isobars (linesThe approach of a well-developed warm front (i.e.,of equalatmosphericpressure)tendtorunparalleltothewhen the warm air is mT)is usuallyheralded not onlybyfronts.Asa waveis formed,thepatten is distorted some-falling pressure,but also by a more-or-less regular se-what, as shown in Figure 3510a. In this illustration, colder airquence of clouds. First, cirrus appear.These give wayisnorth ofwarmerair.InFigures3510a-3510d isobarsaresuccessivelyto cirrostratus,altostratus,altocumulus,anddrawnat4-millibar intervals.nimbostratus.Brief showers may precede the steady rainThe wave tends to travel in the directionof thegeneralaccompanyingthenimbostratus.DIRECTIONOFCLOUDSWAVE MOTION-1008FALLING1004SINGPRESSUREOCRILOWWARMPCOLDFRONT100FRONTQFigure3510a.First stage inthedevelopmentof afrontal wave (topview)
WEATHER ELEMENTS 491 polar air which is colder than the underlying surface. The w and k classifications are primarily indications of stability (i.e., change of temperature with increasing height). If the air is cold relative to the surface, the lower portion of the air mass will be heated, resulting in instability (temperature markedly decreases with increasing height) as the warmer air tends to rise by convection. Conversely, if the air is warm relative to the surface, the lower portion of the air mass is cooled, tending to remain close to the surface. This is a stable condition (temperature increases with increasing height). Two other types of air masses are sometimes recognized. These are monsoon (M), a transitional form between cP and E; and superior (S), a special type formed in the free atmosphere by the sinking and consequent warming of air aloft. 3510. Fronts As air masses move within the general circulation, they travel from their source regions to other areas dominated by air having different characteristics. This leads to a zone of separation between the two air masses, called a frontal zone or front, across which temperature, humidity, and wind speed and direction change rapidly. Fronts are represented on weather maps by lines; a cold front is shown with pointed barbs, a warm front with rounded barbs, and an occluded front with both, alternating. A stationary front is shown with pointed and rounded barbs alternating and on opposite sides of the line with the pointed barbs away from the colder air.The front may take on a wave-like character,becoming a “frontal wave.” Before the formation of frontal waves, the isobars (lines of equal atmospheric pressure) tend to run parallel to the fronts. As a wave is formed, the pattern is distorted somewhat, as shown in Figure 3510a. In this illustration, colder air is north of warmer air. In Figures 3510a–3510d isobars are drawn at 4-millibar intervals. The wave tends to travel in the direction of the general circulation, which in the temperate latitudes is usually in an easterly and slightly poleward direction. Along the leading edge of the wave, warmer air is replacing colder air. This is called the warm front. The trailing edge is the cold front, where colder air is underrunning and displacing warmer air. The warm air, being less dense, tends to ride up greatly over the colder air it is replacing. Partly because of the replacement of cold, dense air with warm, light air, the pressure decreases. Since the slope is gentle, the upper part of a warm frontal surface may be many hundreds of miles ahead of the surface portion. The decreasing pressure, indicated by a “falling barometer,” is often an indication of the approach of such a wave. In a slow-moving, well-developed wave, the barometer may begin to fall several days before the wave arrives. Thus, the amount and nature of the change of atmospheric pressure between observations, called pressure tendency, is of assistance in predicting the approach of such a system. The advancing cold air, being more dense, tends to ride under the warmer air at the cold front, lifting it to greater heights. The slope here is such that the upper-air portion of the cold front is behind the surface position relative to its motion. After a cold front has passed, the pressure increases, giving a rising barometer. In the first stages, these effects are not marked, but as the wave continues to grow, they become more pronounced, as shown in Figure 3510b. As the amplitude of the wave increases, pressure near the center usually decreases, and the low is said to “deepen.” As it deepens, its forward speed generally decreases. The approach of a well-developed warm front (i.e., when the warm air is mT) is usually heralded not only by falling pressure, but also by a more-or-less regular sequence of clouds. First, cirrus appear. These give way successively to cirrostratus, altostratus, altocumulus, and nimbostratus. Brief showers may precede the steady rain accompanying the nimbostratus. Figure 3510a. First stage in the development of a frontal wave (top view)
492WEATHERELEMENTSCOrCLOUDS1008HIGH1001FALLINGPRESSURERAINOWARM--1000FRC1004FRONT1008COLDHIGHFigure3510b.Afullydeveloped frontalwave (topview)OFALLINGPRESSUREHIGHRAINao1100%0-1012-COLDFRONT1016HIGHFigure 3510c.Afrontal wave nearing occlusion (top view)
492 WEATHER ELEMENTS Figure 3510b. A fully developed frontal wave (top view). Figure 3510c. A frontal wave nearing occlusion (top view)