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-