《航海学》课程参考文献（地文资料）CHAPTER 38 WEATHER ROUTING

CHAPTER 38WEATHERROUTINGPRINCIPLESOFWEATHERROUTING3800.Introductionroute,or if theforecasts permit,divertingto a shortertrackto takeadvantage of favorable weather and sea conditions.Ship weatherrouting develops an optimum track forThe greatest potential advantage for this ship weather rout-oceanvoyagesbased on forecastsof weather,sea condi-ing exists when:(1)thepassage is relativelylong,abouttions,and a ship's individual characteristics for a particular1,500miles or more,(2)the waters are navigationallyunretransit.Within specified limits of weather and sea condi-stricted, so that there is a choice of routes;, and (3)weatheris a factorin determining therouteto befollowedtions,thetermoptimumisusedtomeanmaximumsafetyandcrewcomfort,minimumfuelconsumption,minimumUse of this advisory service in no way relieves thetime underway,oranydesiredcombinationofthesefactorscommandingofficerormasterofresponsibilityforprudentThepurpose of this chapter is to acquaint the mariner withseamanship and safe navigation.There is no intent by thethe basicphilosophy and procedures of ship weatherrout-routingagencyto inhibit theexerciseof professional judging as an aid to understanding the routing agency'sment and prerogatives of commandingofficers andrecommendationsmasters.Themariner's first resources for routeplanning in rela-tion to weather are the Pilot Chart Atlases and the Sailing3801.HistoricalPerspectiveDirections (Planning Guides).These publications give cliThe advent ofextended range forecasting and thedevelop-matic data, such as waveheight frequencies and ice limits.forthemajor ocean basinsof theworld.Theyrecommendmentofselectiveclimatology,alongwithpowerful computerspecificroutesbased onprobabilities,butnot on specificmodeling techniques, have made ship routing systems possible.conditionsThe abilityto effectivelyadvise shipstotake advantageoffavor-The ship routingagency,acting as an advisory serviceable weather was hampered previously by forecast limitationsattempts to avoidorreducetheeffects of specificadverseandthelackofaneffectivecommunicationssystem.weather and sea conditions byissuing initial route recomDevelopment work in the area of data accumulation andclimatology has a long history. Benjamin Franklin, as deputymendations priorto sailing,recommendations fortrackpostmaster general of the British Colonies in North America,changes while underway (diversions), and weather adviso-ries to alert the commanding officer or master aboutproducedachartoftheGulf Streamfrominformationsuppliedapproachingunfavorable weather and seaconditionswhichby masters ofNew England whaling ships. This first mapping ofcannot be effectively avoided bya diversion.AdversetheGulfStreamhelped improvethemailpacketservicebetweenweatherandseaconditionsaredefinedasthoseconditionsthe British Colonies and England. In some passages the sailingwhich will cause damage, significant speed reduction, ortimewasreducedbvasmuchas14davsoverroutespreviouslytime loss.sailed.In themid-19th century,MatthewFontaineMaurycom-The initial route recommendation is based on a surveypiled large amounts of atmospheric and oceanographic dataofweatherandseaforecastsbetweenthepointofdeparturefromships'logbooks.Forthefirsttime,aclimatologyofoceanand the destination. It takes into account the hull type, speedweather and currents of the world was available to the mariner.capability,cargo, and loading conditions. The ship'sThis information was used by Maury to develop seasonally rec-progress is continuallymonitored, and, ifadverseweatherommendedroutesforsailingshipsandearlysteampoweredand sea conditions are forecast along the ship's currentvessels inthelatter halfofthe19thcentury.Inmanycases,Mau-track.arecommendationforadiversionorweatheradvisory'schartswereprovedcorrectbythesavingsintransittimery is transmitted to the ship.By this process of initial routeAveragetransittimeon theNewYorktoCaliforniaviaCapeselection and continuedmonitoringof the ship's progressHorn route was reduced from 183 days to 139 days with the useforpossiblechangesintheforecastweatherandseacondi-ofhisrecommendedseasonalroutestions along a route,it is possibleto maximizethe ship'sInthe1950'stheconceptof shipweatherroutingwasspeed and safety.put into operationby several privatemeteorological groupsInprovidingoptimum sailing conditions,theadvisoryand by the U.S.Navy.By applying the available surface andservice also attempts toreducetransit timeby avoidingtheupperairforecaststotransoceanicshipping,itwaspossibleadverseconditionswhichmaybeencounteredonashorterto effectively avoid much heavy weather whilegenerally539

539 CHAPTER 38 WEATHER ROUTING PRINCIPLES OF WEATHER ROUTING 3800. Introduction Ship weather routing develops an optimum track for ocean voyages based on forecasts of weather, sea condi￾tions, and a ship’s individual characteristics for a particular transit. Within specified limits of weather and sea condi￾tions, the term optimum is used to mean maximum safety and crew comfort, minimum fuel consumption, minimum time underway, or any desired combination of these factors. The purpose of this chapter is to acquaint the mariner with the basic philosophy and procedures of ship weather rout￾ing as an aid to understanding the routing agency’s recommendations. The mariner’s first resources for route planning in rela￾tion to weather are the Pilot Chart Atlases and the Sailing Directions (Planning Guides). These publications give cli￾matic data, such as wave height frequencies and ice limits, for the major ocean basins of the world. They recommend specific routes based on probabilities, but not on specific conditions. The ship routing agency, acting as an advisory service, attempts to avoid or reduce the effects of specific adverse weather and sea conditions by issuing initial route recom￾mendations prior to sailing, recommendations for track changes while underway (diversions), and weather adviso￾ries to alert the commanding officer or master about approaching unfavorable weather and sea conditions which cannot be effectively avoided by a diversion. Adverse weather and sea conditions are defined as those conditions which will cause damage, significant speed reduction, or time loss. The initial route recommendation is based on a survey of weather and sea forecasts between the point of departure and the destination. It takes into account the hull type, speed capability, cargo, and loading conditions. The ship’s progress is continually monitored, and, if adverse weather and sea conditions are forecast along the ship’s current track, a recommendation for a diversion or weather adviso￾ry is transmitted to the ship. By this process of initial route selection and continued monitoring of the ship’s progress for possible changes in the forecast weather and sea condi￾tions along a route, it is possible to maximize the ship’s speed and safety. In providing optimum sailing conditions, the advisory service also attempts to reduce transit time by avoiding the adverse conditions which may be encountered on a shorter route, or if the forecasts permit, diverting to a shorter track to take advantage of favorable weather and sea conditions. The greatest potential advantage for this ship weather rout￾ing exists when: (1) the passage is relatively long, about 1,500 miles or more; (2) the waters are navigationally unre￾stricted, so that there is a choice of routes; and (3) weather is a factor in determining the route to be followed. Use of this advisory service in no way relieves the commanding officer or master of responsibility for prudent seamanship and safe navigation. There is no intent by the routing agency to inhibit the exercise of professional judg￾ment and prerogatives of commanding officers and masters. 3801. Historical Perspective The advent of extended range forecasting and the develop￾ment of selective climatology, along with powerful computer modeling techniques, have made ship routing systems possible. The ability to effectively advise ships to take advantage of favor￾able weather was hampered previously by forecast limitations and the lack of an effective communications system. Development work in the area of data accumulation and climatology has a long history. Benjamin Franklin, as deputy postmaster general of the British Colonies in North America, produced a chart of the Gulf Stream from information supplied by masters of New England whaling ships. This first mapping of the Gulf Stream helped improve the mail packet service between the British Colonies and England. In some passages the sailing time was reduced by as much as 14 days over routes previously sailed. In the mid-19th century, Matthew Fontaine Maury com￾piled large amounts of atmospheric and oceanographic data from ships’ log books. For the first time, a climatology of ocean weather and currents of the world was available to the mariner. This information was used by Maury to develop seasonally rec￾ommended routes for sailing ships and early steam powered vessels in the latter half of the 19th century. In many cases, Mau￾ry’s charts were proved correct by the savings in transit time. Average transit time on the New York to California via Cape Horn route was reduced from 183 days to 139 days with the use of his recommended seasonal routes. In the 1950’s the concept of ship weather routing was put into operation by several private meteorological groups and by the U.S. Navy. By applying the available surface and upper air forecasts to transoceanic shipping, it was possible to effectively avoid much heavy weather while generally

540WEATHERROUTINGsailing shorter routes than previously.Ship weather routing services are being offered bymany nations.These include Japan,UnitedKingdom,Rus-Optimum Track Ship Routing (OTSR), the ship rout-sia,Netherlands,Germany,and the United States.Also,ing service of the U.S.Navy,utilizes short range andseveral private firms provide routing services to shippingextendedrangeforecastingtechniquesinrouteselectionand surveillanceprocedures.The shortrange dynamicfore-industryclientscasts of 3 to 5 days are derived from meteorologicalThere are two general types ofcommercial ship routingequations.Theseforecastsarecomputedtwicedailyfromaservices. The first uses techniques similar to the Navy'sdata base ofnorthern hemisphere surface and upperair ob-OTSR system to forecast conditions and compute routingservations,and include surfacepressure,upper air constantrecommendations.Thesecond assembles andprocessespressure heights, and the spectral wave values. A signifi-weatherand seacondition dataandtransmitsthisto shipsatcant increase in data input,particularly from satellitesea for on-board processing and generation ofroute recom-informationoveroceanareas.canextendthetimeperiodmendations.Theformer system allows forgreaterfor which these forecasts are useful.computerpower tobeapplied to the routing task becauseFor extended rangeforecasting,generally 3 to 14 days,powerful computers are available ashore.Thelatter systema computer searches a library of historical northern hemi-allows greater flexibility to the ship's master in changingsphere surface pressureand 500millibaranalyses foranparameters, selecting routes,and displaying data.analogous weather pattern.This is an attemptat selectiveclimatology bymatchingthe curent weatherpattern with3802.ShipAndCargoConsiderationspast weather patterns and providing a logical sequence-of-eventsforecastforthe10to14dayperiodfollowingthedy-Ship and cargo characteristics have a significant influ-namicforecast.ItisperformedforboththeAtlanticandenceontheapplicationof shipweatherrouting.Shipsize,Pacific Oceansusing climatological datafor the entireperi-speed capabilityandtypeofcargoare importantconsider-od ofdata stored in the computer.Forlongeroceantransits,ations in the route selection process priorto sailing and themonthlyvalues of wind, seas,fog,andocean currentsaresurveillance procedure whileunderway.Aship's character-used tofurtherextend thetimerange.isticsidentifyitsvulnerabilitytoadverseconditionsanditsAviation was first in applying the principle of mini-abilitytoavoidthemmum time tracks (MTT)to a changing wind field.But theGenerally,ships with higher speed capability and lessproblem of finding an MTT for a specific flight is muchcargo encumbrances will haveshorter routes and bebettersimplerthanforatransoceanic shippassagebecausean air-ableto maintain near normal SOA's than ships with lowercraft's transit time is much shorter than a ship's. Thus,speed capability or cargoes.Some routes areunique be-marine minimum time tracks require significantly longercauseofthetypeofshiporcargo.Avoidingoneelementofrangeforecaststodevelopanoptimumroute.weatherto reducepounding or rollingmaybeof prime im-Automation has enabled shiproutingagencies tode-portance.Forexample,a2oknotshipwitha heavydeckveloprealistic minimumtimetracks.Computation ofcargo may be severely hampered in its ability tomaintain aminimumtimetracksmakesuseof:20knot SOAinanyseas exceedingmoderateheadorbeamseas becauseofthepossibility ofdamageresultingfromthe1.A navigation system to computeroutedistance,deckload'scharacteristics.Asimilarshipwithastablecartime enroute, estimated times of arrival (ETA's),go under the deck is not as vulnerableand maybe abletoandtoprovide6hourlyDRsynopticpositionsformaintain the20knot SOAin conditions whichwould dras-the range of the dynamic forecasts for the ship'stically slow the deck-loaded vessel. In towing operations, acurrent tracktug is more vulnerable to adverse weather and sea condi-tions,notonly in considerationofthetow,butalsobecause2.A surveillance system to survey wind, seas, fog.and ocean currents obtained from thedynamicandof its alreadylimited speed capability.Its slow speed addsto the difficulty of avoiding adverse weather and seaclimatologicalfields.conditions.3.An environmental constraint systemimposedaspart of the route selection and surveillance process.Shipperformancecurves(speedcurves)areusedtoes-Constraints are theupper limits of wind and seastimate the ship's SOA while transiting the forecast seastates.Thecurves indicatethe effectofhead,beam,andfol-desired for the transit.They are determined bytheship'sloading,speed capability,and vulnerability.lowing seas of various significant wave heights on theTheconstraint systemisan importantpartoftheship's speed.Figure3802 isaperformancecurvepreparedroute selection process and acts as a warning sys-foran18knotvessel.tem when the weather and sea forecast along theWith the speed curves it is possibletodetermine justpresenttrack exceedspredetermined limits.how costlya diversion will be in terms of therequired dis-4.Ship speed characteristics used toapproximatetance and time.A diversion maynot be necessary where theship's speed of advance (SOA) while transiting theduration of the adverseconditions islimited.In this case,itforecast sea states.maybe better to ride outtheweather and seasknowing that

540 WEATHER ROUTING sailing shorter routes than previously. Optimum Track Ship Routing (OTSR), the ship rout￾ing service of the U.S. Navy, utilizes short range and extended range forecasting techniques in route selection and surveillance procedures. The short range dynamic fore￾casts of 3 to 5 days are derived from meteorological equations. These forecasts are computed twice daily from a data base of northern hemisphere surface and upper air ob￾servations, and include surface pressure, upper air constant pressure heights, and the spectral wave values. A signifi￾cant increase in data input, particularly from satellite information over ocean areas, can extend the time period for which these forecasts are useful. For extended range forecasting, generally 3 to 14 days, a computer searches a library of historical northern hemi￾sphere surface pressure and 500 millibar analyses for an analogous weather pattern. This is an attempt at selective climatology by matching the current weather pattern with past weather patterns and providing a logical sequence-of￾events forecast for the 10 to 14 day period following the dy￾namic forecast. It is performed for both the Atlantic and Pacific Oceans using climatological data for the entire peri￾od of data stored in the computer. For longer ocean transits, monthly values of wind, seas, fog, and ocean currents are used to further extend the time range. Aviation was first in applying the principle of mini￾mum time tracks (MTT) to a changing wind field. But the problem of finding an MTT for a specific flight is much simpler than for a transoceanic ship passage because an air￾craft’s transit time is much shorter than a ship’s. Thus, marine minimum time tracks require significantly longer range forecasts to develop an optimum route. Automation has enabled ship routing agencies to de￾velop realistic minimum time tracks. Computation of minimum time tracks makes use of: 1. A navigation system to compute route distance, time enroute, estimated times of arrival (ETA’s), and to provide 6 hourly DR synoptic positions for the range of the dynamic forecasts for the ship’s current track. 2. A surveillance system to survey wind, seas, fog, and ocean currents obtained from the dynamic and climatological fields. 3. An environmental constraint system imposed as part of the route selection and surveillance process. Constraints are the upper limits of wind and seas desired for the transit. They are determined by the ship’s loading, speed capability, and vulnerability. The constraint system is an important part of the route selection process and acts as a warning sys￾tem when the weather and sea forecast along the present track exceeds predetermined limits. 4. Ship speed characteristics used to approximate ship’s speed of advance (SOA) while transiting the forecast sea states. Ship weather routing services are being offered by many nations. These include Japan, United Kingdom, Rus￾sia, Netherlands, Germany, and the United States. Also, several private firms provide routing services to shipping industry clients. There are two general types of commercial ship routing services. The first uses techniques similar to the Navy’s OTSR system to forecast conditions and compute routing recommendations. The second assembles and processes weather and sea condition data and transmits this to ships at sea for on-board processing and generation of route recom￾mendations. The former system allows for greater computer power to be applied to the routing task because powerful computers are available ashore. The latter system allows greater flexibility to the ship’s master in changing parameters, selecting routes, and displaying data. 3802. Ship And Cargo Considerations Ship and cargo characteristics have a significant influ￾ence on the application of ship weather routing. Ship size, speed capability, and type of cargo are important consider￾ations in the route selection process prior to sailing and the surveillance procedure while underway. A ship’s character￾istics identify its vulnerability to adverse conditions and its ability to avoid them. Generally, ships with higher speed capability and less cargo encumbrances will have shorter routes and be better able to maintain near normal SOA’s than ships with lower speed capability or cargoes. Some routes are unique be￾cause of the type of ship or cargo. Avoiding one element of weather to reduce pounding or rolling may be of prime im￾portance. For example, a 20 knot ship with a heavy deck cargo may be severely hampered in its ability to maintain a 20 knot SOA in any seas exceeding moderate head or beam seas because of the possibility of damage resulting from the deck load’s characteristics. A similar ship with a stable car￾go under the deck is not as vulnerable and may be able to maintain the 20 knot SOA in conditions which would dras￾tically slow the deck-loaded vessel. In towing operations, a tug is more vulnerable to adverse weather and sea condi￾tions, not only in consideration of the tow, but also because of its already limited speed capability. Its slow speed adds to the difficulty of avoiding adverse weather and sea conditions. Ship performance curves (speed curves) are used to es￾timate the ship’s SOA while transiting the forecast sea states. The curves indicate the effect of head, beam, and fol￾lowing seas of various significant wave heights on the ship’s speed. Figure 3802 is a performance curve prepared for an 18 knot vessel. With the speed curves it is possible to determine just how costly a diversion will be in terms of the required dis￾tance and time. A diversion may not be necessary where the duration of the adverse conditions is limited. In this case, it may be better to ride out the weather and seas knowing that

541WEATHERROUTINGrouting is normally considered attained if the effects ofa diversion,even if ableto maintainthenormal SOA,willnot overcomethe increaseddistanceand timerequiredwindand seas canbeoptimized.At other times, the diversion track is less costly be-Wind:The effect of wind speed on ship performancecause itavoids an area of adverse weather and seais difficult to determine. In light winds (less than 20-knots),conditions.whilebeingabletomaintainnormalSOAevenships lose speed in headwinds and gain speed slightly in fol-thoughthedistancetodestinationisincreased.Basedonin-lowing winds.For higher wind speeds,ship speed isput data for environmental conditions and ship's behavior,reduced in both head and following winds.This is due to theroute selection and surveillance techniques seek to achieveincreasedwaveaction,whicheven infollowing seas resultsthe optimum balance between time,distance,and acceptin increased drag from steering corrections, and indicatesable environmental and seakeeping conditions.Althoughthe importanceofseaconditions indetermining shipperfor-speed performance curves are an aid to the ship routingmance.In dealing with wind, it is also necessary to knowagency,the responsebymarinersto deterioratingweatherthe ship's sail area. High winds will have a greater adverseandseaconditionsisnotuniform.Somereducespeedvol-effect on alarge,fullyloaded containershipor carcarrieruntarily or change heading sooner than others whenthan a fully loaded tanker of similar length.This effect isunfavorable conditions are encountered.Certain wavesmostnoticeablewhendocking,buttheeffectofbeamwindswith characteristics suchthattheship's bowand stern are inover several days at sea can also be considerable.successive crests and troughs present special problems forWave Height: Wave height is the major factor affect-the mariner.Being nearly equal to the ship's length, suchingshipperformance.Waveaction isresponsibleforshipwavelengths may induce very dangerous stresses.The de-motions which reduce propeller thrust and cause increasedgree ofhogging and sagging andtheassociated dangermaydrag from steering corrections.The relationship of shipbemore apparent tothemariner than to theship routingspeed towavedirection and height is similar to thatofwind.agency.Therefore, adjustment in course and speed for aHead seas reduce ship speed, while following seas increasemore favorable ride may be initiated by the commandingship speed slightly to a certain point, beyond which they re-officerormasterwhenthissituationisencounteredtard it. In heavy seas, exact performance may be difficult topredictbecause oftheadjustments to courseand speedfor3803.EnvironmentalFactorsshiphandling and comfort. Although the effect of sea andswell ismuchgreater than wind, it isdifficultto separatetheEnvironmental factors of importancetoship weathertwo in ship routing.routingarethoseelementsoftheatmosphereandoceanthatInan effort toprovideamoredetailed description ofmayproduce a change in the status of a shiptransit.In shipthe actual and forecast sea state, the U.S.Navy Fleet Nu-routing, consideration is given to wind, seas, fog, ice, andmericalMeteorologyandOceanographyCenterocean currents.While all of the environmental factors areMontereyCalionia,producestheGlobalSpectralOceanimportantfor route selection and surveillance, optimum2018Following seas16BeamSeasaeusHeadseas6F4F1022Wave height (ft.)Figure3802.Performance curves for head, beam, andfollowing seas

WEATHER ROUTING 541 a diversion, even if able to maintain the normal SOA, will not overcome the increased distance and time required. At other times, the diversion track is less costly be￾cause it avoids an area of adverse weather and sea conditions, while being able to maintain normal SOA even though the distance to destination is increased. Based on in￾put data for environmental conditions and ship’s behavior, route selection and surveillance techniques seek to achieve the optimum balance between time, distance, and accept￾able environmental and seakeeping conditions. Although speed performance curves are an aid to the ship routing agency, the response by mariners to deteriorating weather and sea conditions is not uniform. Some reduce speed vol￾untarily or change heading sooner than others when unfavorable conditions are encountered. Certain waves with characteristics such that the ship’s bow and stern are in successive crests and troughs present special problems for the mariner. Being nearly equal to the ship’s length, such wavelengths may induce very dangerous stresses. The de￾gree of hogging and sagging and the associated danger may be more apparent to the mariner than to the ship routing agency. Therefore, adjustment in course and speed for a more favorable ride may be initiated by the commanding officer or master when this situation is encountered. 3803. Environmental Factors Environmental factors of importance to ship weather routing are those elements of the atmosphere and ocean that may produce a change in the status of a ship transit. In ship routing, consideration is given to wind, seas, fog, ice, and ocean currents. While all of the environmental factors are important for route selection and surveillance, optimum routing is normally considered attained if the effects of wind and seas can be optimized. Wind: The effect of wind speed on ship performance is difficult to determine. In light winds (less than 20-knots), ships lose speed in headwinds and gain speed slightly in fol￾lowing winds. For higher wind speeds, ship speed is reduced in both head and following winds. This is due to the increased wave action, which even in following seas results in increased drag from steering corrections, and indicates the importance of sea conditions in determining ship perfor￾mance. In dealing with wind, it is also necessary to know the ship’s sail area. High winds will have a greater adverse effect on a large, fully loaded container ship or car carrier than a fully loaded tanker of similar length. This effect is most noticeable when docking, but the effect of beam winds over several days at sea can also be considerable. Wave Height: Wave height is the major factor affect￾ing ship performance. Wave action is responsible for ship motions which reduce propeller thrust and cause increased drag from steering corrections. The relationship of ship speed to wave direction and height is similar to that of wind. Head seas reduce ship speed, while following seas increase ship speed slightly to a certain point, beyond which they re￾tard it. In heavy seas, exact performance may be difficult to predict because of the adjustments to course and speed for shiphandling and comfort. Although the effect of sea and swell is much greater than wind, it is difficult to separate the two in ship routing. In an effort to provide a more detailed description of the actual and forecast sea state, the U.S. Navy Fleet Nu￾merical Meteorology and Oceanography Center, Monterey, California, produces the Global Spectral Ocean Figure 3802. Performance curves for head, beam, and following seas

WaveModel (GSOWM)forusebytheU.S.Navy'sOpti-relativelylowlatitudes.The important considerations tomumTrackShipRouting(OTSR)service.Thismodelbe evaluated are the difference in distance between aprovides energyvaluesfrom 12 differentdirections (300great-circle route and a route selected for optimum cur-sectors)and15frequencybandsforwaveperiodsfrom6rent, with the expected increase in SOA from theto 26seconds withthetotal waveenergypropagatedfollowing current, and thedecreased probability of a di-throughout thegrid system as a function of direction andversion for weather and seas at the lower latitude.Forfrequency.Itisbasedontheanalyzedandforecastplane-example,ithasprovenbeneficial toremain equatorwardof approximately 22°N for westbound passages betweentary boundary layer model wind fields, and is produced forboth the Northern and Southern Hemispheres out to 72the Canal Zone and southwest Pacific ports. For east-hours.For OTSR purposes,primary and secondary wavesboundpassages,ifthemaximumlatitudeonagreat-circlearederived from thespectral waveprogram,wherethetrackfromthe southwest PacifictotheCanal Zoneis be-primary wave train has the principal energy (direction andlow24°N,a routepassingnear theaxis of the Equatorialfrequency),and the secondary has tobe 20percent oftheCountercurrent is practical because the increaseddistanceprimary,is offset by favorable current.Direction and speed ofocean currents aremore predictablethan wind and seas,Fog: Fog, while not directly affecting ship perfor-but somevariabilitycan be expected.Major ocean cur-mance,should beavoided as muchas feasible,in ordertorents can be disrupted for several days by very intensemaintain normal speed in safeconditions.Extensiveareasweather systems such as hurricanes and byglobal phe-offogduringsummertimecanbeavoidedbyselectinganomenasuchasEINinolowerlatituderoutethan onebased solelyuponwind andIce:The problem of ice is twofold:floating ice (ice-seas.Although the route maybe longer, transittime maybergs)anddeckice.Ifpossible,areas oficebergsorpack icebe less due to not having to reduce speed in reduced visi-should be avoided because of the difficultyof detection andbility.In addition,crew fatigueduetoincreasedthepotential for collision.Deck icemaybemoredifficulttowatchkeeping vigilance can be reduced.contendwithfroma shiproutingpointof viewbecause it isNorthWall Effect:DuringtheNorthernHemispherecaused by freezing weather associated with a large weatherfall and winter, the waters to the north of the Gulf Streamsystem.While mostly a nuisancefactor on large ships,itin the North Atlantic are at their coldest, while the Gulfcausessignificantproblemswiththestabilityofsmall ships.Stream itself remains at a constant relatively warm tem-Latitude: Generally,the higher the latitudeofa route.perature.After passage ofa strong cold front orbehind aeven in the summer, the greaterare the problems with thedeveloping coastal low pressure system,Arctic air isenvironment.Certainoperations should benefitfrom sea-sometimes drawn off the Mid-Atlantic coast of theUnitedsonal planningaswell as optimum routingForexampleStatesand out overthewarmwaters oftheGulf Streambytowingoperationsnorthof about4oolatitude shouldbenortherly winds. This cold air is warmed as it passes overavoided innon-summermonths ifpossible.the Gulf Stream,resulting in rapid and intense deepeningofthelowpressuresystemandhigherthannormal surface3804.SynopticWeatherConsiderationswinds.Higher waves and confused seas result from thesewinds.When thesewinds oppose the northeast set of thecurrent, the result is increased wave heights and a shorten-A ship routing agency should direct its forecastinging of the wave period. If the opposing current isskillsto avoiding or limiting theeffect of weather and seassufficiently strong,the waves will break.These phenome-associated with extratropical lowpressure systems inthena are collectively called the "North Wall Effect"mid and higherlatitudes and thetropical systems in lowlat-referring tothe region of most dramatictemperatureitude.Seasonalormonsoonweatheris alsoafactor inroutechangebetween the cold watertothe north and thewarmselection and diversion in certain areasGulf Stream waterto the south.The most dangerous as-Despite the amount of attention and publicity given topect of this phenomenon is that the strong winds andtropical cyclones,mid-latitude low pressure systems gener-extremelyhigh, steep waves occur in a limited area andally present more difficult problems to a ship routingmay develop without warning.Thus, a ship that is labor-agency.This is primarily dueto thefactthat major shiptraf-ing in near-gale force northerly winds and rough seas.fic is sailing in the latitudes of themigrating lowpressureproceeding on a northerly course,can suddenlyencountersystems,and theamountof potential exposureto intensestormforcewindsanddangerouslyhighbreakingseasweather systems,especially in winter, is much greater.Numerous ships havefoundered off theNorth AmericanLow pressure systems weaker than gale intensitycoast in the approximate position of the Gulf Stream's(windslessthan34knots)arenotasevereproblemformostNorth Wall.A similar phenomenon occurs in the Northships.However,a relatively weak system may generatepro-Pacific near the Kuroshio Current and off the Southeastlongedperiodsof roughseaswhichmayhampernormalAfrican coast neartheAgulhas Current.work aboard ship.Ship weather routing canfrequently limitOcean Currents: Ocean currents do not present arough conditionsto short periods of time and provide moresignificantroutingproblem,but they canbeadeterminingfavorableconditionsformostofthetransit.Relativelysmallfactor in route selection and diversion.This is especiallyships,tugswithtows,lowpowered ships,and ships withtrue when the points of departure and destination are atsensitive cargoes can be significantly affected by weather

Wave Model (GSOWM) for use by the U.S. Navy’s Opti￾mum Track Ship Routing (OTSR) service. This model provides energy values from 12 different directions (30° sectors) and 15 frequency bands for wave periods from 6 to 26 seconds with the total wave energy propagated throughout the grid system as a function of direction and frequency. It is based on the analyzed and forecast plane￾tary boundary layer model wind fields, and is produced for both the Northern and Southern Hemispheres out to 72 hours. For OTSR purposes, primary and secondary waves are derived from the spectral wave program, where the primary wave train has the principal energy (direction and frequency), and the secondary has to be 20 percent of the primary. Fog: Fog, while not directly affecting ship perfor￾mance, should be avoided as much as feasible, in order to maintain normal speed in safe conditions. Extensive areas of fog during summertime can be avoided by selecting a lower latitude route than one based solely upon wind and seas. Although the route may be longer, transit time may be less due to not having to reduce speed in reduced visi￾bility. In addition, crew fatigue due to increased watchkeeping vigilance can be reduced. North Wall Effect: During the Northern Hemisphere fall and winter, the waters to the north of the Gulf Stream in the North Atlantic are at their coldest, while the Gulf Stream itself remains at a constant relatively warm tem￾perature. After passage of a strong cold front or behind a developing coastal low pressure system, Arctic air is sometimes drawn off the Mid-Atlantic coast of the United States and out over the warm waters of the Gulf Stream by northerly winds. This cold air is warmed as it passes over the Gulf Stream, resulting in rapid and intense deepening of the low pressure system and higher than normal surface winds. Higher waves and confused seas result from these winds. When these winds oppose the northeast set of the current, the result is increased wave heights and a shorten￾ing of the wave period. If the opposing current is sufficiently strong, the waves will break. These phenome￾na are collectively called the “North Wall Effect,” referring to the region of most dramatic temperature change between the cold water to the north and the warm Gulf Stream water to the south. The most dangerous as￾pect of this phenomenon is that the strong winds and extremely high, steep waves occur in a limited area and may develop without warning. Thus, a ship that is labor￾ing in near-gale force northerly winds and rough seas, proceeding on a northerly course, can suddenly encounter storm force winds and dangerously high breaking seas. Numerous ships have foundered off the North American coast in the approximate position of the Gulf Stream’s North Wall. A similar phenomenon occurs in the North Pacific near the Kuroshio Current and off the Southeast African coast near the Agulhas Current. Ocean Currents: Ocean currents do not present a significant routing problem, but they can be a determining factor in route selection and diversion. This is especially true when the points of departure and destination are at relatively low latitudes. The important considerations to be evaluated are the difference in distance between a great-circle route and a route selected for optimum cur￾rent, with the expected increase in SOA from the following current, and the decreased probability of a di￾version for weather and seas at the lower latitude. For example, it has proven beneficial to remain equatorward of approximately 22°N for westbound passages between the Canal Zone and southwest Pacific ports. For east￾bound passages, if the maximum latitude on a great-circle track from the southwest Pacific to the Canal Zone is be￾low 24°N, a route passing near the axis of the Equatorial Countercurrent is practical because the increased distance is offset by favorable current. Direction and speed of ocean currents are more predictable than wind and seas, but some variability can be expected. Major ocean cur￾rents can be disrupted for several days by very intense weather systems such as hurricanes and by global phe￾nomena such as El Nino. Ice: The problem of ice is twofold: floating ice (ice￾bergs) and deck ice. If possible, areas of icebergs or pack ice should be avoided because of the difficulty of detection and the potential for collision. Deck ice may be more difficult to contend with from a ship routing point of view because it is caused by freezing weather associated with a large weather system. While mostly a nuisance factor on large ships, it causes significant problems with the stability of small ships. Latitude: Generally, the higher the latitude of a route, even in the summer, the greater are the problems with the environment. Certain operations should benefit from sea￾sonal planning as well as optimum routing. For example, towing operations north of about 40° latitude should be avoided in non-summer months if possible. 3804. Synoptic Weather Considerations A ship routing agency should direct its forecasting skills to avoiding or limiting the effect of weather and seas associated with extratropical low pressure systems in the mid and higher latitudes and the tropical systems in low lat￾itude. Seasonal or monsoon weather is also a factor in route selection and diversion in certain areas. Despite the amount of attention and publicity given to tropical cyclones, mid-latitude low pressure systems gener￾ally present more difficult problems to a ship routing agency. This is primarily due to the fact that major ship traf￾fic is sailing in the latitudes of the migrating low pressure systems, and the amount of potential exposure to intense weather systems, especially in winter, is much greater. Low pressure systems weaker than gale intensity (winds less than 34 knots) are not a severe problem for most ships. However, a relatively weak system may generate pro￾longed periods of rough seas which may hamper normal work aboard ship. Ship weather routing can frequently limit rough conditions to short periods of time and provide more favorable conditions for most of the transit. Relatively small ships, tugs with tows, low powered ships, and ships with sensitive cargoes can be significantly affected by weather

2-:.WEATHERROUTINGOCTORERONEMVARYThere oreno isolinesSof 10%gale frequencyMaythrough September0Figure 3804a. Generalized 10% frequency isolines of gale force winds for October through January

WEATHER ROUTING 543 Figure 3804a. Generalized 10% frequency isolines of gale force winds for October through January

I+营4治3WEATHER ROUTING墅APRILFEBRUARY:JANARY.Thereare noisolinesof10%galefrequencyMaythroughSeptember-Figure 3804b.Generalized 10%frequency isolines of gale force winds for January throughApril

544 WEATHER ROUTING Figure 3804b. Generalized 10% frequency isolines of gale force winds for January through April

33R9E8.1Ro0V102S常3a2-元4-".12010eR.8-5fogo2L0oto02富100R.ea19%m武明50%initialdetectionareaa&&9o%Initialdetectionareaa源R.n.O120%8890RWEATHERROUTINGANISLANDSoFigure 3804c, Area of initial detection of high percentage of tropical cyclones which later developed to tropical storm or hurricaine intensity, 1957-1974545

WEATHER ROUTING 545 Figure 3804c. Area of initial detection of high percentage of tropical cyclones which later developed to tropical storm or hurricaine intensity, 1957-1974

36WEATHERROUTING50%Initialdetectionarea9o%InitialdetectionareeMARCUSISLANDA0WAKISANDeFigure 3804d..Area of initial detection of high percentage of tropical cyclones which later developed to tropical storm or hurricaine intensity,1946-1973

546 WEATHER ROUTING Figure 3804d. Area of initial detection of high percentage of tropical cyclones which later developed to tropical storm or hurricaine intensity, 1946-1973

547WEATHERROUTINGsystems weaker than gale intensity. Using a routing agencyIt has proven equally beneficial to employ similar con-can be beneficial.siderations for routing in themonsoon areas of the IndianOcean and theSouth China Sea.This is accomplished byGales (winds 34 to 47 knots) and storms (winds greaterproviding routes and diversions that generally avoid the ar-than48knots)in theopen seacan generateveryroughoreas of high frequencyof galeforce windsand associatedhigh seas,particularly when an adverse current such as theheavy seas, as much as feasible.Ships can then remain inGulf Stream is involved.This canforce a reduction in speedsatisfactory conditions with limited increases in routeinordertogainamorecomfortableand saferide.Becauseofdistance.the extensive geographic area covered by a well developedlowpressure system, once ship's speed is reduced the abilityDependinguponthepoints of departureanddestinationtoimprovetheship'ssituationisseverelyhampered.Thustherearemanycombinationsofroutesthatcanbeusedwhenexposuretopotentialdamageanddanger isgreatlyincreasedtransiting the northern IndianOcean(Arabian Sea,Bay ofArecommendation for a diversion by a routing agency wellBengal)andtheSouthChinaSea.Forexample,intheArabian Sea during the summer monsoon,routes to and from theinadvanceoftheintenseweatherandassociatedseaswilllimit the duration of exposure of the ship.If effective, shipRed Sea,thewesternPacific,andthe eastern Indian Oceanshould hold equatorward. Ships proceeding to the Persianspeed will notbe reduced and satisfactoryprogress will bemaintained even though the remaining distance to destina-Gulf during this period are held farther south and west toputtion is increased.Overall transit time is usually shorter thanthe heaviest seas on the quarter or stern when transiting theArabian SeaEastbound ships departing thePersian Gulfif notrack changehad been madeand the ship had remainedmayproceed generally east southeasttoward the Indian sub-in heavy weather.In some cases diversions are made to avoidadverseweatherconditionsand shorten thetrackat the samecontinent, then south, to pass north and east of the highestsouthwesterly seas in the Arabian Sea.Westbound ships outtime.Significantsavingsintimeandcostscanresultofthe Persian GulffortheCapeofGood Hope appear to haveIn very intense low pressure systems, with high windslittlechoiceinroutesunlessconsiderabledistanceisaddedtoand long duration overa long fetch, seas will be generatedthe transitby passing east of the highest seas.In thewinterandpropagatedasswelloverconsiderabledistances.Evenonmonsoon,routestoorfromtheRed SeaforthewesternPa-adiversion,itisdifficulttoeffectivelyavoidallunfavorablecific and the Indian Ocean are held farther north in theconditions.Generally,original routes for transoceanic pas-Arabian Sea to avoid thehighest seas.Ships proceeding tosages, issued by the U.s. Navy's ship routing service, arethePersian Gulffrom thewesternPacificand eastern Indianequatorward of the 10% frequency isoline for gale forceOceanmayholdmoreeastwardwhenproceedingnorthinthewinds for the month of transit, as interpreted from the U.SArabian Sea.Shipsdeparting thePersianGulf areawill haveNavy's MarineClimaticAtlasoftheWorld.Theseareshownconsiderablyless difficulty than duringthesummermon-inFigure3804a and Figure 3804b for the Pacific.To avoidsoon.Similar considerations can begivenwhenrouting shipsthearea of significant gale activity intheAtlantic from Octo-proceedingto and fromtheBayof Bengalber to April, the latitude of transit is generally in the lowerInthe South China Sea,transits via thePalawan Pas-thirties.sage are recommended when strong,opposing wind andTheareas,seasons,and theprobabilityofdevelopmentseas are forecast. This is especially true during the winteroftropicalcvclonesarefairlywelldefinedinclimatologicalmonsoon.During periods when the major monsoon flow ispublications.Inlong rangeplanning,considerablebenefitslack,ships can use the shortest track as conditions permit.can begained by limiting the exposureto the potential haz-ardsoftropicalsvstems.3805.Special Weather And EnvironmentalIn the North Pacific, avoid areas with the greatest prob-Considerationsability of tropical cyclone formation.Avoiding existingtropical cycloneswithahistoryof24hoursormoreof6-In addition to the synoptic weather considerations inhourlywarnings is in mostcases relatively straightforward.ship weather routing,there are special environmental prob-However,whentransitingthetropical cyclonegeneratingar-ea, the ship under routing may provide the first report oflemsthatcanbeavoidedbyfollowingrecommendationsand advisories of ship routing agencies.These problemsenvironmental conditions indicatingthatanewdisturbanceisgenerally cover a smaller geographic area and are seasonaldeveloping.In the eastern North Pacific the generating areain nature, but are still importantto ship routing.for a highpercentage oftropical cyclones is relatively com-pact (Figure3804c).Remain south of a line fromlat.9oNIn the North Atlantic, because of heavy shipping traf-long.90°Wtolat.14°N,long.115°W.InthewesternNorthfic,frequent poor visibilityin rain or fog,and restrictedPacific it is advisabletohold north of22°Nwhen no tropicalnavigationparticularlyeastofDoverStrait,somemarinesystems are known to exist. See Figure 3804d.prefer to transit to orfrom the North Sea via Pentland Firthpassingnorthof theBritish Islesratherthanvia theEnglishIn theAtlantic,sail near the axis of the Bermuda high orChannelnorthwardtoavoid thearea offormationoftropical cyclones.Weather routed shipsgenerallyavoid the area of denseOfcourse.avoidinganexistingtropicalcvclonetakesprecefog with low visibility in thevicinity ofthe Grand Banks offdenceoveravoidingageneralarea ofpotential development

WEATHER ROUTING 547 systems weaker than gale intensity. Using a routing agency can be beneficial. Gales (winds 34 to 47 knots) and storms (winds greater than 48 knots) in the open sea can generate very rough or high seas, particularly when an adverse current such as the Gulf Stream is involved. This can force a reduction in speed in order to gain a more comfortable and safe ride. Because of the extensive geographic area covered by a well developed low pressure system, once ship’s speed is reduced the ability to improve the ship’s situation is severely hampered. Thus, exposure to potential damage and danger is greatly increased. A recommendation for a diversion by a routing agency well in advance of the intense weather and associated seas will limit the duration of exposure of the ship. If effective, ship speed will not be reduced and satisfactory progress will be maintained even though the remaining distance to destina￾tion is increased. Overall transit time is usually shorter than if no track change had been made and the ship had remained in heavy weather. In some cases diversions are made to avoid adverse weather conditions and shorten the track at the same time. Significant savings in time and costs can result. In very intense low pressure systems, with high winds and long duration over a long fetch, seas will be generated and propagated as swell over considerable distances. Even on a diversion, it is difficult to effectively avoid all unfavorable conditions. Generally, original routes for transoceanic pas￾sages, issued by the U.S. Navy’s ship routing service, are equatorward of the 10% frequency isoline for gale force winds for the month of transit, as interpreted from the U.S. Navy’s Marine Climatic Atlas of the World. These are shown in Figure 3804a and Figure 3804b for the Pacific. To avoid the area of significant gale activity in the Atlantic from Octo￾ber to April, the latitude of transit is generally in the lower thirties. The areas, seasons, and the probability of development of tropical cyclones are fairly well defined in climatological publications. In long range planning, considerable benefit can be gained by limiting the exposure to the potential haz￾ards of tropical systems. In the North Pacific, avoid areas with the greatest prob￾ability of tropical cyclone formation. Avoiding existing tropical cyclones with a history of 24 hours or more of 6- hourly warnings is in most cases relatively straightforward. However, when transiting the tropical cyclone generating ar￾ea, the ship under routing may provide the first report of environmental conditions indicating that a new disturbance is developing. In the eastern North Pacific the generating area for a high percentage of tropical cyclones is relatively com￾pact (Figure 3804c). Remain south of a line from lat. 9°N, long. 90°W to lat. 14°N, long. 115°W. In the western North Pacific it is advisable to hold north of 22°N when no tropical systems are known to exist. See Figure 3804d. In the Atlantic , sail near the axis of the Bermuda high or northward to avoid the area of formation of tropical cyclones. Of course, avoiding an existing tropical cyclone takes prece￾dence over avoiding a general area of potential development. It has proven equally beneficial to employ similar con￾siderations for routing in the monsoon areas of the Indian Ocean and the South China Sea. This is accomplished by providing routes and diversions that generally avoid the ar￾eas of high frequency of gale force winds and associated heavy seas, as much as feasible. Ships can then remain in satisfactory conditions with limited increases in route distance. Depending upon the points of departure and destination, there are many combinations of routes that can be used when transiting the northern Indian Ocean (Arabian Sea, Bay of Bengal) and the South China Sea. For example, in the Arabi￾an Sea during the summer monsoon, routes to and from the Red Sea, the western Pacific, and the eastern Indian Ocean should hold equatorward. Ships proceeding to the Persian Gulf during this period are held farther south and west to put the heaviest seas on the quarter or stern when transiting the Arabian Sea. Eastbound ships departing the Persian Gulf may proceed generally east southeast toward the Indian sub￾continent, then south, to pass north and east of the highest southwesterly seas in the Arabian Sea. Westbound ships out of the Persian Gulf for the Cape of Good Hope appear to have little choice in routes unless considerable distance is added to the transit by passing east of the highest seas. In the winter monsoon, routes to or from the Red Sea for the western Pa￾cific and the Indian Ocean are held farther north in the Arabian Sea to avoid the highest seas. Ships proceeding to the Persian Gulf from the western Pacific and eastern Indian Ocean may hold more eastward when proceeding north in the Arabian Sea. Ships departing the Persian Gulf area will have considerably less difficulty than during the summer mon￾soon. Similar considerations can be given when routing ships proceeding to and from the Bay of Bengal. In the South China Sea, transits via the Palawan Pas￾sage are recommended when strong, opposing wind and seas are forecast. This is especially true during the winter monsoon. During periods when the major monsoon flow is slack, ships can use the shortest track as conditions permit. 3805. Special Weather And Environmental Considerations In addition to the synoptic weather considerations in ship weather routing, there are special environmental prob￾lems that can be avoided by following recommendations and advisories of ship routing agencies. These problems generally cover a smaller geographic area and are seasonal in nature, but are still important to ship routing. In the North Atlantic, because of heavy shipping traf￾fic, frequent poor visibility in rain or fog, and restricted navigation, particularly east of Dover Strait, some mariners prefer to transit to or from the North Sea via Pentland Firth, passing north of the British Isles rather than via the English Channel. Weather routed ships generally avoid the area of dense fog with low visibility in the vicinity of the Grand Banks off

548WEATHERROUTINGNewfoundlandandtheareaeastof Japannorthof 35°Nprovided when estimated dates of departure (EDD's)are giv-Fishingvesselsinthesetwoareasprovideanaddedhazardtoen to the routing agency well in advanceofdeparture,usuallysafenavigation.This condition existsprimarilyfrom Junea week to several months.Long rangeplanning routes arethrough September. Arctic supply ships en route from thebasedmoreonseasonal andclimatological expectationsthanU.S.eastcoast to the Davis Strait-Baffin Bayarea in the sum-the current weather situation, While planning routes are anmer frequentlytransit via Cabot Strait and the Strait of Belleattemptto makeextended range (morethan a week)or longIsle, where navigation aids are available and icebergs arerange(morethanamonth)forecasts,theserecommendationsare likely to be revised near the time of departure to reflectgenerally grounded.thecurrentweatherpattern.AninitialrouterecommendationIcebergs are a definite hazard in the NorthAtlantic fromis more closelyrelated to the current weatherpatternsbyus-lateFebruarythroughJune,and occasionallylater.Thehaz-ingthelatest dynamicforecaststhan aretheplanningrouteard of floating ice is frequently combined with restrictedrecommendations.These,too,aresubjecttorevisionpriortovisibilityin fog.International Ice Patrol reports and warningssailing, if weather and sea conditions warrant.areincorporated intotheplanning ofroutestosafelyavoiddangerous iceberg areas.It isusuallynecessaryto hold southAdjustment of departuretime is a recommendationofat least45°Nuntilwell southeastof Newfoundland.Thefor delay in departure,or early departure iffeasible,and is in-U.S.Navyship routing officeattheNaval AtlanticMeteorol-tended to avoid or significantly reducethe adverse weatherogyand OceanographyCenter in Norfolkmaintains a safetyand seasforecaston thefirstportion oftheroute,ifsailing onthe original EDD.The initial route is not revised, only themarginofatleast100milesfromicebergsreportedbytheIn-ternational Ice Patrol. Also, in a severe winter, the Denmarktiming of the ship's transit through an area with curently un-favorableweather conditions.AdjustingthedeparturetimeisStraitmaybeclosedbyice.aneffectivemethod ofavoidinga potentiallyhazardous situ-In the northern hemisphere winter, a strong high pres-ation where there is no optimum route for sailing at thesuresystemmovingsoutheastoutoftheRockyMountainsoriginally scheduled timebrings coldairdownacrossCentral AmericaandthewesternGulfof Mexicoproducing galeforce winds in the Gulf of Te-A diversion is an underway adjustment in track and ishuantepec.Thisfall wind is similarto the pampero,mistral,intendedtoavoidorlimittheeffectofadverseweatherconand bora of other areas of the world.An adjustment to ship'sditions forecast to be encountered along the ship's currenttrack can successfully avoid thehighest seas associated withtrack,Ship's speed is expected tobereduced bythe encoun-the"Tehuantepecer."For transits between the Canal Zoneterwiththeheavyweather.Inmostcasesthedistancetoand northwest Pacific ports, littleadditional distance is re-destination is increased in attemptingto avoid the adverseweather,but this is partially overcomebybeing abletomain-quiredtoavoidthisarea(inwinter)byremainingsouthofatleast12°Nwhencrossing97W.Whileavoidingthehighesttainnearnormal SOA.Diversionsarealsorecommendedseas.someunfavorableswellconditionsmaybeencounteredwheresatisfactoryweatherandseaconditionsareforecastonsouth of this line.Ships transiting between thePanama Canalashortertrack.andNorthAmericanwestcoastportscanstayclosealongtheAdjustment of SOA isa recommendation for slowingcoastoftheGulf of Tehuantepecto avoid heavy seas duringorincreasing the ship's speed asmuchas practicable,in angale conditions,but may still encounterhighoffshore windsattemptto avoid an adverse weather situationby adjustingIn the summer, thesemi-permanent high pressure sys-thetimingoftheencounter.Thisisalsoaneffectivemeanstems over the world's oceans producestrong equatorwardofmaintainingmaximumship operating efficiency,whileflowalongthewestcoastsofcontinents.Thisfeatureismostnot diverting from thepresent ship's track.By adjustingthepronouncedoffthecoastofCaliforniaandPortugal intheSOA,amajorweather system can sometimes beavoidedNorthernHemisphereand alongChile,western Australiawith no increase in distance.The development of fast shipsand southwestAfrica in the Southem Hemisphere.Very(SOAgreater than30knots)gives the ship routing agencyrough seas are generated and are considered a definite factorthe potential to“"make the ship's weather"by adjusting theinroute selection or diversion when transiting these areas.ship's speed and track for encounter with favorable weatherconditions.3806.TypesOfRecommendationsAndAdvisoriesEvasion is arecommendationtothecommandingofficerormastertotakeindependentactiontoavoid.asmuchaspossible,apotentiallydangerousweather system.TheshipAn initial route recommendation is issued to a ship orroutingmeteorologistmay recommendageneral directionroutingauthoritynormally48to72hourspriorto sailing,andfor safeevasionbut does not specify anexacttrackThe rec-theprocess of surveillancebegins,Surveillanceisacontinuommendationfor evasion is an indication that the weatherous process, maintained until the ship arrives at itsand sea conditions have deteriorated to a point where shiph-destination.Initial routerecommendationsare a compositeandling and safety are the primary considerations andrepresentation of experience,climatology,weather and seaprogresstowarddestination hasbeentemporarilysuspended,state forecasts,operational concerns,and the ship's seagoingoris at leastof secondaryconsideration.characteristics.Aplanningrouteprovidesabest estimate ofarealistic routefor a specific transit period.Such routes areA weather advisoryis a transmission sentto the ship

548 WEATHER ROUTING Newfoundland and the area east of Japan north of 35°N. Fishing vessels in these two areas provide an added hazard to safe navigation. This condition exists primarily from June through September. Arctic supply ships en route from the U.S. east coast to the Davis Strait-Baffin Bay area in the sum￾mer frequently transit via Cabot Strait and the Strait of Belle Isle, where navigation aids are available and icebergs are generally grounded. Icebergs are a definite hazard in the North Atlantic from late February through June, and occasionally later. The haz￾ard of floating ice is frequently combined with restricted visibility in fog. International Ice Patrol reports and warnings are incorporated into the planning of routes to safely avoid dangerous iceberg areas. It is usually necessary to hold south of at least 45°N until well southeast of Newfoundland. The U.S. Navy ship routing office at the Naval Atlantic Meteorol￾ogy and Oceanography Center in Norfolk maintains a safety margin of at least 100 miles from icebergs reported by the In￾ternational Ice Patrol. Also, in a severe winter, the Denmark Strait may be closed by ice. In the northern hemisphere winter, a strong high pres￾sure system moving southeast out of the Rocky Mountains brings cold air down across Central America and the western Gulf of Mexico producing gale force winds in the Gulf of Te￾huantepec. This fall wind is similar to the pampero, mistral, and bora of other areas of the world. An adjustment to ship’s track can successfully avoid the highest seas associated with the “Tehuantepecer.” For transits between the Canal Zone and northwest Pacific ports, little additional distance is re￾quired to avoid this area (in winter) by remaining south of at least 12°N when crossing 97°W. While avoiding the highest seas, some unfavorable swell conditions may be encountered south of this line. Ships transiting between the Panama Canal and North American west coast ports can stay close along the coast of the Gulf of Tehuantepec to avoid heavy seas during gale conditions, but may still encounter high offshore winds. In the summer, the semi-permanent high pressure sys￾tems over the world’s oceans produce strong equatorward flow along the west coasts of continents. This feature is most pronounced off the coast of California and Portugal in the Northern Hemisphere and along Chile, western Australia, and southwest Africa in the Southern Hemisphere. Very rough seas are generated and are considered a definite factor in route selection or diversion when transiting these areas. 3806. Types Of Recommendations And Advisories An initial route recommendation is issued to a ship or routing authority normally 48 to 72 hours prior to sailing, and the process of surveillance begins. Surveillance is a continu￾ous process, maintained until the ship arrives at its destination. Initial route recommendations are a composite representation of experience, climatology, weather and sea state forecasts, operational concerns, and the ship’s seagoing characteristics. A planning route provides a best estimate of a realistic route for a specific transit period. Such routes are provided when estimated dates of departure (EDD’s) are giv￾en to the routing agency well in advance of departure, usually a week to several months. Long range planning routes are based more on seasonal and climatological expectations than the current weather situation. While planning routes are an attempt to make extended range (more than a week) or long range (more than a month) forecasts, these recommendations are likely to be revised near the time of departure to reflect the current weather pattern. An initial route recommendation is more closely related to the current weather patterns by us￾ing the latest dynamic forecasts than are the planning route recommendations. These, too, are subject to revision prior to sailing, if weather and sea conditions warrant. Adjustment of departure time is a recommendation for delay in departure, or early departure if feasible, and is in￾tended to avoid or significantly reduce the adverse weather and seas forecast on the first portion of the route, if sailing on the original EDD. The initial route is not revised, only the timing of the ship’s transit through an area with currently un￾favorable weather conditions. Adjusting the departure time is an effective method of avoiding a potentially hazardous situ￾ation where there is no optimum route for sailing at the originally scheduled time. A diversion is an underway adjustment in track and is intended to avoid or limit the effect of adverse weather con￾ditions forecast to be encountered along the ship’s current track. Ship’s speed is expected to be reduced by the encoun￾ter with the heavy weather. In most cases the distance to destination is increased in attempting to avoid the adverse weather, but this is partially overcome by being able to main￾tain near normal SOA. Diversions are also recommended where satisfactory weather and sea conditions are forecast on a shorter track. Adjustment of SOA is a recommendation for slowing or increasing the ship’s speed as much as practicable, in an attempt to avoid an adverse weather situation by adjusting the timing of the encounter. This is also an effective means of maintaining maximum ship operating efficiency, while not diverting from the present ship’s track. By adjusting the SOA, a major weather system can sometimes be avoided with no increase in distance. The development of fast ships (SOA greater than 30 knots) gives the ship routing agency the potential to “make the ship’s weather” by adjusting the ship’s speed and track for encounter with favorable weather conditions. Evasion is a recommendation to the commanding offic￾er or master to take independent action to avoid, as much as possible, a potentially dangerous weather system. The ship routing meteorologist may recommend a general direction for safe evasion but does not specify an exact track. The rec￾ommendation for evasion is an indication that the weather and sea conditions have deteriorated to a point where shiph￾andling and safety are the primary considerations and progress toward destination has been temporarily suspended, or is at least of secondary consideration. A weather advisory is a transmission sent to the ship