《航海学》课程参考文献（地文资料）CHAPTER 37 WEATHER OBSERVATIONS

CHAPTER 37WEATHEROBSERVATIONSBASICSOFWEATHEROBSERVATIONS3700.Introductionthin metal cell which is compressed byatmospheric pres-sure, slight changes in air pressure cause the cell to expandWeather forecasts are generallybased upon informa-or contract,while a systemoflevers magnifies and convertstion acquired by observations made at a large number ofthismotion to a reading on a gage or recorderEarlymercurial barometers were calibrated to indicatestations.Ashore,these stations are located so astoprovideadequatecoverageofthe areaofinterest.Mostobservationsthe height, usually in inches or millimeters, of the columnatseaaremadebymariners,wherevertheyhappentobeofmercuryneededtobalancethecolumnofairabovetheSincethe number of observations at sea is small comparedpoint ofmeasurement.While units of inches and millime-tothe numberashore,marine observations areof great im-ters are still widely used, many modern barometers areportance.Data recorded by designated vessels are sent bycalibratedtoindicatethecentimeter-gram-second unitofradio to weather centers ashore, where they are plotted,pressure,themillibar, which is equal to 1,000 dynes peralong with other observations,to providedata for drawingsquare centimeter.A dyne is the force required to acceleratesynoptic charts, which are used to makeforecasts.Com-a mass of one gram at the rate of one centimeterper secondplete weather information gathered at sea by cooperatingper second.A reading in any of the three units of measure-vessels is mailedto the appropriate meteorological servicesmentcanbeconvertedtotheequivalentreadingineitheroffor use in the preparation of weather atlases and in marinethe other units by means oftables, or the conversionfactorsclimatological studies.given in the appendix. However, the pressure readingA special effort should be made to provideroutine syn-should always be reported in millibars.optic reportswhen transiting areas wherefew ships areavailable to report weather observations.This effort is par-3702.TheBarometerticularlyimportant inthetropics,where a vessel's synopticThe mercurial barometerwas invented by Evangelis-weather report may be one of the first indications of a de-veloping tropical cyclone. Even with satellite imagery,ta Torricelli in 1643.In its simplest form it consists of aactual reports are needed toconfirm suspicious patterns andglasstubealittlemorethan30inchesinlengthandofuniprovide actual temperature,pressure, and other measure-form internal diameter.With one end closed, thetube isments.Forecasts can be no better than thedatareceived.filled withmercury,and inverted intoacupofmercury.Themercury in the tubefalls until the column is just supported3701.AtmosphericPressurebythepressureof the atmosphereontheopen cup,leavinga vacuum at the upper end of the tube.The height of the col-The sea ofair surrounding the earth exerts a pressure ofumn indicates atmospheric pressure,greater pressuresabout 14.7 pounds per square inch on the surface of thesupportinghigher columns ofmercury.earth.This atmosphericpressure,sometimes calledbaro-The mercurial barometer is subject torapid variationsmetric pressure,varies from place to place, and at thein height,called pumping,dueto pitch androll of thevessameplace itvariesover time.sel and temporary changes in atmospheric pressure in theAtmosphericpressure isoneofthemostbasicelementsvicinity of the barometer.Because of this,plus the care re-of ameteorological observation.When thepressure ateachquired in the reading the instrument, its bulkiness, and itsstation isplotted on a synopticchart, linesof equal atmo-vulnerability to physical damage, the mercurial barometerhas been replaced at sea by the aneroid barometer.spheric pressure, called isobars,indicate the areas of highand lowpressure.Theseareuseful inmakingweatherpre3703.TheAneroidBarometerdictions,becausecertaintypesofweatherarecharacteristicof eachtypeof area,and the windpatterns overlarge areascanbededucedfrom the isobars.The aneroid barometermeasures theforce exerted byAtmospheric pressure is measured withabarometer.atmosphericpressureonapartly evacuated,thin-metal ele-A mercurial barometermeasurespressurebybalancingmentcalled a sylphon cell (aneroid capsule).A small springthe weight of a column of air against that of a column ofis used, either internally or externally,to partly counteractthe tendency oftheatmospheric pressureto crushthecellmercury.The aneroid barometer has a partly evacuated,521

521 CHAPTER 37 WEATHER OBSERVATIONS BASICS OF WEATHER OBSERVATIONS 3700. Introduction Weather forecasts are generally based upon informa￾tion acquired by observations made at a large number of stations. Ashore, these stations are located so as to provide adequate coverage of the area of interest. Most observations at sea are made by mariners, wherever they happen to be. Since the number of observations at sea is small compared to the number ashore, marine observations are of great im￾portance. Data recorded by designated vessels are sent by radio to weather centers ashore, where they are plotted, along with other observations, to provide data for drawing synoptic charts, which are used to make forecasts. Com￾plete weather information gathered at sea by cooperating vessels is mailed to the appropriate meteorological services for use in the preparation of weather atlases and in marine climatological studies. A special effort should be made to provide routine syn￾optic reports when transiting areas where few ships are available to report weather observations. This effort is par￾ticularly important in the tropics, where a vessel’s synoptic weather report may be one of the first indications of a de￾veloping tropical cyclone. Even with satellite imagery, actual reports are needed to confirm suspicious patterns and provide actual temperature, pressure, and other measure￾ments. Forecasts can be no better than the data received. 3701. Atmospheric Pressure The sea of air surrounding the earth exerts a pressure of about 14.7 pounds per square inch on the surface of the earth. This atmospheric pressure, sometimes called baro￾metric pressure, varies from place to place, and at the same place it varies over time. Atmospheric pressure is one of the most basic elements of a meteorological observation. When the pressure at each station is plotted on a synoptic chart, lines of equal atmo￾spheric pressure, called isobars, indicate the areas of high and low pressure. These are useful in making weather pre￾dictions, because certain types of weather are characteristic of each type of area, and the wind patterns over large areas can be deduced from the isobars. Atmospheric pressure is measured with a barometer. A mercurial barometer measures pressure by balancing the weight of a column of air against that of a column of mercury. The aneroid barometer has a partly evacuated, thin metal cell which is compressed by atmospheric pres￾sure; slight changes in air pressure cause the cell to expand or contract, while a system of levers magnifies and converts this motion to a reading on a gage or recorder. Early mercurial barometers were calibrated to indicate the height, usually in inches or millimeters, of the column of mercury needed to balance the column of air above the point of measurement. While units of inches and millime￾ters are still widely used, many modern barometers are calibrated to indicate the centimeter-gram-second unit of pressure, the millibar, which is equal to 1,000 dynes per square centimeter. A dyne is the force required to accelerate a mass of one gram at the rate of one centimeter per second per second. A reading in any of the three units of measure￾ment can be converted to the equivalent reading in either of the other units by means of tables, or the conversion factors given in the appendix. However, the pressure reading should always be reported in millibars. 3702. The Barometer The mercurial barometer was invented by Evangelis￾ta Torricelli in 1643. In its simplest form it consists of a glass tube a little more than 30 inches in length and of uni￾form internal diameter. With one end closed, the tube is filled with mercury, and inverted into a cup of mercury. The mercury in the tube falls until the column is just supported by the pressure of the atmosphere on the open cup, leaving a vacuum at the upper end of the tube. The height of the col￾umn indicates atmospheric pressure, greater pressures supporting higher columns of mercury. The mercurial barometer is subject to rapid variations in height, called pumping, due to pitch and roll of the ves￾sel and temporary changes in atmospheric pressure in the vicinity of the barometer. Because of this, plus the care re￾quired in the reading the instrument, its bulkiness, and its vulnerability to physical damage, the mercurial barometer has been replaced at sea by the aneroid barometer. 3703. The Aneroid Barometer The aneroid barometer measures the force exerted by atmospheric pressure on a partly evacuated, thin-metal ele￾ment called a sylphon cell (aneroid capsule). A small spring is used, either internally or externally, to partly counteract the tendency of the atmospheric pressure to crush the cell

522WEATHEROBSERVATIONSoWEATER710AROMETEELANEORLPMANUM2552Figure3703.AnaneroidbarometerAtmospheric pressureis indicated directlybya scalechartto indicatethepressureatanytimeandapointerconnectedtothecellbyacombinationofle-Thebarograph is usually mounted on a shelfor desk invers.The linkage provides considerable magnification ofaroom opentotheatmosphere,in alocationwhich mini-the slight motion of the cell, to permit readings to highermizes the effect of the ship's vibration. Shock-absorbingprecision than could be obtained without it.material such as sponge rubber may be placed under the in-Ananeroidbarometershouldbemountedpermanent-strumenttominimizevibration.ly.Prior to installation,the barometer should be carefullyThepen shouldbe checked and the inkwell filled eachset.U.S.shipsoftheVoluntaryObservingShip(VOS)pro-time the chart is changed.gram are set to sea level pressure. Other vessels may be setA marine microbarograph is a precision barographto stationpressureand corrected for heightasnecessary.Anusing greater magnification and an expanded chart.It is de-adjustmentscrewisprovidedforthispurpose.Theerrorofsigned to maintain its precision through the conditionsthe instrument is determined by comparison witha mercu-rial barometerora standard precision aneroid barometer.Ifencounteredaboardship.Twosylphoncellsareused,onemounted over the other in tandem.Minor fluctuations duea qualified meteorologist is not available to make this ad-justment,adjust by first removing only one-half theto shocks or vibrations are eliminated by damping.Sinceapparent error.Thetap the casegentlyto assist the linkageoil-filled dashpots are used forthis purpose, the instrumentto adjust itself, and repeat the adjustment. If the remainingshould never be inverted. The dashpots of the mi-error is not more than half amillibar (0.015 inch), no at-crobarograph should be kept filled with dashpot oil totempt should be made to remove it by further adjustment.within three-eighths inch of the topInstead,a correction should be applied to thereadings.TheShip motions are compensated by damping and springaccuracyof thiscorrectionshouldbecheckedfromtimetoloading which make it possible for the microbarograph totime.betilted upto22°withoutvaryingmorethan0.3millibarsfrom truereading.Microbarographs havebeen almost en-3704.The Barographtirelyreplacedbystandard barographsBoth instruments require checkingfrom time to time toThe barograph is a recording barometer. In principleinsure correct indication ofpressure.The position of theit is the same as a nonrecordinganeroid barometer exceptpen is adjusted by a small knob provided for this purpose.that the pointer carries a pen at its outer end, and the scaleTheadjustment should bemade in stages, eliminatinghalfis replaced by a slowly rotating cylinder around which achart is wrapped.A clock mechanism inside the cylinder ro-the apparent error,tapping the caseto insure linkage adjust-tatesthecylindersothatacontinuouslineistracedonthementto the new setting,andthenrepeating theprocess

522 WEATHER OBSERVATIONS Atmospheric pressure is indicated directly by a scale and a pointer connected to the cell by a combination of le￾vers. The linkage provides considerable magnification of the slight motion of the cell, to permit readings to higher precision than could be obtained without it. An aneroid barometer should be mounted permanent￾ly. Prior to installation, the barometer should be carefully set. U.S. ships of the Voluntary Observing Ship (VOS) pro￾gram are set to sea level pressure. Other vessels may be set to station pressure and corrected for height as necessary. An adjustment screw is provided for this purpose. The error of the instrument is determined by comparison with a mercu￾rial barometer or a standard precision aneroid barometer. If a qualified meteorologist is not available to make this ad￾justment, adjust by first removing only one-half the apparent error. The tap the case gently to assist the linkage to adjust itself, and repeat the adjustment. If the remaining error is not more than half a millibar (0.015 inch), no at￾tempt should be made to remove it by further adjustment. Instead, a correction should be applied to the readings. The accuracy of this correction should be checked from time to time. 3704. The Barograph The barograph is a recording barometer. In principle it is the same as a nonrecording aneroid barometer except that the pointer carries a pen at its outer end, and the scale is replaced by a slowly rotating cylinder around which a chart is wrapped. A clock mechanism inside the cylinder ro￾tates the cylinder so that a continuous line is traced on the chart to indicate the pressure at any time. The barograph is usually mounted on a shelf or desk in a room open to the atmosphere, in a location which mini￾mizes the effect of the ship’s vibration. Shock-absorbing material such as sponge rubber may be placed under the in￾strument to minimize vibration. The pen should be checked and the inkwell filled each time the chart is changed. A marine microbarograph is a precision barograph using greater magnification and an expanded chart. It is de￾signed to maintain its precision through the conditions encountered aboard ship. Two sylphon cells are used, one mounted over the other in tandem. Minor fluctuations due to shocks or vibrations are eliminated by damping. Since oil-filled dashpots are used for this purpose, the instrument should never be inverted. The dashpots of the mi￾crobarograph should be kept filled with dashpot oil to within three-eighths inch of the top. Ship motions are compensated by damping and spring loading which make it possible for the microbarograph to be tilted up to 22° without varying more than 0.3 millibars from true reading. Microbarographs have been almost en￾tirely replaced by standard barographs. Both instruments require checking from time to time to insure correct indication of pressure. The position of the pen is adjusted by a small knob provided for this purpose. The adjustment should be made in stages, eliminating half the apparent error, tapping the case to insure linkage adjust￾ment to the new setting, and then repeating the process. Figure 3703. An aneroid barometer

523WEATHEROBSERVATIONS3705.Adjusting Barometer Readings3706.TemperatureTemperature is ameasure ofheat energy,measured inAtmospheric pressure as indicated bya barometer orbarograph may be subject to several errors.degrees.Several different temperature scales are in use.Instrument error:Inaccuracy due to imperfection orOn the Fahrenheit (F) scale pure water freezes at 320incorrectadjustmentcanbedeterminedbycomparison withand boils at 2120a standard precision instrument.The National Weather Ser-OntheCelsius(C)scalecommonlyusedwiththemet-viceprovides a comparison service.In major U.S.ports aric system, the freezing pointof pure water is 0°and thePort Meteorological Officercarriesaportableprecisionan-boiling point is 10ooThis scale,has beenknownbyvariouseroidbarometerforbarometercomparisonsonboardshipsnames in different countries. In the United States it wasfor-whichparticipateintheVoluntaryObservingShip(VOS)merly called the centigrade scale. The Ninth Generalprogram of theNational Weather Service.The portableba-Conference of Weights and Measures, held in France inrometer is compared with stationbarometersbefore and1948,adopted thenameCelsiustobeconsistentwiththeaftera shipvisit.Ifa barometeristaken to aNationanaming of othertemperature scales after their inventorsWeather Serviceshore station,the comparison can bemadeand toavoid the use of different names indifferent coun-there.The correct sea-level pressure can also be obtained bytries.OntheoriginalCelsiusscale.invented in1742byatelephone.TheshipboardbarometershouldbecorrectedforSwedish astronomer named Anders Celsius,numberingheight,as explained below,beforecomparison withthiswas thereverse of the modern scale, oorepresenting thevalue. If there is reason to believe that the barometer is inboiling point ofwater, and 100oits freezing point.error,it should becompared witha standard,and ifan errorAbsolutezerois consideredtobethelowestpossibleis found,the barometer should be adjusted to the correcttemperature, at which there is no molecular motion and areading, or a correction applied to all readings.bodyhas no heat.For somepurposes,it is convenienttoex-Height error: The atmospheric pressure reading at thepress temperature by a scale at which oo is absolute zero.height of the barometer is called the station pressure andThis is called absolute temperature.If Fahrenheit degreesis subject to a height correction in order to make it a sea lev-are used, it maybecalledRankine (R)temperature;and ifel pressure readingIsobars adequately reflect windCelsius, Kelvin (K) temperature. The Kelvin scale is moreconditions and geographic distribution of pressure onlywidelyused than theRankine.Absolutezero is-459.69°Fwhentheyaredrawnforpressureatconstantheight(ortheor-273.16°C.varyingheightatwhichaconstantpressureexists).Onsyn-Temperatureofone scalecanbeeasilyconverted toan-opticcharts itiscustomaryto showtheequivalentpressureother because of the linear mathematical relationshipat sea level, called sea level pressure.This isfound by ap-between them. Notethat the sequence of calculation isplying a correction to station pressure. The correctionslightly different, algebraic rules must befollowed.depends upon the height of the barometer and the averageC = F-32temperature of the air between this height and the surfaceC =(F-32),or1.8Theoutsideairtemperaturetaken aboard shipis sufficientlyaccurateforthispurpose.This is an importantcorrection0F = 1.8C+32F=C + 32. orwhich should beappliedtoall readingsof anytypebarom-eter.SeeTable31forthis correction.K =C+273.16Gravity error: Mercurial barometers are calibrated forstandard sea-level gravity at latitude 45°32'40".If the gravityR = F + 459.69differsfrom this amount, an error is introduced.The correc-tion to be applied to readings at various latitudes is given inA temperature of-40° is the same by either the CelsiusTable32.This correctiondoesnotapplytoreadingsofanan-eroid barometer or microbarograph.Gravity also changesorFahrenheitscale.Similarformulascanbemadeforconwith height above sea level, but the effect isnegligiblefor theversionofother temperaturescalereadings.TheConversionTableforThermometer Scales (Table29)gives the equiva-firstfew hundredfeet,and so is notneededfor readings takenlentvaluesofFahrenheit,Celsius,andKelvintemperatures.aboard ship.See Table 32 for this correction.Temperature error: Barometers are calibrated at aThe intensity or degree of heat (temperature)should notstandard temperatureof32F.The liquidof amercurialba-be confused with the amount of heat.If the temperature of airrometerexpandsasthetemperatureofthemercuryrises.andorsomeothersubstanceistobeincreased(thesubstancemadecontracts as it decreases.The correction to adjust the readinghotter)by a given number of degrees, the amount of heat thatoftheinstrumenttothetruevalueisgiven inTable33.Thismust be added is dependent upon theamountof the substancetobeheated.Also,equal amounts ofdifferentsubstances recorrectionis appliedtoreadingsofmercurial barometersonly.Modernaneroidbarometersarecompensatedfortem-quiretheaddition ofunequalamounts ofheattoeffectanequalperature changes by the use of different metals havingincreaseintemperaturebecauseoftheirdifferenceof specificheat.Units used for measurement of amount of heat are theunequalcoefficients of linearexpansion

WEATHER OBSERVATIONS 523 3705. Adjusting Barometer Readings Atmospheric pressure as indicated by a barometer or barograph may be subject to several errors. Instrument error: Inaccuracy due to imperfection or incorrect adjustment can be determined by comparison with a standard precision instrument. The National Weather Ser￾vice provides a comparison service. In major U. S. ports a Port Meteorological Officer carries a portable precision an￾eroid barometer for barometer comparisons on board ships which participate in the Voluntary Observing Ship (VOS) program of the National Weather Service. The portable ba￾rometer is compared with station barometers before and after a ship visit. If a barometer is taken to a National Weather Service shore station, the comparison can be made there. The correct sea-level pressure can also be obtained by telephone. The shipboard barometer should be corrected for height, as explained below, before comparison with this value. If there is reason to believe that the barometer is in error, it should be compared with a standard, and if an error is found, the barometer should be adjusted to the correct reading, or a correction applied to all readings. Height error: The atmospheric pressure reading at the height of the barometer is called the station pressure and is subject to a height correction in order to make it a sea lev￾el pressure reading. Isobars adequately reflect wind conditions and geographic distribution of pressure only when they are drawn for pressure at constant height (or the varying height at which a constant pressure exists). On syn￾optic charts it is customary to show the equivalent pressure at sea level, called sea level pressure. This is found by ap￾plying a correction to station pressure. The correction depends upon the height of the barometer and the average temperature of the air between this height and the surface. The outside air temperature taken aboard ship is sufficiently accurate for this purpose. This is an important correction which should be applied to all readings of any type barom￾eter. See Table 31 for this correction. Gravity error: Mercurial barometers are calibrated for standard sea-level gravity at latitude 45°32’40". If the gravity differs from this amount, an error is introduced. The correc￾tion to be applied to readings at various latitudes is given in Table 32. This correction does not apply to readings of an an￾eroid barometer or microbarograph. Gravity also changes with height above sea level, but the effect is negligible for the first few hundred feet, and so is not needed for readings taken aboard ship. See Table 32 for this correction. Temperature error: Barometers are calibrated at a standard temperature of 32°F. The liquid of a mercurial ba￾rometer expands as the temperature of the mercury rises, and contracts as it decreases. The correction to adjust the reading of the instrument to the true value is given in Table 33. This correction is applied to readings of mercurial barometers only. Modern aneroid barometers are compensated for tem￾perature changes by the use of different metals having unequal coefficients of linear expansion. 3706. Temperature Temperature is a measure of heat energy, measured in degrees. Several different temperature scales are in use. On the Fahrenheit (F) scale pure water freezes at 32° and boils at 212°. On the Celsius (C) scale commonly used with the met￾ric system, the freezing point of pure water is 0° and the boiling point is 100°. This scale, has been known by various names in different countries. In the United States it was for￾merly called the centigrade scale. The Ninth General Conference of Weights and Measures, held in France in 1948, adopted the name Celsius to be consistent with the naming of other temperature scales after their inventors, and to avoid the use of different names in different coun￾tries. On the original Celsius scale, invented in 1742 by a Swedish astronomer named Anders Celsius, numbering was the reverse of the modern scale, 0° representing the boiling point of water, and 100° its freezing point. Absolute zero is considered to be the lowest possible temperature, at which there is no molecular motion and a body has no heat. For some purposes, it is convenient to ex￾press temperature by a scale at which 0° is absolute zero. This is called absolute temperature. If Fahrenheit degrees are used, it may be called Rankine (R) temperature; and if Celsius, Kelvin (K) temperature. The Kelvin scale is more widely used than the Rankine. Absolute zero is –459.69°F or –273.16°C. Temperature of one scale can be easily converted to an￾other because of the linear mathematical relationship between them. Note that the sequence of calculation is slightly different; algebraic rules must be followed. A temperature of –40° is the same by either the Celsius or Fahrenheit scale. Similar formulas can be made for con￾version of other temperature scale readings. The Conversion Table for Thermometer Scales (Table 29) gives the equiva￾lent values of Fahrenheit, Celsius, and Kelvin temperatures. The intensity or degree of heat (temperature) should not be confused with the amount of heat. If the temperature of air or some other substance is to be increased (the substance made hotter) by a given number of degrees, the amount of heat that must be added is dependent upon the amount of the substance to be heated. Also, equal amounts of different substances re￾quire the addition of unequal amounts of heat to effect an equal increase in temperature because of their difference of specific heat. Units used for measurement of amount of heat are the C 5 9 = C -( ) F 32 – , or F 32 – 1.8 = - F 9 5 = F 1.8 -C 32 or + , = C + 32 K C 273.16 = + R F 459.69 = +

524WEATHEROBSERVATIONSBritish thermal unit (BTU),the amount of heat needed toThe same process causes moisture to form on the out-raisethetemperature of 1poundof water1Fahrenheit,andside of a container of cold liquid, the liquid cooling the airthe calorie,the amount ofheatneeded to raise thetemperatureintheimmediatevicinityofthecontaineruntilitreachestheof 1 gram of water 1° Celsius.dewpoint.When moisture is deposited on man-madeob-jects, it is usually called sweat. It occurs whenever the3707.Temperature Measurementtemperature of a surface is lower than the dew point of airin contact with it. It is of particular concern to themarinerbecause of its effect upon his instruments,and possibleTemperature is measured with a thermometer.Mostdamagetohis ship oritscargo.Lensesofoptical instruthermometers are based upon the principle that materials ex-ments may sweat, usuallywithsuch small dropletsthatthepand with an increase of temperature, and contract assurface has a“"frosted"appearance. When this occurs, thetemperaturedecreases.In its most usualformathermometerinstrument is said to“fog"or“fog up,"and is useless untilconsistsofabulbfilledwithmercuryand connectedtoatubethemoisture is removed.Damage is often caused bycorro-of very small cross-sectional area.Themercury onlypartlysion or direct water damage when pipes sweat and drip,orfills the tube. In the remainder is a vacuum. Air is driven outwhen the inside of the shell plates of a vessel sweat.Cargoby boiling the mercury,and the top ofthe tube is then sealed.may sweat if it is cooler than the dew point oftheair.As themercury expands orcontracts withchanging temper-Clouds andfog form from condensation of wateronature, the length of the mercury column in the tube changes.minute particles of dust, salt, and other material in the air.Sea surface temperature observations are used in theEachparticleforms anucleus aroundwhichadropletofwa-forecastingoffog and furnish important information aboutterforms.If air is completely free from solid particles onthedevelopmentand movement oftropical cyclones.Com-which water vapor may condense, the extra moisture re-mercial fishermen are interested in the sea surfacemains inthevapor state,andtheairis saidtobetemperature as an aid in locating certain species of fish.supersaturated.There are severalmethods of determining seawatertemper-Relative humidity and dew point are measured witha hyature.Theseincludeengineroomintakereadings,condensergrometer.The most common type, called a psychrometer,intake readings, thermistor probes attached to the hull, andreadings from bucketsrecovered from overtheside.Al-consistsoftwothermometersmountedtogetheronasinglestripofmaterial.Oneofthethermometersismountedalittlethoughthecondenserintakemethodisnotatruemeasureoflowerthantheother,and has itsbulb covered withmuslin.surface water temperature, the error is generally smallWhen themuslin covering is thoroughly moistened and theIf the surfacetemperature isdesired,a sample shouldthermometerwell ventilated,evaporationcoolsthebulbofthebeobtained by bucket,preferablya canvas bucket,fromathermometer, causing it to indicate a lower reading than theforward position well clear of any discharge lines.The sam-other.A sling psychrometer is ventilated by whirling theple should be taken immediately to a place where it isthermometers.Thedifferencebetweenthedrv-bulbandwet-sheltered from wind and sun. The water should then bebulb temperatures is used to enter psychrometric tables (Ta-stirred with the thermometer,keeping thebulb submerged,ble35andTable36)tofindtherelativehumidityanddewuntil a constant reading is obtained.point. If the wet-bulb temperature is abovefreezing,reason-Aconsiderablevariationinseasurfacetemperaturecanably accurateresultscan be obtained by a psychrometerbe experienced in a relatively shortdistance of travel.Thisconsistingof dry-and wet-bulb thermometersmounted soisespeciallytruewhen crossingmajor ocean currents suchthat air can circulate freely around them without special ven-astheGulfStreamand theKuroshioCurrent.Significanttilation.Thistypeofinstallation is common aboard shipvariations also occur where large quantities of freshwaterare discharged from rivers.A clevernavigator will noteExample:The dry-bulbtemperature is 65°F,and thethesechangesasinindicationofwhentoallowforsetandwet-bulb temperature is 61°F.drift in dead reckoning.Required: (l)Relative humidity, (2)dew point.Solution: The difference between readings is 4o. En-3708.HumidityteringTable35withthisvalue,andadry-bulbtemperatureof65°,therelativehumidityisfoundtobe80percent.FromHumidity is a measure ofthe atmosphere's water vaporTable36thedewpoint is58°content. Relative humidity is the ratio, stated as a percent-Answers:()Relativehumidity80percent,(2)dewage,ofthepressureofwatervaporpresent intheatmospherepoint58°tothesaturationvaporpressureatthesametemperatureAs air temperature decreases, the relative humidity in-creases.Atsomepoint,saturationtakesplace,andanyfurtherAlso in use aboard many ships is the electric psy-cooling results in condensation of some of themoisture.Thechrometer.This is a hand held, battery operated instrumenttemperature at which this occurs is called the dew point, andwith two mercury thermometers for obtaining dry-and wet-bulbtemperature readings.Itconsists of a plastichousingthemoisture deposited upon objects is called dewif it forms intheliquid state, orfrost ifitforms in thefrozen state.that holds the thermometers, batteries, motor, and fan

524 WEATHER OBSERVATIONS British thermal unit (BTU), the amount of heat needed to raise the temperature of 1 pound of water 1° Fahrenheit; and the calorie, the amount of heat needed to raise the temperature of 1 gram of water 1° Celsius. 3707. Temperature Measurement Temperature is measured with a thermometer. Most thermometers are based upon the principle that materials ex￾pand with an increase of temperature, and contract as temperature decreases. In its most usual form a thermometer consists of a bulb filled with mercury and connected to a tube of very small cross-sectional area. The mercury only partly fills the tube. In the remainder is a vacuum. Air is driven out by boiling the mercury, and the top of the tube is then sealed. As the mercury expands or contracts with changing temper￾ature, the length of the mercury column in the tube changes. Sea surface temperature observations are used in the forecasting of fog and furnish important information about the development and movement of tropical cyclones. Com￾mercial fishermen are interested in the sea surface temperature as an aid in locating certain species of fish. There are several methods of determining seawater temper￾ature. These include engine room intake readings, condenser intake readings, thermistor probes attached to the hull, and readings from buckets recovered from over the side. Al￾though the condenser intake method is not a true measure of surface water temperature, the error is generally small. If the surface temperature is desired, a sample should be obtained by bucket, preferably a canvas bucket, from a forward position well clear of any discharge lines. The sam￾ple should be taken immediately to a place where it is sheltered from wind and sun. The water should then be stirred with the thermometer, keeping the bulb submerged, until a constant reading is obtained. A considerable variation in sea surface temperature can be experienced in a relatively short distance of travel. This is especially true when crossing major ocean currents such as the Gulf Stream and the Kuroshio Current. Significant variations also occur where large quantities of freshwater are discharged from rivers. A clever navigator will note these changes as in indication of when to allow for set and drift in dead reckoning. 3708. Humidity Humidity is a measure of the atmosphere’s water vapor content. Relative humidity is the ratio, stated as a percent￾age, of the pressure of water vapor present in the atmosphere to the saturation vapor pressure at the same temperature. As air temperature decreases, the relative humidity in￾creases. At some point, saturation takes place, and any further cooling results in condensation of some of the moisture. The temperature at which this occurs is called the dew point, and the moisture deposited upon objects is called dew if it forms in the liquid state, or frost if it forms in the frozen state. The same process causes moisture to form on the out￾side of a container of cold liquid, the liquid cooling the air in the immediate vicinity of the container until it reaches the dew point. When moisture is deposited on man-made ob￾jects, it is usually called sweat. It occurs whenever the temperature of a surface is lower than the dew point of air in contact with it. It is of particular concern to the mariner because of its effect upon his instruments, and possible damage to his ship or its cargo. Lenses of optical instru￾ments may sweat, usually with such small droplets that the surface has a “frosted” appearance. When this occurs, the instrument is said to “fog” or “fog up,” and is useless until the moisture is removed. Damage is often caused by corro￾sion or direct water damage when pipes sweat and drip, or when the inside of the shell plates of a vessel sweat. Cargo may sweat if it is cooler than the dew point of the air. Clouds and fog form from condensation of water on minute particles of dust, salt, and other material in the air. Each particle forms a nucleus around which a droplet of wa￾ter forms. If air is completely free from solid particles on which water vapor may condense, the extra moisture re￾mains in the vapor state, and the air is said to be supersaturated. Relative humidity and dew point are measured with a hy￾grometer. The most common type, called a psychrometer, consists of two thermometers mounted together on a single strip of material. One of the thermometers is mounted a little lower than the other, and has its bulb covered with muslin. When the muslin covering is thoroughly moistened and the thermometer well ventilated, evaporation cools the bulb of the thermometer, causing it to indicate a lower reading than the other. A sling psychrometer is ventilated by whirling the thermometers. The difference between the dry-bulb and wet￾bulb temperatures is used to enter psychrometric tables (Ta￾ble 35 and Table 36) to find the relative humidity and dew point. If the wet-bulb temperature is above freezing, reason￾ably accurate results can be obtained by a psychrometer consisting of dry- and wet-bulb thermometers mounted so that air can circulate freely around them without special ven￾tilation. This type of installation is common aboard ship. Example: The dry-bulb temperature is 65°F, and the wet-bulb temperature is 61°F. Required: (1) Relative humidity, (2) dew point. Solution: The difference between readings is 4°. En￾tering Table 35 with this value, and a dry-bulb temperature of 65°, the relative humidity is found to be 80 percent. From Table 36 the dew point is 58°. Answers: (1) Relative humidity 80 percent, (2) dew point 58°. Also in use aboard many ships is the electric psy￾chrometer. This is a hand held, battery operated instrument with two mercury thermometers for obtaining dry- and wet￾bulb temperature readings. It consists of a plastic housing that holds the thermometers, batteries, motor, and fan

525WEATHEROBSERVATIONS3709.Wind Measurementhas an apparent speed equal to the speed ofthe vessel. Thus,iftheactual ortrue wind is zero and the speed ofthevessel isWindmeasurement consists of determination ofthedi-10knots, theapparent wind is fromdead ahead at 10knotsrection and speed of the wind.Direction is measured byaIfthetruewind isfrom dead ahead at15knots,andthe speedwind vane, and speed by an anemometer.of the vessel is 10 knots, the apparent wind is 15 + 10 = 25Several types of wind speed and direction sensors areknots from dead ahead.If the vessel reverses course,theap-available, using vanes to indicate wind direction and rotat-parent wind is 15-10=5 knots, from dead astern.ingcups orpropellers for speed sensing.Manyships haveThe apparentwind isthevector sum of thetrue windreliable wind instruments installed,and inexpensive windand the reciprocal of the vessel's course and speed vector.instruments areavailableforeven thesmallestyacht.If noSincewind vanes and anemometers measure apparentanemometeris available,windspeedcanbeestimatedbyitswind, theusual problem aboarda vessel equipped with aneffect upon the seaand nearbyobjects.Thedirection can beanemometeristoconvertapparentwindtotruewind.Therecomputed accurately,even on a fastmovingvessel, byma-are several ways of doing this. Perhaps the simplest is byneuveringboardorTable30thegraphical solution illustrated inthefollowing example:3710.TrueAndApparentWindExampleI:A ship isproceedingon course240°ataspeed of 18knots.Theapparentwind is from 040°relativeAn observeraboard a vessel proceedingthrough still airat30knots.experiences an apparent wind which is from dead ahead andRequired:The direction and speed of the true wind.MANEUVERINGBOARDSCALESH130AopereetWiod-Figure3710.Finding true wind byManeuveringBoard

WEATHER OBSERVATIONS 525 3709. Wind Measurement Wind measurement consists of determination of the di￾rection and speed of the wind. Direction is measured by a wind vane, and speed by an anemometer. Several types of wind speed and direction sensors are available, using vanes to indicate wind direction and rotat￾ing cups or propellers for speed sensing. Many ships have reliable wind instruments installed, and inexpensive wind instruments are available for even the smallest yacht. If no anemometer is available, wind speed can be estimated by its effect upon the sea and nearby objects. The direction can be computed accurately, even on a fast moving vessel, by ma￾neuvering board or Table 30. 3710. True And Apparent Wind An observer aboard a vessel proceeding through still air experiences an apparent wind which is from dead ahead and has an apparent speed equal to the speed of the vessel. Thus, if the actual or true wind is zero and the speed of the vessel is 10 knots, the apparent wind is from dead ahead at 10 knots. If the true wind is from dead ahead at 15 knots, and the speed of the vessel is 10 knots, the apparent wind is 15 + 10 = 25 knots from dead ahead. If the vessel reverses course, the ap￾parent wind is 15 – 10 = 5 knots, from dead astern. The apparent wind is the vector sum of the true wind and the reciprocal of the vessel’s course and speed vector. Since wind vanes and anemometers measure apparent wind, the usual problem aboard a vessel equipped with an anemometer is to convert apparent wind to true wind. There are several ways of doing this. Perhaps the simplest is by the graphical solution illustrated in the following example: Example 1: A ship is proceeding on course 240° at a speed of 18 knots. The apparent wind is from 040° relative at 30 knots. Required: The direction and speed of the true wind. Figure 3710. Finding true wind by Maneuvering Board

526WEATHEROBSERVATIONSSolution:First startingfrom the centerofa maneuver-If a vessel is proceeding at 12 knots, 6 knots constitutesingboard,plottheship'svectorer,at240,length18knotsone-half(0.5)unit,12knotsone unit, 18knots1.5 units,24(using the 3-I scale).Next plot the relative wind's vectorknotstwounits,etc.fromr,inadirectionof100(thereciprocalof280°)length30knots.Thetruewind is fromthecenter to theendofthisExample2:Ashipisproceedingon course270°atavector or line ew.speed of 10 knots.The apparent wind is from 10o off theAlternatively,you can plot the ship's vectorfrom theport bow, speed 30 knots.center,then plottherelative wind's vector toward the cen-Required: The relative direction, true direction, andter, and see the true wind's vector from the end of this linespeed of the true wind by table.totheend of theship's vector.Useparallel rulerstotrans-fer the wind vector to the center for an accurate reading.Solution: The apparent wind speed isAnswer:Truewind isfrom315°at20knots.30 - 3.0 ships speed units10On a moving ship, the direction of the true wind is al-ways on the same sideand aft of thedirection of theEnterTable30with3.0and10°and find therelativedirec-apparent wind.Thefaster the ship moves,the more the ap-parent wind draws ahead of the true windtionofthetruewindtobe15°offtheportbow(345°relative)Solution can also bemadewithout plotting,inthe fol-andthespeedtobe2.02timestheship'sspeed,or20knots,approximately.Thetruedirectionis345°+2700=2550lowingmanner:On a maneuveringboard,label thecircles5,10,15,20,etc.,from the center,and drawvertical lines tan-Answers:Truewindfrom345°relative=255°true,at20knots.gent to these circles. Cut out the 5:1 scale and discard thatpart having graduations greater than the maximum speed ofthe vessel.Keep this sheetfor all solutions. (For durabilityByvariations ofthisproblem,onecan findtheappar-thetwopartscanbemountedoncardboardorothersuitableent wind from thetrue wind, the course or speed requiredmaterial.)Tofind true wind, spot in point 1 byeye.Placetheto produce an apparent wind from a given direction orzero of the 5:1 scale on this point and align the scale (invert-speed, or the course and speed to produce an apparented) using the vertical lines. Locate point 2 at the speed of thewind ofagiven speed fromagiven direction.Suchprob-vesselasindicatedonthe5:1scale.Itisalwavsverticallybe-lems often arise in aircraft carrier operationsand in somerescue situations. See “Pub. 217, Maneuvering Boardlowpoint1.Readtherelativedirectionandthespeedofthetrue wind, using eye interpolation if needed.Manual"formoredetailed information.Atabular solution canbe made using Table30,Direc-When wind speed and direction are determined by thetion and Speed of True Wind in Units of Ship's Speed.Theappearanceofthesea,theresultistruespeedanddirec-entering valuesfor this table are the apparent wind speed intion. Waves move in the same direction as the generatingunits of ship's speed,and thedifferencebetween thehead-wind, and are not deflected by earth's rotation.If a winding and theapparent wind direction.Thevalues takenfromvane is used,the direction of the apparent wind thus deter-the table are the relative direction (right or left) of the truemined can be used with the speed of the true wind towind, and the speed of the true wind in units of ship's speed.determinethedirection of thetruewind byvectordiagram.WINDANDWAVES3711.EffectsOfWindOnTheSeaThese pictures (courtesy of Environment Canada)present the results ofa project carried out on board the Ca-There is a direct relationshipbetween the speed of thenadianOceanWeatherShipsVANCOUVERandQUADRA at Ocean Weather Station PAPA (50°N.,wind and the state of the sea.This is useful in predicting the145°W),betweenApril1976andMay1981.Theaimoftheseaconditionsto beanticipated whenfuturewindspeedforecastsareavailable.Itcanalsobeusedtoestimatetheproject was to collect colorphotographs of the sea surfacespeed of the wind, which may be necessary when an ane-as it appears under the influence of the various ranges ofmometer is notavailable.wind speed, as defined byTheBeaufort Scaleof WindWind speeds are usually grouped in accordance with theForce.The photographs represent as closely as possibleBeaufort scale,named afterAdmiral Sir FrancisBeaufortsteady-state sea conditionsovermanyhours for each Beau-(1774-1857),who devised it in 1806.As adopted in 1838fort windforce,exceptForce12,forwhichnophotographsBeaufort numbers ranged from 0 (calm)to 12 (hurricane).Theare available.They weretaken from heights ranging from12-17metersabovethe sea surface;anemometer heightwasBeaufort wind scaleand sea statephotographswhichare atthe28meters.end ofthischapter canbeused toestimatewind speed

526 WEATHER OBSERVATIONS Solution: First starting from the center of a maneuver￾ing board, plot the ship’s vector er, at 240°, length 18 knots (using the 3–1 scale). Next plot the relative wind’s vector from r, in a direction of 100° (the reciprocal of 280°) length 30 knots. The true wind is from the center to the end of this vector or line ew. Alternatively, you can plot the ship’s vector from the center, then plot the relative wind’s vector toward the cen￾ter, and see the true wind’s vector from the end of this line to the end of the ship’s vector. Use parallel rulers to trans￾fer the wind vector to the center for an accurate reading. Answer: True wind is from 315° at 20 knots. On a moving ship, the direction of the true wind is al￾ways on the same side and aft of the direction of the apparent wind. The faster the ship moves, the more the ap￾parent wind draws ahead of the true wind. Solution can also be made without plotting, in the fol￾lowing manner: On a maneuvering board, label the circles 5, 10, 15, 20, etc., from the center, and draw vertical lines tan￾gent to these circles. Cut out the 5:1 scale and discard that part having graduations greater than the maximum speed of the vessel. Keep this sheet for all solutions. (For durability, the two parts can be mounted on cardboard or other suitable material.) To find true wind, spot in point 1 by eye. Place the zero of the 5:1 scale on this point and align the scale (invert￾ed) using the vertical lines. Locate point 2 at the speed of the vessel as indicated on the 5:1 scale. It is always vertically be￾low point 1. Read the relative direction and the speed of the true wind, using eye interpolation if needed. A tabular solution can be made using Table 30, Direc￾tion and Speed of True Wind in Units of Ship’s Speed. The entering values for this table are the apparent wind speed in units of ship’s speed, and the difference between the head￾ing and the apparent wind direction. The values taken from the table are the relative direction (right or left) of the true wind, and the speed of the true wind in units of ship’s speed. If a vessel is proceeding at 12 knots, 6 knots constitutes one-half (0.5) unit, 12 knots one unit, 18 knots 1.5 units, 24 knots two units, etc. Example 2: A ship is proceeding on course 270° at a speed of 10 knots. The apparent wind is from 10° off the port bow, speed 30 knots. Required: The relative direction, true direction, and speed of the true wind by table. Solution: The apparent wind speed is Enter Table 30 with 3.0 and 10° and find the relative direc￾tion of the true wind to be 15° off the port bow (345° relative), and the speed to be 2.02 times the ship’s speed, or 20 knots, ap￾proximately. The true direction is 345° + 270° = 255°. Answers: True wind from 345° relative = 255° true, at 20 knots. By variations of this problem, one can find the appar￾ent wind from the true wind, the course or speed required to produce an apparent wind from a given direction or speed, or the course and speed to produce an apparent wind of a given speed from a given direction. Such prob￾lems often arise in aircraft carrier operations and in some rescue situations. See “Pub. 217, Maneuvering Board Manual”, for more detailed information. When wind speed and direction are determined by the appearance of the sea, the result is true speed and direc￾tion. Waves move in the same direction as the generating wind, and are not deflected by earth’s rotation. If a wind vane is used, the direction of the apparent wind thus deter￾mined can be used with the speed of the true wind to determine the direction of the true wind by vector diagram. WIND AND WAVES 3711. Effects Of Wind On The Sea There is a direct relationship between the speed of the wind and the state of the sea. This is useful in predicting the sea conditions to be anticipated when future wind speed forecasts are available. It can also be used to estimate the speed of the wind, which may be necessary when an ane￾mometer is not available. Wind speeds are usually grouped in accordance with the Beaufort scale, named after Admiral Sir Francis Beaufort (1774-1857), who devised it in 1806. As adopted in 1838, Beaufort numbers ranged from 0 (calm) to 12 (hurricane). The Beaufort wind scale and sea state photographs which are at the end of this chapter can be used to estimate wind speed. These pictures (courtesy of Environment Canada) present the results of a project carried out on board the Ca￾nadian Ocean Weather Ships VANCOUVER and QUADRA at Ocean Weather Station PAPA (50°N., 145°W), between April 1976 and May 1981. The aim of the project was to collect color photographs of the sea surface as it appears under the influence of the various ranges of wind speed, as defined by The Beaufort Scale of Wind Force. The photographs represent as closely as possible steady-state sea conditions over many hours for each Beau￾fort wind force, except Force 12, for which no photographs are available. They were taken from heights ranging from 12-17 meters above the sea surface; anemometer height was 28 meters. 30 10- 3.0 = ships speed units

527WEATHEROBSERVATIONS3712.EstimatingTheWindAt Seastrongcurrents,shallowwater,swell,precipitation,ice,andwind shifts.Their effects will be described laterObservers on board ships at sea usually determinetheA wind of a given Beaufort Force will, therefore, pro-speed of thewind by estimatingBeaufortForce,asmer-duceacharacteristicappearanceofthe seasurfaceprovidedchant ships may not be equipped with wind measuringthat it has been blowing for a sufficient length of time,andinstruments.Through experience,ships'officers havede-over a sufficiently long fetch.velopedvarious methods ofestimatingthisforce.TheIn practice,themariner observes the sea surface,not-effect of the wind on the observer himself, the ship's rig-ingthesizeofthewaves,the white caps,spindrift,etc.,andging,flags, etc.,is used as a guide, but estimatesbased onthen finds the criterion which best describes the sea surfacethese indications give therelative wind whichmust be cor-as he saw it. This criterion is associated with a Beaufortrected forthe motionof theshipbeforean estimate of thenumber,forwhich a correspondingmeanwind speed andtruewindspeedcanbeobtainedrange in knots are given.Since meteorological reports re-The most common method involves the appearance ofquirethat wind speeds be reported inknots,the mean speedthe sea surface. The state of the sea disturbance, i.e. the di-forthe Beaufort number maybe reported,or anexperiencedmensionsofthewaves,thepresenceofwhitecaps,foam,orobserver may judge that the sea disturbance is such that aspray,depends principally on three factors:higherorlowerspeed withintherangefor theforce is moreaccurate.1.Thewind speed.Thehigherthespeed of thewind,This method should beused with caution.The sea con-the greater is the sea disturbance.ditions described for each BeaufortForce are"steady-state"2.The wind's duration. At any point on the sea, theconditions; ie.the conditions which result when the winddisturbance will increase the longer the wind blowshasbeenblowingfora relativelylong time,and over agreatat a given speed, until a maximum state of distur-stretch of water. At any particular time at sea, though, thebance is reached.duration of the wind or the fetch, or both, may not have3.Thefetch.This is thelength ofthe stretchof waterbeen great enough to produce these"steady-state"condi-over whichthe windactsonthe sea surfacefromtions.When a high wind springs up suddenly afterthe same direction.previously calm or near calm conditions, it will requiresome hours, depending on the strength ofthe wind, to gen-For a given wind speed and duration, the longer theerate waves of maximum height.The height of the wavesfetch, thegreater is the sea disturbance.Ifthefetch is short,increasesrapidlyinthefirstfewhoursafterthecommence-suchas a fewmiles, the disturbancewill be relatively smallmentof theblow,but increases at a much slowerratelaterno matter how great the wind speed is or how long it hason.beenblowing.There are other factors which can modify the appear-At the beginning of thefetch (such as at a coastlineanceofthesea surfacecausedbywindalone.Thesearewhen the wind is offshore)after the wind has been blowingTheoreticalFetch (nautical miles), withBeaufortDuration ofwinds,(hours),maximumwaveunlimited duration offorceofwith unlimited fetch, toheight (ft)blow, to produce percentwind.producepercentof maxi-unlimiteddurationof maximum wave heightmum wave height indicated.indicated.and fetch.50%75%90%50%75%90%535281.58313258123.510306071222205.52175150975540162515028093285117019200450Table3712.Durationofwindsand length of fetchesrequiredforvariouswindforces

WEATHER OBSERVATIONS 527 3712. Estimating The Wind At Sea Observers on board ships at sea usually determine the speed of the wind by estimating Beaufort Force, as mer￾chant ships may not be equipped with wind measuring instruments. Through experience, ships’ officers have de￾veloped various methods of estimating this force. The effect of the wind on the observer himself, the ship’s rig￾ging, flags, etc., is used as a guide, but estimates based on these indications give the relative wind which must be cor￾rected for the motion of the ship before an estimate of the true wind speed can be obtained. The most common method involves the appearance of the sea surface. The state of the sea disturbance, i.e. the di￾mensions of the waves, the presence of white caps, foam, or spray, depends principally on three factors: 1. The wind speed. The higher the speed of the wind, the greater is the sea disturbance. 2. The wind’s duration. At any point on the sea, the disturbance will increase the longer the wind blows at a given speed, until a maximum state of distur￾bance is reached. 3. The fetch. This is the length of the stretch of water over which the wind acts on the sea surface from the same direction. For a given wind speed and duration, the longer the fetch, the greater is the sea disturbance. If the fetch is short, such as a few miles, the disturbance will be relatively small no matter how great the wind speed is or how long it has been blowing. There are other factors which can modify the appear￾ance of the sea surface caused by wind alone. These are strong currents, shallow water, swell, precipitation, ice, and wind shifts. Their effects will be described later. A wind of a given Beaufort Force will, therefore, pro￾duce a characteristic appearance of the sea surface provided that it has been blowing for a sufficient length of time, and over a sufficiently long fetch. In practice, the mariner observes the sea surface, not￾ing the size of the waves, the white caps, spindrift, etc., and then finds the criterion which best describes the sea surface as he saw it. This criterion is associated with a Beaufort number, for which a corresponding mean wind speed and range in knots are given. Since meteorological reports re￾quire that wind speeds be reported in knots, the mean speed for the Beaufort number may be reported, or an experienced observer may judge that the sea disturbance is such that a higher or lower speed within the range for the force is more accurate. This method should be used with caution. The sea con￾ditions described for each Beaufort Force are “steady-state” conditions; i.e. the conditions which result when the wind has been blowing for a relatively long time, and over a great stretch of water. At any particular time at sea, though, the duration of the wind or the fetch, or both, may not have been great enough to produce these “steady-state” condi￾tions. When a high wind springs up suddenly after previously calm or near calm conditions, it will require some hours, depending on the strength of the wind, to gen￾erate waves of maximum height. The height of the waves increases rapidly in the first few hours after the commence￾ment of the blow, but increases at a much slower rate later on. At the beginning of the fetch (such as at a coastline when the wind is offshore) after the wind has been blowing Beaufort force of wind. Theoretical maximum wave height (ft) unlimited duration and fetch. Duration of winds, (hours), with unlimited fetch, to produce percent of maxi￾mum wave height indicated. Fetch (nautical miles), with unlimited duration of blow, to produce percent of maximum wave height indicated. 50% 75% 90% 50% 75% 90% 3 2 1.5 5 8 3 13 25 5 8 3.5 8 12 10 30 60 7 20 5.5 12 21 22 75 150 9 40 7 16 25 55 150 280 11 70 9 19 32 85 200 450 Table 3712. Duration of winds and length of fetches required for various wind forces

528WEATHEROBSERVATIONSforalongtime,the wavesarequitesmall near shore,and in-rion were used under these conditions without considerationcrease inheight rapidly overthe first 50 miles or so of theoftheshortfetch,thewind speed wouldbeunderestimatedfetch.Farther offshore, the rate of increase in height withWithanoffshorewindtheseacriterionmaybeusedwithdistanceslowsdown,andafter500milesorsofromthebe-confidence ifthedistancetothecoast is greaterthan theval-ginning of the fetch, there is little or no increase in height.uesgiven in theextreme right-hand column of Table3712:again,provided that thewind hasbeen blowing offshoreforTable 3712 illustrates the duration of winds and thea sufficient length of time.lengthof fetchesrequired forvariouswindforcesto buildseas to50percent, 75percent,and 90percent of their theo-3713.Special Wind Effectsretical maximum heights.The theoretical maximum wave heights represent theTidal and Other Currents: A wind blowing against aaverage heights of the highest third of the waves, as thesewaves are most significant.tideor strongcurrentcausesagreaterseadisturbancethan nor-It will beseen thatwindsofforce5orless canbuild seasmal,whichmayresultinanoverestimateofthewindspeedOn theother hand,a wind blowing in thesame directionas ato 90 percent of their maximum height, in less than 12 hours,tideorstrongcurrentcauseslessseadisturbancethannormalprovided the fetch is long enough.Higher winds require amuch greater time-force 11 winds requiring 32 hours to buildandmayresultinanunderestimateofthewindspeedwavesto90percentoftheirmaximumheight.ThetimesgivenShallow Water: Waves running into shallowwater in-inTable3712represent thoserequired tobuildwaves startingcrease in steepness, and hence, their tendency to break.frominitiallycalmseaconditions.IfwavesarealreadypresentWith an onshore wind there will,therefore,bemorewhite-attheonsetoftheblowthetimeswouldbesomewhatlessdecaps over the shallow waters than over the deeper waterpending on the initial wave heights and their direction relativefarther offshore.It is only over relatively deep water thatto the direction ofthewind which has sprung up.the sea criterion can be used with confidence.The first consideration when using the sea criterion toSwell:Swell is the namegiven to waves,generally ofestimate wind speed, therefore, is to decide whether theconsiderable length,which were raised in somedistant areawind has been blowing long enough from the same direc-by winds blowing there, and which have moved into the vi-tion to produce a steady state sea condition.Ifnot,then it iscinity of the ship: or to waves raised nearby and whichpossiblethat the wind speed may beunderestimated.continuetoadvanceafterthewindattheshiphasabatedorExperience has shown that the appearance of white-changed direction.The direction of swell waves is usuallycaps, foam, spindrift, etc., reaches a steady state conditiondifferent from the direction of the wind and the sea wavesSwell waves are not considered when estimating windbeforetheheightofthewavesattaintheirmaximumvaluespeed and direction.Onlythose waves raised by the windIt is a safeassumption that the appearance of the sea(suchblowing at the time are of any significance.The wind-driv-as white-caps, etc.)will reach a steady state in the time re-quiredtobuild thewaves to 50-75percent of theirenwavesshowa greatertendencytobreakwhenmaximum height.Thus, from Table 3712, it is seen that asuperimposed on the crests of swell, and hence, morewhitecaps maybeformed than iftheswell wereabsent.Un-force5windcouldrequire8hoursatmosttoproduceader these conditions, theuse of the sea criterion may resultcharacteristic appearance of the sea surfacein a slight overestimate of the wind speed.A second consideration, when using the sea criterion, isPrecipitation: Heavy rain has a damping or smoothingthe lengthofthefetch over whichthe wind has been blowingtoproducethepresentstateofthesea.Ontheopenseaun-effectontheseasurfacewhichmustbemechanicalinchar-lessthemarinerhasthelatest synopticweathermapacter.Since the sea surface will therefore appear lessavailable,thelengthofthefetch will notbeknown.Itwill bedisturbed than would be the case without therain, the windseen from Table 3712, though, that only relatively shortspeedmaybe underestimatedunless the smoothing effect isfetches are requiredforthe lowerwind forcesto generatetakenintoaccounttheircharacteristicseas.Ontheopensea,thefetchesassoci-Ice:Even small concentrationsoficefloatingonthe seaated withmoststorms andotherweather systems areusuallysurfacewill dampenwavesconsiderably,and concentra-long enough so that even winds uptoforce9canbuild seastions greater than about seven-tenths averagewill eliminateupto90percentormoreoftheirmaximumheight,providingwaves altogether.Young sea ice, which in theearly stages ofthe wind blows from the samedirection long enough.formation has a thick soupy consistency,and later takes onWhen navigating closeto a coast, or inrestricted waarubberyappearance,isveryeffective indampeningwaves.ters, however,it maybenecessaryto makeallowancesforConsequently.theseacriterioncannotbeusedwithanydetheshorterstretchesofwateroverwhichthewindblowsgree of confidence when sea ice is present. In higherForexample,referringtoTable3712,if theshipis22mileslatitudes,thepresenceofan icefieldsomedistancetowind-from acoast, and anoffshorewindwith an actual speed ofwardoftheshipmaybesuspectedifwhentheshipisnotforce7 is blowing,thewaves at the ship will never attainclose to any coast, the wind is relatively strong but the seasmorethan50percentoftheirmaximumheightforthisspeedabnormally underdeveloped.The edge of the icefield actsnomatter howlongthewindblows.Hence,ifthesea crite-like a coastline, and the short fetch between the ice and the

528 WEATHER OBSERVATIONS for a long time, the waves are quite small near shore, and in￾crease in height rapidly over the first 50 miles or so of the fetch. Farther offshore, the rate of increase in height with distance slows down, and after 500 miles or so from the be￾ginning of the fetch, there is little or no increase in height. Table 3712 illustrates the duration of winds and the length of fetches required for various wind forces to build seas to 50 percent, 75 percent, and 90 percent of their theo￾retical maximum heights. The theoretical maximum wave heights represent the average heights of the highest third of the waves, as these waves are most significant. It will be seen that winds of force 5 or less can build seas to 90 percent of their maximum height, in less than 12 hours, provided the fetch is long enough. Higher winds require a much greater time-force 11 winds requiring 32 hours to build waves to 90 percent of their maximum height. The times given in Table 3712 represent those required to build waves starting from initially calm sea conditions. If waves are already present at the onset of the blow, the times would be somewhat less de￾pending on the initial wave heights and their direction relative to the direction of the wind which has sprung up. The first consideration when using the sea criterion to estimate wind speed, therefore, is to decide whether the wind has been blowing long enough from the same direc￾tion to produce a steady state sea condition. If not, then it is possible that the wind speed may be underestimated. Experience has shown that the appearance of white￾caps, foam, spindrift, etc., reaches a steady state condition before the height of the waves attain their maximum value. It is a safe assumption that the appearance of the sea (such as white-caps, etc.) will reach a steady state in the time re￾quired to build the waves to 50-75 percent of their maximum height. Thus, from Table 3712, it is seen that a force 5 wind could require 8 hours at most to produce a characteristic appearance of the sea surface. A second consideration, when using the sea criterion, is the length of the fetch over which the wind has been blowing to produce the present state of the sea. On the open sea, un￾less the mariner has the latest synoptic weather map available, the length of the fetch will not be known. It will be seen from Table 3712, though, that only relatively short fetches are required for the lower wind forces to generate their characteristic seas. On the open sea, the fetches associ￾ated with most storms and other weather systems are usually long enough so that even winds up to force 9 can build seas up to 90 percent or more of their maximum height, providing the wind blows from the same direction long enough. When navigating close to a coast, or in restricted wa￾ters, however, it may be necessary to make allowances for the shorter stretches of water over which the wind blows. For example, referring to Table 3712, if the ship is 22 miles from a coast, and an offshore wind with an actual speed of force 7 is blowing, the waves at the ship will never attain more than 50 percent of their maximum height for this speed no matter how long the wind blows. Hence, if the sea crite￾rion were used under these conditions without consideration of the short fetch, the wind speed would be underestimated. With an offshore wind, the sea criterion may be used with confidence if the distance to the coast is greater than the val￾ues given in the extreme right-hand column of Table 3712; again, provided that the wind has been blowing offshore for a sufficient length of time. 3713. Special Wind Effects Tidal and Other Currents: A wind blowing against a tide or strong current causes a greater sea disturbance than nor￾mal, which may result in an overestimate of the wind speed. On the other hand, a wind blowing in the same direction as a tide or strong current causes less sea disturbance than normal, and may result in an underestimate of the wind speed. Shallow Water: Waves running into shallow water in￾crease in steepness, and hence, their tendency to break. With an onshore wind there will, therefore, be more white￾caps over the shallow waters than over the deeper water farther offshore. It is only over relatively deep water that the sea criterion can be used with confidence. Swell: Swell is the name given to waves, generally of considerable length, which were raised in some distant area by winds blowing there, and which have moved into the vi￾cinity of the ship; or to waves raised nearby and which continue to advance after the wind at the ship has abated or changed direction. The direction of swell waves is usually different from the direction of the wind and the sea waves. Swell waves are not considered when estimating wind speed and direction. Only those waves raised by the wind blowing at the time are of any significance. The wind-driv￾en waves show a greater tendency to break when superimposed on the crests of swell, and hence, more whitecaps may be formed than if the swell were absent. Un￾der these conditions, the use of the sea criterion may result in a slight overestimate of the wind speed. Precipitation: Heavy rain has a damping or smoothing effect on the sea surface which must be mechanical in char￾acter. Since the sea surface will therefore appear less disturbed than would be the case without the rain, the wind speed may be underestimated unless the smoothing effect is taken into account. Ice: Even small concentrations of ice floating on the sea surface will dampen waves considerably, and concentra￾tions greater than about seven-tenths average will eliminate waves altogether. Young sea ice, which in the early stages of formation has a thick soupy consistency, and later takes on a rubbery appearance, is very effective in dampening waves. Consequently, the sea criterion cannot be used with any de￾gree of confidence when sea ice is present. In higher latitudes, the presence of an ice field some distance to wind￾ward of the ship may be suspected if, when the ship is not close to any coast, the wind is relatively strong but the seas abnormally underdeveloped. The edge of the ice field acts like a coastline, and the short fetch between the ice and the

529WEATHEROBSERVATIONSship is not sufficientfor the wind to fully develop the seas.gives the sea a “choppy"or confused appearance. It is dur-Wind Shifts: Following a rapid change in the directioning the firstfewhours following thewind shiftthattheofthewind,asoccursatthepassageofacoldfront,thenewappearance ofthe sea surfacemay not providea reliable in-wind will flatten out to a greatextent thewaves which weredication of wind speed. The wind is normally stronger thanpresent before the wind shift.This happens because the di-the sea would indicate,as old waves are beingflattened out,rection of the windafterthe shift may differby90°or moreand new waves are beginning to be developed.from the direction of the waves, which does not change.Night Observations: On a dark night, when it is im-Hence,the windmayopposethe progress ofthe waves andpossible to see the sea clearly,the observer mayestimatedampen them out quickly.Atthe same time, the new windthe apparent wind from its effect on the ship's riggingbegins to generate its own waves on top of this dissipatingflags, etc.,or simply the“feel"ofthe wind.swell, and itisnot longbeforethecrosspatternof wavesCLOUDS3714.CloudFormationcolor, and often of a silky appearance (Figure 3715a andFigure 3715d).Their fibrous and feathery appearance isClouds consist of innumerabletiny droplets of water.causedbytheir composition oficecrystals.Cirrus appear inor ice crystals,formed by condensation of water vaporvariedformssuchasisolatedtufts;long,thinlinesacrossaround microscopic particles in the air.Fog is a cloud inthe sky, branching, feather-like plumes; curved wispscontact with the surface of the earth.whichmay end in tufts,and other shapes.These clouds mayThe shape,size,height,thickness,andnature ofa cloudbearranged inparallel bands which cross thesky in greatdepend upon the conditions under which itis formedcircles,and appearto convergetoward a point on the hori-Therefore,clouds are indicators ofvarious processes occurzon. This may indicate the general direction of a lowring inthe atmosphere.The abilitytorecognize differentpressurearea.Cirrusmaybebrilliantlycolored at sunrisetypes,and aknowledge of the conditions associated withand sunset.Because oftheir height, they become illuminat-them, are useful in predicting future weather.ed before other clouds in the morning,and remain lightedAlthoughthevariety ofclouds is virtually endless,theyafter others at sunset. Cirrus are generally associated withmay be classified according to general type. Clouds arefair weather, but if they arefollowed by lower and thickergrouped generally into three"families"according to com-clouds, they are often the forerunner of rain or snowmon characteristics.High clouds have a mean lower levelCirrocumulus (Ce)are high clouds composed ofabove20.000 feet.They are composed principallyof icesmall whiteflakes orscales,orofvery small globular mass-crystals.Middlecloudshaveameanlevelbetween6,500es, usuallywithout shadows and arranged in groups ofand20,000feet.Theyarecomposed largelyofwaterdrop-lines,ormoreoften inripplesresembling sandonthesea-lets,although the higher ones havea tendency toward iceshore (Figure 3715b).One form of cirrocumulus isparticles.Low clouds have a mean lower level of less thanpopularlyknown as“mackerel sky"because the pattern re-6.500 feet.These clouds are composed entirelyof watersembles the scales on the back of a mackerel. Like cirrus.dropletscirrocumulus arecomposedofice crystalsand aregenerallyWithinthese3families are 10principal cloud typesassociatedwithfairweather,butmayprecedea stormifThe names oftheseare composed of various combinationsthey thicken and lower.They may turngray and appearandformsofthefollowingbasicwords,all fromLatinhard before thickening.Cirrostratus (Cs) are thin, whitish, high clouds (Fig.Cirrus,meaning“curl, lock,or tuft ofhair."3715c)sometimes covering the sky completely and giving itCumulus,meaning“heap,a pile, an accumulation."a milky appearance and at other times presenting, more orStratus, meaning “spread out, flatten, cover with aless distinctly,a formation like a tangled web.The thin veillaver."isnotsufficientlydensetoblurtheoutlineofsunormoonAlto, meaning “high, upper air."However,theicecrystals ofwhichthecloud is composedre-Nimbus, meaning"rainy cloud."fract the light passing through to form halos with the sun ormoon at the center.Figure3715d shows cirrus thickeningIndividual cloud types recognize certain characteris.and changing into cirrostratus.In this form it is popularlytics,variations,or combinations of these.The10principalknown as“mares'tails."Ifit continues to thicken and lower,cloudtypes andtheircommonlyused symbols arethe ice crystals melting to form water droplets, the cloud for-mation isknown as altostratus.When this occurs,rain may3715.HighCloudsnormallybe expected within24hours.Themorebrush-likeCirrus (Ci)aredetached high clouds of delicate andfi-the cirrus when the sky appears as in Figure 3715d, the stron-brous appearance, without shading, generally white inger wind at the level of the cloud

WEATHER OBSERVATIONS 529 ship is not sufficient for the wind to fully develop the seas. Wind Shifts: Following a rapid change in the direction of the wind, as occurs at the passage of a cold front, the new wind will flatten out to a great extent the waves which were present before the wind shift. This happens because the di￾rection of the wind after the shift may differ by 90° or more from the direction of the waves, which does not change. Hence, the wind may oppose the progress of the waves and dampen them out quickly. At the same time, the new wind begins to generate its own waves on top of this dissipating swell, and it is not long before the cross pattern of waves gives the sea a “choppy” or confused appearance. It is dur￾ing the first few hours following the wind shift that the appearance of the sea surface may not provide a reliable in￾dication of wind speed. The wind is normally stronger than the sea would indicate, as old waves are being flattened out, and new waves are beginning to be developed. Night Observations: On a dark night, when it is im￾possible to see the sea clearly, the observer may estimate the apparent wind from its effect on the ship’s rigging, flags, etc., or simply the “feel” of the wind. CLOUDS 3714. Cloud Formation Clouds consist of innumerable tiny droplets of water, or ice crystals, formed by condensation of water vapor around microscopic particles in the air. Fog is a cloud in contact with the surface of the earth. The shape, size, height, thickness, and nature of a cloud depend upon the conditions under which it is formed. Therefore, clouds are indicators of various processes occur￾ring in the atmosphere. The ability to recognize different types, and a knowledge of the conditions associated with them, are useful in predicting future weather. Although the variety of clouds is virtually endless, they may be classified according to general type. Clouds are grouped generally into three “families” according to com￾mon characteristics. High clouds have a mean lower level above 20,000 feet. They are composed principally of ice crystals. Middle clouds have a mean level between 6,500 and 20,000 feet. They are composed largely of water drop￾lets, although the higher ones have a tendency toward ice particles. Low clouds have a mean lower level of less than 6,500 feet. These clouds are composed entirely of water droplets. Within these 3 families are 10 principal cloud types. The names of these are composed of various combinations and forms of the following basic words, all from Latin: Cirrus, meaning “curl, lock, or tuft of hair.” Cumulus, meaning “heap, a pile, an accumulation.” Stratus, meaning “spread out, flatten, cover with a layer.” Alto, meaning “high, upper air.” Nimbus, meaning “rainy cloud.” Individual cloud types recognize certain characteris￾tics, variations, or combinations of these. The 10 principal cloud types and their commonly used symbols are: 3715. High Clouds Cirrus (Ci) are detached high clouds of delicate and fi￾brous appearance, without shading, generally white in color, and often of a silky appearance (Figure 3715a and Figure 3715d). Their fibrous and feathery appearance is caused by their composition of ice crystals. Cirrus appear in varied forms such as isolated tufts; long, thin lines across the sky; branching, feather-like plumes; curved wisps which may end in tufts, and other shapes. These clouds may be arranged in parallel bands which cross the sky in great circles, and appear to converge toward a point on the hori￾zon. This may indicate the general direction of a low pressure area. Cirrus may be brilliantly colored at sunrise and sunset. Because of their height, they become illuminat￾ed before other clouds in the morning, and remain lighted after others at sunset. Cirrus are generally associated with fair weather, but if they are followed by lower and thicker clouds, they are often the forerunner of rain or snow. Cirrocumulus (Cc) are high clouds composed of small white flakes or scales, or of very small globular mass￾es, usually without shadows and arranged in groups of lines, or more often in ripples resembling sand on the sea￾shore (Figure 3715b). One form of cirrocumulus is popularly known as “mackerel sky” because the pattern re￾sembles the scales on the back of a mackerel. Like cirrus, cirrocumulus are composed of ice crystals and are generally associated with fair weather, but may precede a storm if they thicken and lower. They may turn gray and appear hard before thickening. Cirrostratus (Cs) are thin, whitish, high clouds (Fig. 3715c) sometimes covering the sky completely and giving it a milky appearance and at other times presenting, more or less distinctly, a formation like a tangled web. The thin veil is not sufficiently dense to blur the outline of sun or moon. However, the ice crystals of which the cloud is composed re￾fract the light passing through to form halos with the sun or moon at the center. Figure 3715d shows cirrus thickening and changing into cirrostratus. In this form it is popularly known as “mares’ tails.” If it continues to thicken and lower, the ice crystals melting to form water droplets, the cloud for￾mation is known as altostratus. When this occurs, rain may normally be expected within 24 hours. The more brush-like the cirrus when the sky appears as in Figure 3715d, the stron￾ger wind at the level of the cloud

530WEATHEROBSERVATIONSFigure 3715a. Cirrus.Figure3715b.Cirrocumulus.Figure3715d.Cirrus and cirrostratus.Figure3715c.CirrostratusFigure3716b.Altocumulus inbands.Figure3716a.Altocumulus inpatches.Figure 3716c.Turreted altocumulusFigure3716d.AltostratusFigure 3717a. Stratocumulus.Figure 3717b. Stratus.Figure3717cCumulusFigure 3717d.Cumulonimbus

530 WEATHER OBSERVATIONS Figure 3715a. Cirrus. Figure 3715c. Cirrostratus. Figure 3716a. Altocumulus in patches. Figure 3716c. Turreted altocumulus. Figure 3717a. Stratocumulus. Figure 3717cCumulus. Figure 3715b. Cirrocumulus. Figure 3715d. Cirrus and cirrostratus. Figure 3716b. Altocumulus in bands. Figure 3716d. Altostratus. Figure 3717b. Stratus. Figure 3717d. Cumulonimbus