148TIDESANDTIDALCURRENTStions areboth importantfactors and thetide is characterizedlow waters are about the same.At Seattle the greater ine-by a large inequality in the high water heights, low waterqualities aretypicallyin the lowwaters,while at Honoluluit is thehigh waters that have the greater inequalitiesheights, or inboth.There are usually twohigh and two lowwaters each day,but occasionally the tide may become di-906.SolarTideurnal.Suchtides areprevalentalongthePacificcoast of theUnited States and in many other parts of theworld.Exam-The natural period of oscillation of a body of waterples ofmixed types oftide are shown in Figure 905c. At LosAngeles, it is typical that the inequalities in the high andmay accentuate either the solar or the lunar tidal oscilla-8728286728812181212P#ESHOURSHOURSHOURSF.-4F-7-10F6LUEE2nNFoF4F4LOSANGELESSEATTLEHONOLULUFigure 905c.Mixed tidetions. Though as a general rule the tides follow the moon,riod.Thepractical effect is tocreate alonger periodof standthe relative importanceofthesolareffect varies indifferentat high or lowtide.Thetidetables list theseand other pecu-areas.Thereareafewplaces,primarily in the SouthPacificliaritieswheretheyoccur.and the Indonesian areas, where the solar oscillation is themore important,andatthoseplaces thehigh and lowwaters908.VariationsInRangeoccur at about the same time each day.At Port Adelaide,Though the tide at a particular place can be classifiedAustralia the solar and lunar semidiurnal oscillations areequal andnullifyone anotherat neaps.as to type, it exhibits many variations during the month(Figure 908a).The range of the tide varies according to the907.SpecialTidalEffectsintensity ofthetide-producingforces,though theremaybea lag ofa day or two between a particular astronomic causeand thetidal effectAs a waveentersshallowwater,its speed isdecreased.The combined lunar-solar effect is obtained by addingSince the trough is shallower than the crest, it is retardedmore,resulting in a steepening of the wave front.In a fewestuaries,the advanceof thelow water trough is so muchretarded that the crest of the rising tideovertakes the low,and advances upstream as a breaking wave called a bore.Bores that are large and dangerous at times of large tidalranges maybe mere ripples at thosetimes of the monthwhentherangeis small.ExamplesoccurinthePetitcodiacRiver in theBay of Fundy,and at Haining,China, in theTsientang Kaing. The tide tables indicate where boresoccurOther special features are thedoublelowwater (as atHoek Van Holland)and the double high water (as atSouthampton, England).At such places there is often aslight fall or rise in the middle of the high or low water pe-
148 TIDES AND TIDAL CURRENTS tions are both important factors and the tide is characterized by a large inequality in the high water heights, low water heights, or in both. There are usually two high and two low waters each day, but occasionally the tide may become diurnal. Such tides are prevalent along the Pacific coast of the United States and in many other parts of the world. Examples of mixed types of tide are shown in Figure 905c. At Los Angeles, it is typical that the inequalities in the high and low waters are about the same. At Seattle the greater inequalities are typically in the low waters, while at Honolulu it is the high waters that have the greater inequalities. 906. Solar Tide The natural period of oscillation of a body of water may accentuate either the solar or the lunar tidal oscillations. Though as a general rule the tides follow the moon, the relative importance of the solar effect varies in different areas. There are a few places, primarily in the South Pacific and the Indonesian areas, where the solar oscillation is the more important, and at those places the high and low waters occur at about the same time each day. At Port Adelaide, Australia the solar and lunar semidiurnal oscillations are equal and nullify one another at neaps. 907. Special Tidal Effects As a wave enters shallow water, its speed is decreased. Since the trough is shallower than the crest, it is retarded more, resulting in a steepening of the wave front. In a few estuaries, the advance of the low water trough is so much retarded that the crest of the rising tide overtakes the low, and advances upstream as a breaking wave called a bore. Bores that are large and dangerous at times of large tidal ranges may be mere ripples at those times of the month when the range is small. Examples occur in the Petitcodiac River in the Bay of Fundy, and at Haining, China, in the Tsientang Kaing. The tide tables indicate where bores occur. Other special features are the double low water (as at Hoek Van Holland) and the double high water (as at Southampton, England). At such places there is often a slight fall or rise in the middle of the high or low water period. The practical effect is to create a longer period of stand at high or low tide. The tide tables list these and other peculiarities where they occur. 908. Variations In Range Though the tide at a particular place can be classified as to type, it exhibits many variations during the month (Figure 908a). The range of the tide varies according to the intensity of the tide-producing forces, though there may be a lag of a day or two between a particular astronomic cause and the tidal effect. The combined lunar-solar effect is obtained by adding Figure 905c. Mixed tide
149TIDESANDTIDALCURRENTSAuE9N<-鑫oHE022MNSMLWSFMHSHLWSTADELADFMHHA10MLLWMINNMEW.FHHWWw*NOLULMHHWMLLWPEI-HATeLLWOrewnoonDfetaartO.tilmeeClautcusterE,ontheFohNSmaostarthestrerthotsouhottheEouteerpeign:Oy.sunatahumnalegsinaxter,A,P,ToochartdnumFigure908a.Monthlytidalvariationsatvariousplaces
TIDES AND TIDAL CURRENTS 149 Figure 908a. Monthly tidal variations at various places
150TIDESANDTIDALCURRENTSthemoon'stractiveforcesvectoriallytothesun'stractiveFavThreQuarteQvar(A)E(B)Priamirg accurswhen essee hbeteee sewae firv(A)qearteraadbetversfoasdhirdqeartncHighideoocursbeferetrawsitwaocn.Figure 908b. (A) Spring tides occur at times of new and fullmoon. Range of tide is greater than average since solar andlunar tractiveforces act in samedirection.(B)Neap tidesoccur at times of first and third quarters.Range of tide islessthanaveragesincesolarand lunartractiveforcesactatright angles.(B)Lagirigoecurswhen msee s beteecnfintguaraadfelandbetverethindqaartrsndarkHightidoucersahertramsitefmoas.Figure 908c. Priming and lagging the tides.forces.The resultant tidal bulge will be predominantly lu-nar with modifying solar effects upon both the height of thetide and the direction of the tidal bulge. Special cases of in-terest occur during the times of new and full moon (Figure908b).Withthe earth,moon,and sunlyingapproximatelyon the same line, the tractiveforces ofthe sun are acting inthesamedirectionasthemoon'stractiveforces(modifiedby declination effects).The resultant tides are called springtides, whose ranges are greater than average.Between the spring tides, the moon is at first and thirdquarters.Atthosetimes,thetractiveforces of the sun areacting at approximately right angles to the moon's tractiveforces.Theresults aretides calledneaptides, whoserangesarelessthanaverageWith the moon in positions between quadrature andnew or full, the effect of the sun is to cause the tidal bulgeto either lag or precede the moon (Figure 908c). These ef-fects are called priming and lagging the tides.Thus, when the moon is at the point in its orbit nearestthe earth (atperigee),the lunar semidiurnal range is increasedand perigean tides occur.When the moon is farthestfrom
150 TIDES AND TIDAL CURRENTS the moon’s tractive forces vectorially to the sun’s tractive forces. The resultant tidal bulge will be predominantly lunar with modifying solar effects upon both the height of the tide and the direction of the tidal bulge. Special cases of interest occur during the times of new and full moon (Figure 908b). With the earth, moon, and sun lying approximately on the same line, the tractive forces of the sun are acting in the same direction as the moon’s tractive forces (modified by declination effects). The resultant tides are called spring tides, whose ranges are greater than average. Between the spring tides, the moon is at first and third quarters. At those times, the tractive forces of the sun are acting at approximately right angles to the moon’s tractive forces. The results are tides called neap tides, whose ranges are less than average. With the moon in positions between quadrature and new or full, the effect of the sun is to cause the tidal bulge to either lag or precede the moon (Figure 908c). These effects are called priming and lagging the tides. Thus, when the moon is at the point in its orbit nearest the earth (at perigee), the lunar semidiurnal range is increased and perigean tides occur. When the moon is farthest from Figure 908b. (A) Spring tides occur at times of new and full moon. Range of tide is greater than average since solar and lunar tractive forces act in same direction. (B) Neap tides occur at times of first and third quarters. Range of tide is less than average since solar and lunar tractive forces act at right angles. Figure 908c. Priming and lagging the tides