Back to the Moon with Nuclear Rockets

Destructive surf, late in thesequence, during the tsunami,April 7, 1946, breakingthrough 100-foot palmson Coconut Island, Hilo,Hawaii.embayments or small con-: fined basins. Their lengths; are as long as 1,500 miles,their heights are from the seafloor to the surface, and theirspeeds are hundreds ofmiles per hour. The crest[ widths are so wide (upwardsof 50 miles) that they are lost• from view in the open oceanamong all of the wind-• waves and ocean swell.i The impact of these hugewaves can be dramatic onI coasts exposed to their full: force. The stories of the destructionby the tsunami createdwhen Krakatoa erupted! in 1887 among the islands! of Indonesia have becomelegendary. There were alsoexcellent data and informationobtained during and after the 1946 tsunami that struckthe Hawaiian Islands. At that time, the late Dr. Francis P.Shepard joined Army Engineers, Navy personnel, and geologistsfrom the University of Hawaii (Doak Cox and GordonMacDonald) to help in the most extensive examination everconducted on the destructive effects of a tsunami on a coastor island (see photo above and p. 38).Obviously, the occurrence of tsunamis in the PacificOcean is of keen interest to the population both on the islandsand the continental shores. Research, warning systems,and evacuation plans were initiated after the 1946 disaster.The Wind-Derived Wavy SurfaceWinds blowing across the sea form waves on the surface.Both gravity and surface tension act simultaneously, so thatthe length of the wave being formed by wind action is determinedby which dominates at any given time. Capillarywaves, or ripples, with lengths of 0.4 cm, or less, are basicallycontrolled by surface tension, gravity being totally insignificant.But in the case of waves with lengths greater than10 cm, gravity is the dominant factor and, for that reason,they are called "gravity waves."At wind speeds of less than 1 meter/sec, capillary wavesform on an otherwise calm surface. They arise quickly in responseto wind gusts, forming "cats paws" on the surface.They disappear just as quickly as the little gusts pass by, molecularviscosity taking over to smooth the water's surface.Gusty conditions usually lead to a surface that has both gravitywaves and ripples scattered widely across their surfaces.Winds blow ng at greater than 1 m/sec begin to form gravitywaves. Th< characteristics of gravity waves are determinedby thei' length, amplitude, and the depth of waterthrough whicr they are moving. In depths greater than halfthe wavelengtl, individual water particles rotate in basicallycircular orbits, hardly mixing or moving forward in the directionthe wave i; progressing, with the orbits decreasing exponentiallywith iepth. The maximum height attained by windwaves depenc s on the wind speed, the amount of time itcontinues to b ow, and the distance of ocean over which itblows (called t ie fetch). Basically, at any given wind speed, itis the length of time and the fetch that determine the height ofthe resulting wives.As wind wa/es grow under the attack of the wind, theirdistribution is landom; the winds are never constant in eitherdirection or sp;ed. Therefore, any description of them is statistical,giving i range of wave spectra, heights, frequencies,and directions of propagation. The speed of propagation ofwind waves is close to the average of the wind that prevailsat the time. In a typical range of winds, say 10 to 30 m/sec,wavelengths n nge from 60 to 600 m, wave periods from 5 to20 sec, and he ghts from 2 to 20 m.Waves with greater heights and lengths than those havebeen experien :ed (even recorded in a few cases) during hurricanesin which winds blow around 50 m/sec. Although it isnot an easy m ^asurement, reliable data indicate waves canform up to a hi 'ight of 40 m with lengths approaching 1,000+m. Such wave are restricted to those violent storms, though,and to the 40 c latitudes (the Roaring Forties) of the southernocean. For the most, part, waves rarely exceed 12 m in theAtlantic Ocear and 15 m in the North Pacific.Whitecaps. Observations of whitecaps atop wind wavesare readily m^de by astronauts, but only resolved by excellentphotography. No sensing systems other than the "eyeball"and cameras, preferably with focal lengths longer than250 mm, have the necessary resolution.The distribu ion of whitecapped seas is but poorly known,from space, o • any other kind of observations. This lack isprobably becc use of the conditions under which whitecapsare formed. Tl e whitecaps have to be fairly large to be seenfrom low-Earti orbit. In general, that means a wind speedgreater than 1 5 m/sec (Beaufort Force 7), to produce "foamfrom the breal ing crests blown in streaks along the directionof the wind." 'et, under such wind conditions, all the spray,foam, and aeiosols thrown into the marine air reduces thevisibility, no r latter what the observing direction many be.Furthermore, \ rinds of such speeds are usually born of a goodcyclonic stom with an extensive cloud cover.The best ch ince of observing a foamy, whitecapped sea isbehind a cold front, when the wind speeds remain high andthe overlying itmosphere is clear and dry (See photo, p. 43,top left.) Or, ft am katabatic, down-slope winds blowing fromthe Alps throu ^h the elongated seas of the northern Mediterranean.Wher ever astronauts observe whitecaps, they are atfirst surprised :hat the ocean surface appears "indistinct," asthough the sp; cecraft window has become hazy. Almost immediately,he wever, they realize that any nearby coastalContinued on page 4221st CENTURY Summer 1999 39