FACTORS THAT INFLUENCE TIDES1 Tides are a natural phenomenon involving the alternating rise and fall in the earth"s large bodies of water caused by the gravitational forces of the moon and the sun. The combination of these two variable forces produces the complexrecurrentcycle known as the astronomical tides. Tides occur in oceans and seas, to a limited extent in large lakes and the atmosphere, and, to a very minute degree, in the earth"s solid crust.2 The force that generates tides is the interaction of two different forces: the gravitational attraction of the moon acting upon the earth"s waters and the centrifugal force produced by the revolution of the earth around the center of gravity of the earth-moon system. Although the moon is only 384,395 kilometers from the earth, compared with the sun"s much greater distance of 149,598,181 kilometers, the moon"s closer distance outranks its much smaller mass, and thus the moon"s tide-raising force is twice more than that of the sun.3 The tide-generating forces of the moon and sun cause a maximum accumulation of the ocean waters at two opposite positions on the earth"s surface. This means that at any given time there are two high tides on the planet. One is the direct tide on the side facing the moon; the other is the indirect tide on the opposite side. At the same time these two high tides occur, compensating amounts of water are drawn from all points that are 90 degrees away from thesebulges. As the earth rotates, a sequence of two high tides and two low tides is produced each day. Successive high tides occur about 12.4 hours apart. The direct high tide at any given location occurs when the moon is overhead; low tide occurs when the moon is at either horizon.4 The highest and lowest levels of high tide, called spring tide and neap tide, each occurs twice in every lunar month of about 27.5 days. A spring tide occurs at the new moon and at the full moon, when the moon and earth are aligned with the sun, and thus the moon"s pull is reinforced by the sun"s pull. At spring tide, the difference between high and low tides is the greatest. A neap tide, the lowest level of high tide, occurs when the sun-to-earth direction is at right angles to the moon-to-earth direction. When this happens, the gravitational forces of the moon and suncounteracteach other; thus, the moon"s pull is at minimum strength, and the difference between high and low tides is the least.Spring and neap tides at any given location have a range of about 20 percent more or less, respectively, than the average high tide.5 The vertical range of tides—the difference between high and low—varies according to the size, surface shape, and bottom topography of the basin in which tidal movement occurs. Typically, the tidal range in the open ocean is less than it is near the coasts. For example, in the open water of the central Pacific, the range is no more than 0.3 meters, but in the relatively small, shallow North Sea, it is about 3.6 meters. Along the narrow channel of theBay of Fundyin Nova Scotia, the difference between high and low tides may reach 16 meters under spring tide conditions—the world"s widest tidal range. At New Orleans, which is at the mouth of the Mississippi River, the periodic rise and fall of the tides varies with the river"s stage, being about 0.24 meters at low stage and zero at high stage. In every case, actual high or low tide can vary considerably from the average.6 Several physical factors influence tidal ranges, such as abrupt changes in atmospheric pressure,prolongedperiods of extreme high or low pressure, the density and volume of seawater, and variations in ocean-current velocities. Any of these factors alone can alter sea level; however, in combination with earthquakes, hurricanes, or other phenomena, they are particularly forceful. Hurricane winds, for instance, can heap ocean water into powerful storm surges that increase the impact of the astronomical tides. When a storm surge coincides with the astronomical high tide, the resulting storm tide may reach up to six meters or more above the normal high tide. The greater and more rapid the change of water level, the greater the erosive effect of the tidal action. Currents created by storm tides combine with the waves to erode beaches, coastal highways, and foundations of buildings. The word prolonged in paragraph 6 is closest in meaning to

A. extended
B. surprising
C. predicted
D. dangerous