NAUI Master Scuba Diver 40 The Diving Environment your knees bent, and shuffle quickly to shore. You should duck beneath any oncoming waves that are higher than chest level. To exit through plunging breakers, again time the surf beat and ride the back of a wave to the beach. Beware of being sucked into the wave and “over the falls.” Swim hard to pass quickly through the surf zone before the next wave breaks. Do not try to stand - stay horizontal and literally crawl clear of the water before standing (figure 2-8). Keep all equipment in place until clear of the water when exiting through surf. During surf exits, avoid having a float between yourself and oncoming waves or the float could be pushed into and over you. Push it ahead of you as you exit, or, if it is light, it may be more convenient to carry it clear of the water. TIDES Tides are the predictable, periodic rising and falling of waters primarily due to the gravitational attraction of the moon and secondarily of the sun upon the earth (figure 2-9). The moon, being much closer to the earth, influences the tides about twice as much as the sun, despite its relatively small size. On seacoasts, tides are regular and rhythmic and are essentially long-period waves with a wave length equal to half the circumference of the earth. The gravitational attraction of the moon results in a tide-producing force on the earth’s hemisphere nearest the moon. On the hemisphere opposite the moon, the tide-producing force is in the opposite direction (away from the moon). The resulting effect is two bulges of water on opposite sides of the earth. Visualize the bulges remaining relatively stationary as the earth rotates on its axis beneath the water. Ideally, most points on the earth should experience semi-diurnal (twice daily) tides. However, the position of the moon relative to the equator, because of the tilt of the earth, results in an inequality of tidal patterns in many locations. Quadrature Neap Tide Full Moon New Moon Earth Spring Tide Spring Tide Quadrature First Quarter Last Quarter Neap Tide FIGURE 2-9. TIDE CYCLE The sun acts similarly upon the waters, but with lesser effect. The total tide-producing force is the result of both the sun and the moon. Sun tides increase or reduce lunar tides. When the earth, sun, and moon are aligned (in phase) and when the three are at right angles to each other (out of phase) special situations occur. When the effects are in phase, the solar tide reinforces and amplifies the lunar tide to cause higher than usual spring tides, which occur at new and full moons. Neap tides, lower than usual tidal changes, occur when the sun and moon are out of phase. Tidal range is further influenced by the proximity of the moon to the earth. When the moon is in its orbit nearest the earth (at perigee), tides are higher; when the moon is farthest from the earth (at apogee), tides are lower. When spring tides coincide with a perigee, the highest tides of the year are produced, and when neap tides coincide with an apogee, the lowest tides of the year occur. Note that “spring,” in this context, has nothing to do with the seasons. Although the tidal forces exert themselves consistently over the earth, the configurations of the ocean basins and the interference of land masses prevent the tides from assuming a simple, regular pattern. As water flows over
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