NAUI Master Scuba Diver 44 The Diving Environment There are three basic parts of a rip current: • Mouth–This is the shoremost part known as a feeder zone. It can be fed by longshore current and by the buildup of water on the incline of a beach. • Neck–This is the midpoint of a rip where the offshore motion has its greatest velocity. • Head–The area where the rip current dissipates its energy and ceases to flow offshore. Rip currents may be recognized by a fan-shaped buildup of water on a beach, a stream of dirty water extending outward from the shore, foam on the surface extending beyond the surf zone, and a distinct lack of surf where the current flows outward. Detection of a rip is most difficult on windy days when the surf is choppy. Modest rip currents may be used to aid offshore movement, but should only be used by those who are very familiar with these currents. If you realize you are in a rip and wish to get out, do so by swimming perpendicular to the current - they are rarely more than 30.5 m (100 ft) wide. If a longshore current exists as well, swim out of the rip on the down current side. The velocity of a rip current is often greater than 0.5 m/sec (one knot) and should not be fought. Experienced divers sometimes use rips to get offshore quickly. They ride the current to its head and exit on the down current side. If you are not familiar with rip currents, it is best to avoid them entirely. Wind Currents Offshore currents in large bodies of water are caused by the wind. They are essentially streams of water moving within a larger body of water. Temperature differences create convection currents in the oceans, although the convection currents have very low velocities. The stress of wind blowing across water causes the surface layer of water to move. This motion is transmitted to succeeding layers of water beneath the surface. The rate of motion decreases with depth. A wind current does not flow exactly in the direction of the wind due to the force of the rotation of the earth, or coriolis force. Deflection by coriolis force is clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere. Major ocean currents tend, therefore, to flow clockwise above the equator and counter-clockwise below the equator, with relation to the earth as viewed from above. The coriolis force is greater in higher latitudes and most effective in deep water. Current direction varies from about 15 degrees from the wind along shallow coastal areas to a maximum of 45 degrees in the deep oceans. The velocity of a wind-produced current depends on the speed of the wind, its constancy, the length of time it blows, and other factors. A wind blowing for 12 hours or more in the same direction will cause a surface current equal to about two percent of the wind speed. The set and drift of a current refer to the current’s direction and velocity, respectively. The strength of a current is affected by: 1. Water depth - The speed of a wind-driven current decreases rapidly with depth. 2. Bottom formations - The bottom acts to resist water movement. Therefore, a current is generally weakest at the bottom. 3. Water temperature - Water becomes colder with depth. As the temperature decreases, the density and weight of the water increases, and heavier water resists movement more than lighter water. HOW TO DEAL WITH CURRENTS Shore Dives If unexpectedly caught downstream from your planned exit point, the preferred order of options is: • Return upstream on the bottom if air is available. Pulling hand over hand may be required. • Swim perpendicular to the current to try to get clear of it. • Exit at a pre-planned alternate exit location. • Obtain positive buoyancy, signal for assistance, and wait to be rescued. Useful devices to signal rescuers include a whistle, a mask face plate which can be used as a reflector, or an inflatable locator or signal tube - sometimes called a “sausage.” Waving both arms overhead is also a
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