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A rip current, commonly referred to simply as a rip, or by the misnomer "rip tide", is one specific kind of water current that can be found near beaches. It is a strong, localized, and rather narrow current of water. It is strongest near the surface of the water, and it moves directly away from the shore, cutting through the lines of breaking waves.〔 Rip currents can occur at any beach where there are breaking waves: on oceans, seas, and large lakes. The location of rip currents can be unpredictable: while some tend to recur always in the same place, others can appear and disappear suddenly at various locations near the beach.〔(【引用サイトリンク】title=United States Lifesaving Association's - Rip Currents )〕 A rip current forms because breaking waves push water towards the land. Water that has been pushed up near the beach flows together (as feeder currents), and this water finds a place where it can flow back out to sea. The water then flows out at a right angle to the beach in a tight current called the "neck" of the rip, where the flow is most rapid. When the water in the rip current reaches outside of the lines of breaking waves, the flow loses power, and dissipates in what is known as the "head" of the rip. Sometimes tendrils of left-over current then actually curve back towards the shore. Rip currents can be hazardous to people who are in the water. Swimmers or floaters who are caught in a rip and who do not understand what is going on, may not have the necessary water skills, may panic, or may exhaust themselves by trying to swim directly against the flow of water. Because of these factors, rips are the leading cause of rescues by lifeguards at beaches, and in the US rips are responsible for an average of 46 deaths from drowning each year. A rip current is ''not'' the same thing as undertow, although some people use the latter term when they mean a rip current. Contrary to popular belief, neither rip nor undertow can pull a person vertically down and hold them under the water surface; A rip simply carries floating objects, including people, to an area outside the zone of the breaking waves. ==Causes and occurrence== When wind and waves push water toward the shore, that water is often forced sideways by the oncoming waves. This water streams along the shoreline until it finds a suitable exit route back out to the open water, straight out, at a right angle to the beach. This return flow of water is called a rip current. Rip currents are usually quite narrow, and are located in places such as where there is a gap in a sandbar, under piers, and along jetties. A fairly common misconception is that rip currents are somehow able to pull a swimmer vertically down, under the surface of the water. This is not true, and in reality a rip current is strongest close to the surface. This strong surface flow tends to damp the effect of incoming waves, leading to the illusion of a calm part of the sea, without waves, which may possibly attract some swimmers to that area. The offshore path taken by a rip current can be demonstrated by placing colored dye at the start of a current at the shoreline.〔(Don't get sucked in by the rip... ) Youtube.com〕 A more detailed description involves a quantity known as radiation stress. This is the force (or momentum flux) exerted on the water column by the presence of the wave. As a wave shoals and increases in wave height prior to breaking, radiation stress increases. To balance this, the mean surface (the water level with the wave averaged out) decreases—this is known as ''setdown''. As the wave breaks and continues to reduce in height, the radiation stress decreases. To balance this force, the mean surface increases—this is known as ''setup''. As a wave propagates over a sandbar with a gap (as shown in the lead image), the wave breaks on the bar, leading to setup. However, the part of the wave that propagates over the gap does not break, and thus setdown will continue. Thus, the mean surface over the bars is higher than that over the gap, and a strong flow issues outward through the gap. Rip currents can potentially occur wherever strong longshore variability in wave breaking exists. This variability may be caused by such features as sandbars (as shown in the animated diagram above), by piers and jetties, and even by crossing wave trains. Rips tend to be more common, wider and faster, wherever (and whenever) breaking waves are large and powerful. Varying underwater topography makes some beaches more likely to have rip currents; a few beaches are notorious in this respect. Although rip tide is a misnomer, in areas of significant tidal range the rip currents may only occur at certain points of the tide. Too much water over the bar will not cause waves to break over the bar as, of course, will too little. If the shoreline shelves gently, the area behind the bar may vary from a few feet to a mile or more in the course of an hour or two. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Rip current」の詳細全文を読む スポンサード リンク
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